Download Igneous Rocks and Plate Boundaries

Igneous Rocks and Plate Boundaries
A rock is usually composed of an aggregate of minerals. Some rocks contain one major mineral.
For example, marble is almost entirely composed of the mineral, calcite. Rocks which are made
up of one mineral only are referred to as being monomineralic. However, most rocks usually
contain more than one mineral.
So, what is a mineral?
In Earth science, a mineral is a naturally occurring, inorganic, crystalline substance with a
definite chemical composition.
Define the following terms.
• naturally occurring
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• inorganic
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crystalline
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• definite chemical composition
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A naturally occurring substance is not a synthetic material or a material made by humankind.
This means synthetic rubies and cubic zirconia, which are widely used in jewellery, are not
considered minerals. An inorganic substance has not originated from a living thing. This means
that coral and pearl would not be considered minerals, nor would coal because coal originates
from the remains of plant matter. This is an interesting point since coal in New South Wales is
considered a major mineral export!
Crystalline means that the atoms that make up the substance have a regular arrangement. If
the mineral has room to grow, this regular arrangement would give the mineral a regular shape,
called a crystal. Most of the time minerals don’t get the chance to form these beautiful
crystal shapes.
Definite chemical composition means that the mineral isn’t a mixture like concrete or beach
sand. The mineral can be a single element, like a nugget of gold (Au) or it can have a fixed
chemical composition such as quartz which is silicon dioxide, (SiO2).
A mineral may even have a composition which varies within a fixed range. Olivine, for example,
can be a magnesium silicate, iron silicate, or even a combination of iron and magnesium
silicates, [(Mg,Fe)2SiO4]. In all three cases, the composition of the mineral is defined.
List the three groups of rocks.
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Of these three groups, you will be looking in some detail at igneous rocks.
Igneous rocks
Use this list of words to complete in the text below: volcanic rocks, volcanoes, magma, melt,
plutonic.
When molten material, or (…………………………......), is underground it is called (…………………………..).
When the molten material reaches the surface, it is called lava. (……………………………………. )
provide one way for molten rock to reach the Earth’s surface. Rocks formed by solidified lava
are called (………………………………………………). Rocks formed by magma solidifying underground are
called (……………………………………………….) rocks. (This name was derived from Pluto, the Greek god of
the underworld).
Size of minerals in igneous rocks
Different types of igneous rocks have different mineral grain size. The size of minerals in
igneous rocks is very much dependant on the rate at which the molten rock cooled.
Fine-grained volcanic rocks
Lava erupts from a volcano at high temperatures (after all, it has to be hot enough to melt
rock!), usually between 700 and 1300°C. Heat is very quickly lost to the atmosphere, so lava
cools and solidifies rapidly. This means that atoms do not have time to form regular
arrangements and so large crystals can’t form. Basalt is an example of a fine-grained volcanic
rock. Volcanic rocks often have some glass in them. This is simply due to the rapid cooling of
non-crystalline lava. Obsidian is a black rock made up of volcanic glass.
Coarse-grained plutonic rocks
On the other hand magma cools down extremely slowly. This is because it is difficult for heat
to escape from beneath the Earth’s surface. Rocks are excellent insulators, which is why some
houses are made with walls of sandstone or bluestone. Remember, bricks are a form of
artificial rock. A large body of magma may take hundreds of thousands of years to cool and
solidify completely. As a result, plutonic rocks are quite coarse grained. Their minerals are
much larger than minerals in volcanic rocks and are typically 5 mm or more in length. This
makes mineral identification much easier.
Classifying igneous rocks
Igneous rocks are usually classified by two properties:
• the size of the minerals known as the grain size
• the types of minerals in them.
There are about 2000 known minerals. Only about ten of these minerals make up most of the
rocks in the Earth’s lithosphere. These are called the rock-forming minerals and are the
basis of most rock classifications. In the classification diagram below, plutonic rocks are
shown in bold. This indicates large grain size. The volcanic rocks that form from the same melt
are unbolded (and smaller) to indicate small grain size.
Gabbro is coarse-grained. If gabbro magma reached the surface, then the lava that erupts
cools quickly to form basalt. The overall composition of gabbro and basalt are the same.
The only difference is that gases are able to bubble out and escape once lava erupts, because
the pressure of overlying rock is released, and the lava cools quickly. The release of gas from
lava is similar to opening a bottle of soft drink. Carbon dioxide gas is dissolved in soft drink
under pressure. Once the cap is undone, the pressure is released and the carbon dioxide
escapes (the fizz). The main igneous rock-forming minerals are arranged around the sides of
the classification table. Gabbro, for instance, can be seen to have no quartz, lots of
plagioclase, lots of pyroxene and some olivine.
Answer the following questions
a) Using the igneous rock classification scheme described in this module, name the three
minerals you would expect to find in granite.
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b) Name two minerals you would not expect to find, or would find in very small amounts in
granite.
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c) Write a description of andesite, using the information above.
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d) What do rhyolite and basalt have in common?
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e) What differences are there between rhyolite and basalt?
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Igneous rocks and plate boundaries
Now that you know more about igneous rocks you will need to link different igneous rocks to
particular tectonic settings. There are two igneous rocks characteristic of certain types of
plate boundaries.
As you have already learnt in the Dynamic Earth module, the rock typically produced at
divergent boundaries does not vary much at all and is composed of almost entirely one rock
type – basalt. This mid-oceanic ridge basalt or MORB as it is known, comprises the entire seafloor around the world and covers some 70% of the Earth’s surface.
At convergent boundaries where sea-floor is being subducted, whether that be
oceanic/oceanic convergence or oceanic/continental convergence, andesite is often produced.
In fact, it is unusual to find andesite in other tectonic situations. Although andesite is
characteristic of convergent plate boundaries, many other types of igneous and metamorphic
rocks are also produced at these boundaries, unlike divergent boundaries.
f) Name the plutonic rock that would form within the crust, beneath a volcano erupting
basalt?
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g) Name the plutonic rock that would form within the crust, beneath a volcano erupting
andesite?
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Homework
Read pages 7-14 in your text book and answer the questions on page 14 under the heading
review activities and extension activities.