Geology And Geophysics

Understanding the three Basic Types of Rock



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Like many things in our natural world rocks follow a cycle. Magma from within the mantle of the earth rises to the surface and solidifies to form igneous rocks. Weathering and erosion breaks down igneous rocks to form sediments. Sediments compact and become cemented to form sedimentary rocks. Sedimentary rocks can be further compacted and heated to form metamorphic rocks. Subduction at destructive plate margins returns the minerals forming all three rock types into the mantle of the earth where they melt to form new magma.

There are many different types of igneous rocks. Their composition depends on the source of the magma. The size of the mineral crystals making up the rock depends on the length of time the rocks took to solidify. Mineral crystals in rapidly cooling rocks will be smaller than those formed in rocks that cooled slowly. Plate tectonics plays an important part in the formation of igneous rocks.

Basaltic rocks contain a high proportion of basic (acidic) minerals. Such rocks make up the oceanic plates and form primarily at the constructive plate margins known as mid oceanic ridges. These fine-grained rocks cooled rapidly in the cold depths of the ocean.

The magma forming igneous rocks on or within the continental crust contains minerals that are more alkaline. This magma forms by the partial melting of existing rocks. The alkaline minerals such as feldspar and quartz melt at lower temperatures and pressures than basic minerals such as pyroxene or olivine. This allows a more alkaline magma, which produces the rocks forming much of the continental plates, to form. Magmas that reach the surface of the earth to be erupted as lava from volcanoes will cool relatively quickly yielding small crystals. These are called extrusive igneous rocks. Some magma never reaches the surface but cools slowly at depth allowing formation of larger crystals. These large crystals are evident in granite, which is an example of an intrusive igneous rock.

Any rock type on the surface of the earth is subject to weathering and erosion by the elements. The weathering by water, ice and wind will produce small rocks, pebbles and grains, which can be transported great distances away from the original rock source. The size and shape of the grains with depend on the type of minerals forming the rock and their environment of deposition. Once deposited the individual grains can be compressed and cemented together to form sedimentary rocks.

Examination of sedimentary rocks can tell us about their environment of deposition. High-energy environments such as a beach will favour large grain sizes and a large range of grain sizes, as seen in some sandstones and conglomerates. A low energy environment will such as a lake or lagoon will favour the formation of rocks with a small grain size such as siltstones or mudstones. Desert sandstones formed by wind-blown sands have rounded grains, which are all of a similar size.

The presence of fossils within the rocks can also tell much about the environment of deposition as well as assist in dating the rocks. Ammonites were marine animals so their presence indicates that the rock formed in a marine rather than a freshwater environment. The ammonite also provides a very good dating tool as they evolved steadily over time before becoming extinct at the end of the Cretaceous era about 65 million years ago. The type of ammonite found within a rock will tell a paleontologist when in the geological time scale the sediments were deposited.

Some sedimentary rocks derive their sediments from a biological source. Limestone for example derives its sediments from the remains of the aquatic shellfish.

Rocks that are subjected to high temperature, high pressure or both are changed into a new type of rock known at metamorphic rocks. Minerals within the rock start to migrate in direct relationship to the external pressure exerted. This migration leads to the formation of cleavage planes in rocks such as slate. Cleavage planes form by the realignment of crystals of the flat platy mineral mica. High pressures also cause the formation of new minerals such as garnet or kyanite. The pressures and temperatures at which these minerals form are known. Examination of the different mineral assemblages found in metamorphic rocks such as schist and gneiss indicates their approximate depth of burial at the time of their formation

Where a rock consists of only one type of mineral the new rock will have a similar chemical composition but a different structure as the original sedimentary rock. Limestone consisting of primarily of calcium carbonate when subjected to high pressure metamorphoses into marble while sandstone will become quartzite.

At extremes of temperature and pressure, many rocks will be subject to partial melting to form magma and this can rise to the earth's surface to form new igneous rocks. With this, the rock cycle starts again.

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