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3.6: Igneous Rocks - Geosciences

3.6: Igneous Rocks - Geosciences


INTRODUCTION

Magma is molten rock inside the earth. By carefully analyzing igneous rocks and interpreting the information they contain, we can deduce processes that take place within the earth and we can understand volcanic processes that take place on the earth’s surface.

The study of igneous rocks enables us to understand the igneous part of geologic history. For example, at the end of the Triassic period, 245 million years ago, the greatest mass extinction ever known took place, wiping out more life forms on earth than the mass extinction that led to the demise of dinosaurs 65 million years ago at the end of the Cretaceous. At the end of the Triassic, a huge amount of basalt erupted onto the earth. Many geologists think that the gases and particles released into the atmosphere by those eruptions may have been a major factor in the end of Triassic mass extinction. Those scientists are studying the information contained in the basalts of that age to further test their hypotheses.

Igneous rocks contain three essential sources of information: their minerals, their overall chemical composition, and their igneous texture. Igneous rock names are based on specific combinations of these features. Igneous rocks also contain isotopic information that is used in determining absoloute ages and in further characterizing the origin of the magma. Special equipment and expertise is required to conduct isotopic and precise chemical analyses. Fortunately, with some basic training and practice anyone can learn to identify the minerals, composition and texture of an igneous rock; name the rock; and interpret key information about its origins.

All igneous rocks, other than pure volcanic glass, contain minerals. The minerals provide details on the chemical composition of the rock, and on the conditions in which the magma originated, cooled, and solidified. Geologists conduct chemical analyses of minerals to determine the temperatures and pressures at which they formed and to identify the dissolved gases and chemical elements that were present in the magma.

Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals. The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. Once magma has formed inside the earth, its composition may be modified. Minerals can grow from the magma and separate from it, changing the chemistry of the remaining liquid. Or, one body of magma can mix with another that has a different composition.

Magmas come in a range of compositions, from rich in silica and poor and iron and magnesium (felsic) to moderate in silica and high in iron and magnesium (mafic). Felsic igneous rocks, as a whole rock, tend to have light colors or shades: white, pink, light brown, light gray. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Most mafic magma originates by melting of rocks in the mantle that are extremely rich in iron and magnesium. Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust.

The igneous texture tells us how the magma cooled and solidified. Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock). Or, magma may flow out onto surface of the earth as a lava flow. Another way that igneous rock forms is by magma erupting explosively into the air and falling to earth in pieces known as pyroclastic material, also called tephra. Lava flows and pyroclastic material are volcanic igneous rock (also known as extrusive igneous rock).

The igneous texture of a rock is not how it feels in your hand, not whether it is rough or smooth. The igneous texture describes whether the rock has mineral crystals or is glassy, the size of the mineral grains, and the rock’s porosity (empty spaces).

This basics page focuses on igneous rocks and gives you the background needed to understand the terms used in the igneous rock classification table.

HOW ARE IGNEOUS ROCKS CLASSIFIED?

There are two main types of igneous rocks: (1) plutonic (intrusive) rocks, which form by solidification of molten rock deep within the earth, and (2) volcanic (extrusive) rocks, which solidify from molten rock erupted to the surface. Volcanic rocks break down into two more categories: (a) lava flows and (b) tephra (pyroclastic material).

Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions.For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. The way that magma turns into a solid rock gives it a distinctive igneous texture. For example, magma that becomes a pluton by slowly crystallizing (growing minerals) within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock.

Igneous Rock Textures

The texture of an igneous rock results from the cooling, crystallization, and solidification history of the magma that formed it. Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive, lava flow or pyroclastic.

Texture in this context is not whether the rock feels rough or smooth to the touch. Igneous texture terms have objective definitions that refer only to igneous rocks.

Volcanic Rocks

Let us start with textures associated with rocks formed by lava flows. Magmas that erupt as lava onto the earth’s surface cool and solidify rapidly. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture.

Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix. This texture is called porphyritic.

If lava has bubbles of gas escaping from it as it solidifies, it will end up with “frozen bubble holes” in it. These “frozen bubble holes” are called vesicles, and the texture of a rock containing them is said to be vesicular.

If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture.

If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals (glass by definition is not a mineral, because it does not have a crystal lattice). Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black.

Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. However, instead of originating from lava that flowed on the earth’s surface, tephra is volcanic material that was hurled through the air during a volcanic eruption.

A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in. The term “fine-grained, fragmental” is easy to confuse with the term fine-grained (aphanitic). An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff.

A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture. However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. The bigger chunks of material in a volcanic breccia are more than 1 cm (5/8 inch) across, and sometimes are much bigger.

Plutonic Rocks

When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture.

Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. This hydrothermal fluid gets into cracks and voids in the earth’s crust, and as it cools it may grow very large minerals from the dissolved chemical elements. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter (and sometimes is much larger). The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite.

Igneous Rock Compositions

The most common igneous compositions can be summarized in three words: mafic (basaltic), intermediate (andesitic), and felsic (granitic).

Felsic composition is higher in silica (SiO2) and low in iron (Fe) and magnesium (Mg). Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions.

Composition and Color

Composition influences the color of igneous rocks. Felsic rocks tend to be light in color (white, pink, tan, light brown, light gray). Mafic rocks tend to be dark in color (black, very dark brown, very dark gray, dark green mixed with black). The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors (green, gray, brown).

The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition. If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic.

This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly.

The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. Geologists working in the field carry a rock hammer, so they can break off the weathered, outer parts of rocks to see the “fresh,” unweathered rock inside.

If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. Igneous rocks with quartz in them are usually felsic. Igneous rocks with olivine in them are usually mafic. Igneous rocks with neither quartz nor olivine in them are most commonly intermediate.

