types of rocks
jarod shellenbarger pd 4 11/18/14
As described in some of the previous topics, igneous rocks are produced by the crystallization and solidification of molten magma. Magma forms when rock is heated to high temperatures (between 625 and 1200° Celsius) beneath the Earth's surface. The exact temperature needed to melt rock is controlled by several factors. Chemistry of the rock material, pressure, presence of gases (like water vapor) all influence when melting occurs. Most of the heat required to melt rock into magma comes from the Earth's central internal region known as the core. Scientists estimate that the temperature of the Earth's core is about 5000° Celsius. Heat moves from the Earth's core towards the solid outer crust by convection and conduction. Convection moves hot plumes of magma vertically from the lower mantle to the upper mantle. Some of these plumes melt through the Earth's solid lithosphere and can produce intrusive igneous features and extrusive igneous features on the surface. Heat can also be generated in the lower lithosphere through friction. The tectonic movement of subducted crustal plates can generate enough heat (and pressure) to melt rock. This fact explains the presence of volcanoes along the margin of some continental plates.
Metamorphism involves the alteration of existing rocks by either excessive heat and pressure, or through the chemical action of fluids. This alteration can cause chemical changes or structural modification to the minerals making up the rock. Structural modification may involve the simple reorganization of minerals into layers or the aggregation of minerals into specific areas within the rock.
Much of the Earth's continental crust is composed of metamorphic and igneous rocks. Together, these two rock types form the base material at the core of the Earth's major continental masses. Overlying this core are often thick layers of sedimentary rocks. In some regions, this base rock is exposed to the atmosphere and is known as shields. On the Canadian Shield we can find some of the oldest rocks found on the planet (3.96 billion years old). These very old rocks are primarily metamorphic. Metamorphic rocks also are the rock type found at the core of the world's various mountain ranges.
Sedimentary rocks can be categorized into three groups based on sediment type. Most sedimentary rocks are formed by the lithification of weathered rock debris that has been physically transported and deposited. During the transport process, the particles that make up these rocks often become rounded due to abrasion or can become highly sorted. Examples of this type of sedimentary rock include conglomerate and sandstone. Scientists sometimes call this general group of sedimentary rocks clastic. The remaining types of sedimentary rocks are created either from chemical precipitation and crystallization, or by the lithification of once living organic matter. We identify these sedimentary rocks as being non-clastic.