ORIGINS OF IGNEOUS ROCKS

Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed. For example, a coarse-grained, felsic igneous rock is not only a granite, it is an intrusive igneous rock that formed from slow cooling and crystallization of a body of magma within the earth’s crust. The intrusion of large bodies of granite – batholiths – is usually part of the origin of a mountain range. Similarly, a fine-grained, mafic igneous rock is not only a basalt, it is an extrusive igneous rock that formed from rapid cooling and crystallization of a lava flow at earth’s surface.

HOW TO IDENTIFY IGNEOUS ROCKS

Igneous rocks can be distinguished from sedimentary rocks by the lack of beds, lack of fossils, and lack of rounded grains in igneous rocks, and the presence of igneous textures. A granite, for example, can be distinguished from a sandstone because rather than being a mixture of weathered, rounded grains compressed and cemented together, granite consists of a small number of minerals in shiny black, white, or pink colors, with excellent crystal forms, grown together into a completely interlocking pattern. Sandstones, by contrast, have sedimentary bedding (layers) and consist of rounded grains with some spaces between the grains, which you can see with a hand lens or magnifying glass.

Igneous rocks can be distinguished from most regional metamorphic rocks by the lack of foliation (layering) in igneous rocks. Unfoliated metamorphic rocks lack igneous textures and usually contain minerals not found in igneous rocks.

Granite may look like gneiss at first glance, but granite has no layering, no preferred orientation of the minerals. The minerals in a granite grow randomly in all directions, rather than tending to grow parallel to each other.

Igneous rocks are classified on the basis of their texture and their composition. See the previous sections for descriptions of the different igneous textures and compositions.

The igneous rock classification tables that accompany this section are arranged on the basis of igneous textures first, and further broken down on the basis of igneous composition. Remember that igneous composition is estimated on the basis of color: light = felsic composition, medium = intermediate composition, and dark = mafic composition.

Igneous Rock Classification

Pegmatitic Texture (Extremely Coarse-Grained)
Originates from water-rich intrusions, which cool and crystallize underground
CompositionMost Common MineralsRock Name
felsicNa-plagioclase, orthoclase, quartz, biotite, amphibole, muscovitepegmatite
Phanertitic Texture (Coarse-Grained)
Originates in deep intrusions, which cool and crystallize slowly underground
CompositionMost Common MineralsRock Name
felsicNa-plagioclase, orthoclase, quartz, biotite, amphibole, muscovitegranite
intermediateNa-plagioclase, quartz, orthoclase, amphibole, biotitegranodiorite
Na-plagioclase, amphibole, pyroxene, biotitediorite
maficCa-plagioclase, pyroxene, olivine, amphibolegabbro
Aphanitic Texture (Fine-Grained)
Originates in lava flows (or very shallow intrusions), which cool rapidly
CompositionMost Common MineralsRock Name
felsicNa-plagioclase, orthoclase, quartz, biotite, amphibole, muscoviterhyolite
intermediateNa-plagioclase, quartz, orthoclase, amphibole, biotitedacite
Na-plagioclase, amphibole, pyroxene, biotiteandesite
maficCa-plagioclase, pyroxene, olivine, amphibolebasalt
Frothy Texture (Porous, Pumiceous)
Originates in gas-charged volcanic eruptions, commonly pyroclastic
CompositionMost Common MineralsRock Name
felsicglass (may contain a few minerals typical of felsic rocks)pumice
maficglass (may contain a few mineral typical of mafic rocks)scoria
Note: Basalt with fewer holes, known as vesicles, is called vesicular basalt. Scoria has more holes and may be black or red in color.
Glassy Texture
Originates from cooling too rapid to allow crystal lattices to form
CompositionMost Common MineralsRock Name
felsic to maficglass (no minerals)obsidian
Note: Obsidian that is transparent at thin edges and has good conchoidal fracture is probably felsic.
Fragmental Texture—Coarse (Contains Large Rock Fragments)
Originates from pyroclastic (explosive) eruptions
CompositionMost Common MineralsRock Name
felsic to maficvariable (depending on rock fragments and ash content)volcanic breccia
Fragmental Texture—Fine (Mainly Volcanic Ash)
Originates from pyroclastic (explosive) eruptions
CompositionMost Common MineralsRock Name
felsicmay contain a few minerals typical of felsic rocksrhyolitic tuff
mediummay contain a few minerals typical of intermediate rocksandesitic tuff
maficmay contain a few minerals typical of mafic rocksandesitic tuff

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Volcano World

Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.

Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock.

Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable.

Granite is quarried in many places in the world including the United States. The State of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite.

Dacite is an extrusive igneous rock. The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende.

Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Notice in the photo to the left how it fractures. Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form.

When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way.

Obsidian is usually black or a very dark green, but it can also be found in an almost clear form.

Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations.

Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.

Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.

Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes.

Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals.

Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes.


3.6: Igneous Rocks - Geosciences

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Igneous rocks

Igneous rocks are those formed within the Earth at temperatures that are sufficiently high to produce a liquid component. As the liquid component increases it moves towards the surface because of its lower density than the enclosing solid rock. Many factors determine whether the partly molten rock (magma) remains within the crust (to form rocks such as granite) or whether it reaches the surface and is extruded as a lava. Temperature, fluid content (volatiles) and bulk composition of the magma are factors that strongly influence the behaviour of the magma.

Because of the infinite variables that control magma compositions no two igneous rocks are ever identical even though there might be some visual resemblance. Small variations in the mineral composition and proportions of the source material (often controlled by the degree of melting) will be reflected in the final product. If the magma generated in the "melting pot" becomes contaminated by crustal material (such as a descending slab) or coalesces with other melting pots, or traverses pockets of melt at higher levels during its rise to the surface its original composition will be modified and will be reflected in the eventual crystalline rock. Research into the different stages of rock formation is a fascinating science.

There are two principal igneous rock classes - those that remain under the crust when they crystallize (plutonic such as granites) and those that come out of the crust (volcanic such as lavas). Within each group there are two major sub-classes depending on their bulk composition, that is, felsic (quartz-bearing) and mafic (little or no quartz). These two sub-classes are generally reflected in colour with the former being light and the latter being dark. Some classifications include a more-nebulous intermediate group.


Chapter 3 Intrusive Igneous Rocks

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Describe the rock cycle and the types of processes that lead to the formation of igneous, sedimentary, and metamorphic rocks, and explain why there is an active rock cycle on Earth.
  • Explain the concept of partial melting and describe the geological processes that lead to melting.
  • Describe, in general terms, the range of chemical compositions of magmas.
  • Discuss the processes that take place during the cooling and crystallization of magma, and the typical order of crystallization according to the Bowen reaction series.
  • Explain how magma composition can be changed by fractional crystallization and partial melting of the surrounding rocks.
  • Apply the criteria for igneous rock classification based on mineral proportions.
  • Describe the origins of phaneritic, porphyritic, and pegmatitic rock textures.
  • Identify plutons on the basis of their morphology and their relationships to the surrounding rocks.
  • Explain the origin of a chilled margin.

Figure 3.0.1 A fine-grained mafic dyke (dark green) intruded into a felsic dyke (pink) and into coarse diorite (grey), Quadra Island, B.C. All of these rocks are composed of more than one type of mineral. The mineral components are clearly visible in the diorite, but not in the other two rock types.

A rock is a consolidated mixture of minerals. By consolidated, we mean hard and strong real rocks don’t fall apart in your hands! A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral (Figure 3.0.1). A rock can be composed of only one type of mineral (e.g., limestone is commonly made up of only calcite), but most rocks are composed of several different minerals. A rock can also include non-minerals, such as fossils or the organic matter within a coal bed or in some types of mudstone.

Rocks are grouped into three main categories based on how they form:

  1. Igneous: formed from the cooling and crystallization of magma (molten rock)
  2. Sedimentary : formed when weathered fragments of other rocks are buried, compressed, and cemented together, or when minerals precipitate directly from solution
  3. Metamorphic : formed by alteration (due to heat, pressure, and/or chemical action) of a pre-existing igneous or sedimentary rock

Media Attributions

Waves form on the ocean and on lakes because energy from the wind is transferred to the water. The stronger the wind, the longer it blows, and the larger the area of water over which it blows (the fetch ), the larger the waves are likely to be.

The important parameters of a wave are the wavelength (the horizontal distance between two crests or two troughs), the amplitude (the vertical distance between a trough and a crest ), and the wave velocity (the speed at which wave crests move across the water) (Figure 17.1.1).

Figure 17.1.1 The parameters of water waves.

The typical sizes and speeds of waves in situations where they have had long enough to develop fully are summarized in Table 17.1. In a situation where the fetch is short (say 19 km on a lake) and the wind is only moderate (19 km/h), the waves will develop fully within 2 hours, but they will remain quite small (average amplitude about 27 cm, wavelength 8.5 m). On a large body of water (the ocean or a very large lake) with a fetch of 139 km and winds of 37 km/h, the waves will develop fully in 10 hours the average amplitude will be around 1.5 m and average wavelength around 34 m. In the open ocean, with strong winds (92 km/h) that blow for at least 69 hours, the waves will average nearly 15 m high and their wavelengths will be over 200 m. Small waves (amplitudes under a metre) tend to have relatively shallow slopes (amplitude is 3% to 4% of wavelength), while larger waves (amplitudes over 10 m) have much steeper slopes (amplitude is 6% to 7% of wavelength). In other words, not only are large waves bigger than small ones, they are also generally more than twice as steep, and therefore many times more impressive—and potentially dangerous. It is important to recognize, however, that amplitudes decrease with distance from the area where the waves were generated. Waves on our coast that are generated by a storm near Japan will have similar wavelengths but lower amplitudes than those generated by a comparable storm just offshore.

Table 17.1 The parameters of wind waves in situations where the wind blows in roughly the same direction for long enough for the waves to develop fully. The duration times listed are the minimum required for the waves to develop fully. [1]
[Skip Table]
Wind Speed (kilometres per hour) Fetch (kilometres) Duration (hours) Amplitude (metres) Wavelength (metres) Wave Period (seconds) Wave Velocity (metres per second) Wave Velocity (kilometres per hour)
19 19 2 0.27 8.5 3.0 2.8 10.2
37 139 10 1.5 33.8 5.7 5.9 19.5
56 518 23 4.1 76.5 8.6 8.9 32.0
74 1,313 42 8.5 136 11.4 11.9 42.9
92 2,627 69 14.8 212 14.3 14.8 53.4

Exercise 17.1 Wave height versus length

This table shows the typical amplitudes and wavelengths of waves generated under different wind conditions. The steepness of a wave can be determined from these numbers and is related to the ratio: amplitude/wavelength.

  1. Calculate these ratios for the waves shown. The first one is done for you.
  2. How would these ratios change with increasing distance from the wind that produced the waves?

Relatively small waves move at up to about 10 km/h and arrive on a shore about once every 3 seconds. Very large waves move about five times faster (over 50 km/h), but because their wavelengths are so much longer, they arrive less frequently—about once every 14 seconds.

As a wave moves across the surface of the water, the water itself mostly just moves up and down and only moves a small amount in the direction of wave motion. As this happens, a point on the water surface describes a circle with a diameter that is equal to the wave amplitude (Figure 17.1.2). This motion is also transmitted to the water underneath, and the water is disturbed by a wave to a depth of approximately one-half of the wavelength. Wave motion is illustrated quite clearly on the Wikipedia “Wind wave” site. If you look carefully at that animation, and focus on the small white dots in the water, you should be able to see how the amount that they move decreases with depth.

Figure 17.1.2 The orbital motion of a parcel of water (black dot) as a wave moves across the surface.

The one-half wavelength depth of disturbance of the water beneath a wave is known as the wave base . Since ocean waves rarely have wavelengths greater than 200 m, and the open ocean is several thousand metres deep, the wave base does not normally interact with the bottom of the ocean. However, as waves approach the much shallower water near the shore, they start to “feel” the bottom, and they are affected by that interaction (Figure 17.1.3). The wave “orbits” are both flattened and slowed by dragging, and the implications are that the wave amplitude (height) increases and the wavelength decreases (the waves become much steeper). The ultimate result of this is that the waves lean forward, and eventually break (Figure 17.1.4).

Figure 17.1.3 The effect of waves approaching a sandy shore. Figure 17.1.4 Waves breaking on the shore at Greensand Beach, Hawaii (the sand is green because it is made up mostly of the mineral olivine eroded from the nearby volcanic rocks). Figure 17.1.5 Waves approaching the shore of Long Beach in Pacific Rim National Park. As the waves (depicted by white lines) approach shore, they are refracted to become more parallel to the beach, and their wavelength decreases.

Waves normally approach the shore at an angle, and this means that one part of the wave feels the bottom sooner than the rest of it, so the part that feels the bottom first slows down first. This process is illustrated in Figure 17.1.5, which is based on an aerial photograph showing actual waves approaching Long Beach on Vancouver Island. When the photo was taken, the waves (with crests shown as white lines in the diagram) were approaching at an angle of about 20° to the beach. The waves first reached shore at the southern end ("a" on the image). As they moved into shallow water they were slowed, and since the parts of the waves still in deep water ("b" on the image) were not slowed they were able catch up, and thus the waves became more parallel to the beach.

Figure 17.1.6 The generation of a longshore current by waves approaching the shore at an angle.

In open water, these waves had wavelengths close to 100 m. In the shallow water closer to shore, the wavelengths decreased to around 50 m, and in some cases, even less.

Even though they bend and become nearly parallel to shore, most waves still reach the shore at a small angle, and as each one arrives, it pushes water along the shore, creating what is known as a longshore current within the surf zone (the areas where waves are breaking) (Figure 17.1.6).

Exercise 17.2 Wave refraction

Figure 17.1.7

A series of waves (dashed lines) is approaching the coast on the map shown here.

The location of the depth contour that is equivalent to 1/2 of the wavelength is shown as a red dashed line.

Draw in the next several waves, showing how their patterns will change as they approach shallow water and the shore.

Show, with arrows, the direction of the resulting longshore current.

Another important effect of waves reaching the shore at an angle is that when they wash up onto the beach, they do so at an angle, but when that same wave water flows back down the beach, it moves straight down the slope of the beach (Figure 17.1.8). The upward-moving water, known as the swash, pushes sediment particles along the beach, while the downward-moving water, the backwash, brings them straight back. With every wave that washes up and then down the beach, particles of sediment are moved along the beach in a zigzag pattern.

The combined effects of sediment transport within the surf zone by the longshore current and sediment movement along the beach by swash and backwash is known as longshore drift. Longshore drift moves a tremendous amount of sediment along coasts (both oceans and large lakes) around the world, and it is responsible for creating a variety of depositional features that we’ll discuss in section 17.3.

Figure 17.1.9 The formation of rip currents on a beach with strong surf.

A rip current is another type of current that develops in the nearshore area, and has the effect of returning water that has been pushed up to the shore by incoming waves. As shown in Figure 17.1.9, rip currents flow straight out from the shore and are fed by the longshore currents. They die out quickly just outside the surf zone, but can be dangerous to swimmers who get caught in them. If part of a beach does not have a strong unidirectional longshore current, the rip currents may be fed by longshore currents going in both directions.

Figure 17.1.10 Rip currents on Tunquen Beach in central Chile.

Rip currents are visible in Figure 17.1.10, a beach at Tunquen in Chile near Valparaiso. As is evident from the photo, the rips correspond with embayments in the beach profile. Three of them are indicated with arrows, but it appears that there may be several others farther along the beach.

Tides are related to very long-wavelength but low-amplitude waves on the ocean surface (and to a much lesser extent on very large lakes) that are caused by variations in the gravitational effects of the Sun and Moon. Tide amplitudes in shoreline areas vary quite dramatically from place to place. On the west coast of Canada, the tidal range is relatively high, in some areas as much as 6 m, while on most of the east coast the range is lower, typically around 2 m. A major exception is the Bay of Fundy between Nova Scotia and New Brunswick, where the daily range can be as great as 16 m. Anomalous tides like that are related to the shape and size of bays and inlets, which can significantly enhance the amplitude of the tidal surge. The Bay of Fundy has a natural oscillation cycle of 12.5 hours, and that matches the frequency of the rise and fall of the tides in the adjacent Atlantic Ocean. Ungava Bay, on Quebec’s north coast, has a similarly high tidal range.

As the tides rise and fall they push and pull a large volume of water in and out of bays and inlets and around islands. They do not have as significant an impact on coastal erosion and deposition as wind waves do, but they have an important influence on the formation of features within the intertidal zone, as we’ll see in the following sections.


Classification of Igneous Rocks:

Igneous rocks are classified according to their mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Two important variables that are used for the classification of igneous rocks are particle size and the mineral composition of the rock. Feldspar, quartz, olivines, micas, etc., are all important minerals in the formation of igneous rocks, and are important to their classification.

Types of igneous rocks with other essential minerals are very rare. In simplified classification, igneous rocks are separated by the type of feldspar present, the presence or absence of quartz, and – in cases where feldspar or quartz are not present – by the type of iron or magnesium minerals present. Rocks containing quartz are silica-oversaturated, while rocks with feldspathoids are silica-undersaturated.

Igneous rocks which have crystals large enough to be seen with the unaided eye are classified as phaneritic, while those with crystals too small to be seen are aphanitic. Typically, rocks belonging to the phaneritic class are intrusive in origin, while aphanitic rocks are extrusive.

An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is classified as porphyry. Porphyritic textures develop when lava cools unevenly, causing of some of the crystals to grow before the main mass of the molten rock.

So the next time you find yourself somewhere, just standing about, remember that the ground you walk on was formed under from a pretty hellish process. It began deep in the Earth, where silicate rock, tormented by extreme heat and intense pressure, became a hot, oozing mess. Once it was churned up to the surface. it either exploded into the atmosphere, or melted a path across the landscape before cooling in place.

In short, our world was born of conditions that make Dante’s Inferno look boring and cheerful by comparison!

We have written many articles about igneous rocks for Universe Today. Here’s an article on How Rocks are Formed, What is the Earths’ Mantle Made From?, and What is the Difference Between Magma and Lava?

And for a more detailed look at the Earth, here’s What is the Lithosphere?, and What are the Earth’s Layers?

If you’d like more info on igneous rocks, check out U.S. Geological Survey Website. And here’s a link to Geology.com.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.


3.4 Classification of Igneous Rock

As has already been described, igneous rocks are classified into four categories: felsic, intermediate, mafic, and ultramafic, based on either their chemistry or their mineral composition. The diagram in Figure 3.4.1 can be used to help classify igneous rocks by their mineral composition. An important feature to note on this diagram is the red line separating the non-ferromagnesian silicates in the lower left (K-feldspar, quartz, and plagioclase feldspar) from the ferromagnesian silicates in the upper right (biotite, amphibole, pyroxene, and olivine). In classifying intrusive igneous rocks, the first thing to consider is the percentage of ferromagnesian silicates. In most igneous rocks the ferromagnesian silicate minerals are clearly darker than the others, but it is still quite difficult to estimate the proportions of minerals in a rock.

Based on the position of the red line in Figure 3.4.1, it is evident that felsic rocks can have between 1% and 20% ferromagnesian silicates (the red line intersects the left side of the felsic zone 1% of the distance from the top of the diagram, and it intersects the right side of the felsic zone 20% of the distance from the top). Intermediate rocks have between 20% and 50% ferromagnesian silicates, and mafic rocks have 50% to 100% ferromagnesian silicates. To be more specific, felsic rocks typically have biotite and/or amphibole intermediate rocks have amphibole and, in some cases, pyroxene and mafic rocks have pyroxene and, in some cases, olivine.

Figure 3.4.1 A simplified classification diagram for igneous rocks based on their mineral compositions. [Image Description]

If we focus on the non-ferromagnesian silicates, it is evident that felsic rocks can have from 0% to 35% K-feldspar, from 25% to 35% quartz (the vertical thickness of the quartz field varies from 25% to 35%), and from 25% to 50% plagioclase (and that plagioclase will be sodium-rich, or albitic). Intermediate rocks can have up to 25% quartz and 50% to 75% plagioclase. Mafic rocks only have plagioclase (up to 50%), and that plagioclase will be calcium-rich, or anorthitic.

Exercise 3.5 Mineral proportions in igneous rocks

Figure 3.4.2

The dashed blue lines (labelled a, b, c, d) in Figure 3.4.2 represent four igneous rocks. Complete the table by estimating the mineral proportions (percent) of the four rocks (to the nearest 10%).

Hint: Rocks b and d are the easiest start with those.

Rock Biotite/amphibole Pyroxene Olivine Plagioclase Quartz K-feldspar
a
b
c
d

Figure 3.4.3 provides a diagrammatic representation of the proportions of dark minerals in light-coloured rocks. You can use that when trying to estimate the ferromagnesian mineral content of actual rocks, and you can get some practice doing that by completing Exercise 3.6. Be warned! Geology students almost universally over-estimate the proportion of dark minerals.

Figure 3.4.3 A guide to estimating the proportions of dark minerals in light-coloured rocks.

Exercise 3.6 Proportions of ferromagnesian silicates

The four igneous rocks shown below have differing proportions of ferromagnesian silicates. Estimate those proportions using the diagrams in Figure 3.4.3, and then use Figure 3.4.1 to determine the likely rock name for each one.

___% ___% ___% ___%
__________ __________ __________ __________

Igneous rocks are also classified according to their textures. The textures of volcanic rocks will be discussed in Chapter 4, so here we’ll only look at the different textures of intrusive igneous rocks. Almost all intrusive igneous rocks have crystals that are large enough to see with the naked eye, and we use the term phaneritic (from the Greek word phaneros meaning visible) to describe that. Typically that means they are larger than about 0.5 millitmeres (mm) — the thickness of a strong line made with a ballpoint pen. (If the crystals are too small to distinguish, which is typical of most volcanic rocks, we use the term aphanitic (from the Greek word aphanos – unseen) The intrusive rocks shown in Figure 3.3.5 are all phaneritic, as are those shown in Exercise 3.6.

In general, the size of crystals is proportional to the rate of cooling. The longer it takes for a body of magma to cool, the larger the crystals can grow. It is not uncommon to see an intrusive igneous rock with crystals up to 1 centimetre (cm) long. In some situations, especially toward the end of the cooling stage, the magma can become water rich. The presence of liquid water (still liquid at high temperatures because it is under pressure) promotes the relatively easy movement of ions, and this allows crystals to grow large, sometimes to several centimetres (Figure 3.4.4). Finally, as already described, if an igneous rock goes through a two-stage cooling process, its texture will be porphyritic (Figure 3.3.7).

Figure 3.4.4 A pegmatitic rock with large crystals

Image Descriptions

Figure 3.4.1 image description: Mineral composition of igneous rocks
Igneous Rocks Felsic Intermediate Mafic Ultramafic
K-feldspar 0 to 35% 0% 0% 0%
Quartz 25 to 35% 0 to 25% 0% 0%
Plagioclase feldspar 25 to 50% 50 to 70% 0 to 50% 0%
Biotite and/or Amphibole 0 to 20% 20 to 40% 0 to 30% 0%
Pyroxene 0% 0 to 20% 20 to 75% 0% to 75%
Olivine 0% 0% 0 to 25 % 25% to 100%
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite

Attributions

Some coastal areas are dominated by erosion, an example being the Pacific coast of Canada and the United States, while others are dominated by deposition, examples being the Atlantic and Caribbean coasts of the United States. But on almost all coasts, both deposition and erosion are happening to varying degrees most of the time, although in different places. This is clearly evident in the Tofino area of Vancouver Island (Figure 17.0.1), where erosion is the predominant process on the rocky headlands, while depositional processes predominate within the bays. On deposition-dominant coasts, the coastal sediments are still being eroded from some areas and deposited in others.

A key factor in determining if a coast is dominated by erosion or deposition is its history of tectonic activity. A coast like that of British Columbia is tectonically active, and compression and uplift have been going on for tens of millions of years. This coast has also been uplifted during the past 15,000 years by isostatic rebound due to deglaciation. The coasts of the United States along the Atlantic and the Gulf of Mexico have not seen significant tectonic activity in a few hundred million years, and except in the northeast, have not experienced post-glacial uplift. These areas have relatively little topographic relief, and there is now minimal erosion of coastal bedrock. Another important factor is the supply of sediments. Unless there is a continuous supply of sandy and coarser sediment to a coast it will not be a depositional coast.

On coasts that are dominated by depositional processes, most of the sediment being deposited typically comes from large rivers. An obvious example is where the Mississippi River flows into the Gulf of Mexico at New Orleans another is the Fraser River at Vancouver. There are no large rivers bringing sandy sediments to the west coast of Vancouver Island, but there are still long and wide sandy beaches there. In this area, most of the sand comes from glaciofluvial sand deposits situated along the shore behind the beach, and some comes from the erosion of the rocks on the headlands.

The components of a typical beach are shown in Figure 17.3.1. On a sandy marine beach, the beach face is the area between the low and high tide levels. A berm is a flatter region beyond the reach of high tides this area stays dry except during large storms.

Figure 17.3.1 The components of a sandy marine beach. [Image Description] Figure 17.3.2 The differences between summer and winter on beaches in areas where the winter conditions are rougher and waves have a shorter wavelength but higher energy. In winter, sand from the beach is stored offshore.

Most beaches go through a seasonal cycle because conditions change from summer to winter. In summer, sea conditions are relatively calm with long-wavelength, low-amplitude waves generated by distant winds. Winter conditions are rougher, with shorter-wavelength, higher-amplitude waves caused by strong local winds. As shown in Figure 17.3.2, the heavy seas of winter gradually erode sand from beaches, moving it to an underwater sandbar offshore from the beach. The gentler waves of summer gradually push this sand back toward the shore, creating a wider and flatter beach.

The evolution of sandy depositional features on sea coasts is primarily influenced by waves and currents, especially longshore currents. As sediment is transported along a shore, either it is deposited on beaches, or it creates other depositional features. A spit , for example is an elongated sandy deposit that extends out into open water in the direction of a longshore current. A good example is Goose Spit at Comox on Vancouver Island (Figure 17.3.3). At this location, the longshore current typically flows toward the southwest, and the sand eroded from a 60 m high cliff of Pleistocene glaciofluvial Quadra Sand is pushed in that direction and then out into Comox Harbour.

Figure 17.3.3 The formation of Goose Spit at Comox on Vancouver Island. The sand that makes up Goose Spit is derived from the erosion of Pleistocene Quadra Sand (a thick glaciofluvial sand deposit, as illustrated in the photo on the right).

The Quadra Sand at Comox is visible in Figure 17.3.4. There are numerous homes built at the top of the cliff, and the property owners have gone to considerable expense to reinforce the base of the cliff with large angular rocks ( rip-rap ) and concrete barriers so as to limit further erosion of their properties. One result of this will be to starve Goose Spit of sediments and eventually contribute to its erosion. Of course the rocks and concrete barriers are only temporary they will be eroded by strong winter storms over the next few decades and the Quadra Sand will once again contribute to the maintenance of Goose Spit.

Figure 17.3.4 The Quadra Sand cliff at Comox, and the extensive concrete and rip-rap barrier that has been constructed to reduce erosion. Note that the waves (dashed lines) are approaching the shore at an angle, contributing to the longshore drift. Figure 17.3.5 A depiction of a baymouth bar and a tombolo.

A spit that extends across a bay to the extent of closing, or almost closing it off, is known as a baymouth bar . Most bays have streams flowing into them, and since this water has to get out, it is rare that a baymouth bar will completely close the entrance to a bay. In areas where there is sufficient sediment being transported, and there are near-shore islands, a tombolo may form (Figure 17.3.5).

Tombolos are common around the southern part of the coast of British Columbia, where islands are abundant, and they typically form where there is a wave shadow behind a nearshore island (Figure 17.3.6). This becomes an area with reduced energy, and so the longshore current slows and sediments accumulate. Eventually enough sediments accumulate to connect the island to the mainland with a tombolo. There is a good example of a tombolo in Figure 17.0.1, and another in Figure 17.3.7.

Figure 17.3.6 The process of formation of a tombolo in a wave shadow behind a nearshore island. Figure 17.3.7 A stack (with a wave-cut platform) connected to the mainland by a tombolo, Gabriola Island, B.C.

In areas where coastal sediments are abundant and coastal relief is low (because there has been little or no recent coastal uplift), it is common for barrier islands to form. Barrier islands are elongated islands composed of sand that form a few kilometres away from the mainland. They are common along the U.S. Gulf Coast from Texas to Florida, and along the U.S. Atlantic Coast from Florida to Massachusetts (Figure 17.3.8). North of Boston, the coast becomes rocky, partly because that area has been affected by post-glacial crustal rebound.

Figure 17.3.8 Assateague Island on the Maryland coast, U.S. This barrier island is about 60 km long and only 1 km to 2 km wide. The open Atlantic Ocean is to the right and the lagoon is to the left. This part of Assateague Island has recently been eroded by a tropical storm, which pushed massive amounts of sand into the lagoon.

Figure 17.3.9

On the map, sketch where you would expect the following to form:

What conditions might lead to the formation of barrier islands in this area?

Some coasts in tropical regions (between 30° S and 30° N) are characterized by carbonate reefs . Reefs form in relatively shallow marine water within a few hundred to a few thousand metres of shore in areas where the water is clear because there is little or no input of clastic sediments from streams, and marine organisms such as corals, algae, and shelled organisms can thrive. The associated biological processes are enhanced where upwelling currents bring chemical nutrients from deeper water (but not so deep that the water is cooler than about 25°C) (Figure 17.3.10). Sediments that form in the back reef (shore side) and fore reef (ocean side) are typically dominated by carbonate fragments eroded from the reef and from organisms that thrive in the back-reef area that is protected from wave energy by the reef.

Figure 17.3.10 Cross-section through a typical barrier or fringing reef.

Image descriptions

Figure 17.3.1 image description: A berm, the part of a beach that is beyond the reach of high tide, is part of the backshore. The beach face, the part of the beach between low tide and high tide level, includes the swash zone and the foreshore. Beyond the swash zone is the surf zone and beyond that is the breaker zone. [Return to Figure 17.3.1]

Media Attributions

Some coastal areas are dominated by erosion, an example being the Pacific coast of Canada and the United States, while others are dominated by deposition, examples being the Atlantic and Caribbean coasts of the United States. But on almost all coasts, both deposition and erosion are happening to varying degrees most of the time, although in different places. This is clearly evident in the Tofino area of Vancouver Island (Figure 17.0.1), where erosion is the predominant process on the rocky headlands, while depositional processes predominate within the bays. On deposition-dominant coasts, the coastal sediments are still being eroded from some areas and deposited in others.

A key factor in determining if a coast is dominated by erosion or deposition is its history of tectonic activity. A coast like that of British Columbia is tectonically active, and compression and uplift have been going on for tens of millions of years. This coast has also been uplifted during the past 15,000 years by isostatic rebound due to deglaciation. The coasts of the United States along the Atlantic and the Gulf of Mexico have not seen significant tectonic activity in a few hundred million years, and except in the northeast, have not experienced post-glacial uplift. These areas have relatively little topographic relief, and there is now minimal erosion of coastal bedrock. Another important factor is the supply of sediments. Unless there is a continuous supply of sandy and coarser sediment to a coast it will not be a depositional coast.

On coasts that are dominated by depositional processes, most of the sediment being deposited typically comes from large rivers. An obvious example is where the Mississippi River flows into the Gulf of Mexico at New Orleans another is the Fraser River at Vancouver. There are no large rivers bringing sandy sediments to the west coast of Vancouver Island, but there are still long and wide sandy beaches there. In this area, most of the sand comes from glaciofluvial sand deposits situated along the shore behind the beach, and some comes from the erosion of the rocks on the headlands.

The components of a typical beach are shown in Figure 17.3.1. On a sandy marine beach, the beach face is the area between the low and high tide levels. A berm is a flatter region beyond the reach of high tides this area stays dry except during large storms.

Figure 17.3.1 The components of a sandy marine beach. [Image Description] Figure 17.3.2 The differences between summer and winter on beaches in areas where the winter conditions are rougher and waves have a shorter wavelength but higher energy. In winter, sand from the beach is stored offshore.

Most beaches go through a seasonal cycle because conditions change from summer to winter. In summer, sea conditions are relatively calm with long-wavelength, low-amplitude waves generated by distant winds. Winter conditions are rougher, with shorter-wavelength, higher-amplitude waves caused by strong local winds. As shown in Figure 17.3.2, the heavy seas of winter gradually erode sand from beaches, moving it to an underwater sandbar offshore from the beach. The gentler waves of summer gradually push this sand back toward the shore, creating a wider and flatter beach.

The evolution of sandy depositional features on sea coasts is primarily influenced by waves and currents, especially longshore currents. As sediment is transported along a shore, either it is deposited on beaches, or it creates other depositional features. A spit , for example is an elongated sandy deposit that extends out into open water in the direction of a longshore current. A good example is Goose Spit at Comox on Vancouver Island (Figure 17.3.3). At this location, the longshore current typically flows toward the southwest, and the sand eroded from a 60 m high cliff of Pleistocene glaciofluvial Quadra Sand is pushed in that direction and then out into Comox Harbour.

Figure 17.3.3 The formation of Goose Spit at Comox on Vancouver Island. The sand that makes up Goose Spit is derived from the erosion of Pleistocene Quadra Sand (a thick glaciofluvial sand deposit, as illustrated in the photo on the right).

The Quadra Sand at Comox is visible in Figure 17.3.4. There are numerous homes built at the top of the cliff, and the property owners have gone to considerable expense to reinforce the base of the cliff with large angular rocks ( rip-rap ) and concrete barriers so as to limit further erosion of their properties. One result of this will be to starve Goose Spit of sediments and eventually contribute to its erosion. Of course the rocks and concrete barriers are only temporary they will be eroded by strong winter storms over the next few decades and the Quadra Sand will once again contribute to the maintenance of Goose Spit.

Figure 17.3.4 The Quadra Sand cliff at Comox, and the extensive concrete and rip-rap barrier that has been constructed to reduce erosion. Note that the waves (dashed lines) are approaching the shore at an angle, contributing to the longshore drift. Figure 17.3.5 A depiction of a baymouth bar and a tombolo.

A spit that extends across a bay to the extent of closing, or almost closing it off, is known as a baymouth bar . Most bays have streams flowing into them, and since this water has to get out, it is rare that a baymouth bar will completely close the entrance to a bay. In areas where there is sufficient sediment being transported, and there are near-shore islands, a tombolo may form (Figure 17.3.5).

Tombolos are common around the southern part of the coast of British Columbia, where islands are abundant, and they typically form where there is a wave shadow behind a nearshore island (Figure 17.3.6). This becomes an area with reduced energy, and so the longshore current slows and sediments accumulate. Eventually enough sediments accumulate to connect the island to the mainland with a tombolo. There is a good example of a tombolo in Figure 17.0.1, and another in Figure 17.3.7.

Figure 17.3.6 The process of formation of a tombolo in a wave shadow behind a nearshore island. Figure 17.3.7 A stack (with a wave-cut platform) connected to the mainland by a tombolo, Gabriola Island, B.C.

In areas where coastal sediments are abundant and coastal relief is low (because there has been little or no recent coastal uplift), it is common for barrier islands to form. Barrier islands are elongated islands composed of sand that form a few kilometres away from the mainland. They are common along the U.S. Gulf Coast from Texas to Florida, and along the U.S. Atlantic Coast from Florida to Massachusetts (Figure 17.3.8). North of Boston, the coast becomes rocky, partly because that area has been affected by post-glacial crustal rebound.

Figure 17.3.8 Assateague Island on the Maryland coast, U.S. This barrier island is about 60 km long and only 1 km to 2 km wide. The open Atlantic Ocean is to the right and the lagoon is to the left. This part of Assateague Island has recently been eroded by a tropical storm, which pushed massive amounts of sand into the lagoon.

Figure 17.3.9

On the map, sketch where you would expect the following to form:

What conditions might lead to the formation of barrier islands in this area?

Some coasts in tropical regions (between 30° S and 30° N) are characterized by carbonate reefs . Reefs form in relatively shallow marine water within a few hundred to a few thousand metres of shore in areas where the water is clear because there is little or no input of clastic sediments from streams, and marine organisms such as corals, algae, and shelled organisms can thrive. The associated biological processes are enhanced where upwelling currents bring chemical nutrients from deeper water (but not so deep that the water is cooler than about 25°C) (Figure 17.3.10). Sediments that form in the back reef (shore side) and fore reef (ocean side) are typically dominated by carbonate fragments eroded from the reef and from organisms that thrive in the back-reef area that is protected from wave energy by the reef.

Figure 17.3.10 Cross-section through a typical barrier or fringing reef.

Image descriptions

Figure 17.3.1 image description: A berm, the part of a beach that is beyond the reach of high tide, is part of the backshore. The beach face, the part of the beach between low tide and high tide level, includes the swash zone and the foreshore. Beyond the swash zone is the surf zone and beyond that is the breaker zone. [Return to Figure 17.3.1]


Formation of the Syenite

Formation of syenites are products of alkaline igneous activity, usually formed in thick continental crustal areas, or in Cordilleran subduction zones. Producing Syenite is necessary to melt a granitic or igneous protolith to a fairly low degree of partial melting. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite, adamellite or tonalite.

At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime.

Conversely in certain conditions, large volumes of anorthite crystals may precipitate from thoroughly molten magma in a cumulate process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.


What are some of the oldest rocks so far discovered on Earth?

Scientists have found rocks exceeding 3.5 billion years of age on all the Earth’s continents. But the oldest rocks uncovered so far are the Acasta Gneisses in north-western Canada near Creat Slave Lake, which has been dated at about 4.03 billion years old. Others that are not as old include the lsua Supracrustal rocks in West Greenland (3.7 to 3.8 billion years old), rocks from the Minnesota River Valley and northern Michigan (3.5 to 3.7 billion years old), rocks in Swaziland (3.4 to 3.5 billion years old), and rocks from western Australia (3.4 to 3.6 billion years old). These ancient rocks are mostly from lava flows and shallow water sedimentary processes. This seems to indicate that they were not from the original crust, but formed afterward.

The oldest materials found on Earth to date are tiny, single zircon crystals uncovered in younger sedimentary layers of rock. These crystals, found in western Australia, have been dated at 4.3 billion years old, but the source of the crystals has not yet been discovered.


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