Rocks:

What is a rock?

Mineral matter of variable composition, consolidated or unconsolidated, assembled in masses or considerable quantities in nature, as by the action of heat or water

Rock Types

Scientists are always looking for ways to put things into categories, and rocks are no exception. Rocks are solid masses occurring naturally as part of our planet. As it turns out, rocks can be put into three fundamentally different types:

• Igneous rock
• Sedimentary rock
• Metamorphic rock

Igneous Rocks

Igneous rocks - for example, basalt and granite - are crystalline solids that form directly from cooling magma.

Magma is molten rock made by the partial melting of rocks in the earth's interior under conditions of high temperature and high pressure. As rock melts, it becomes less dense and rises towards the earth's surface. Magma that reaches the earth's surface is called lava. Most lava flows are quiet, but some can be violent, such as the eruption of Mount St. Helens back in 1980.

Igneous rock that forms at the surface of the earth is referred to as extrusive or volcanic rock. These are named after the fire god, Vulcan. Basalt is a good example of extrusive igneous rock.

Igneous rock formed before it reaches the earth's surface is named intrusive or plutonic after the god of the lower world, Pluto. These rocks cool over long periods of time and, thus, develop large crystal structures, as we see in granite. Some intrusive rocks are subsequently exposed to the surface - but only after uplifting or erosion of the earth's surface, otherwise they would remain buried.

Sedimentary Rocks

While igneous rocks make up the bulk of the earth's crust, they're often covered by relatively thin sheets of sedimentary rocks. As the name suggests, sedimentary rock is formed from sediments or debris transported by liquid water, ice, or wind that become compacted and cemented together.

Sedimentary rocks are secondary rocks, as they are formed from the accumulation of small pieces of pre-existing rock - that is, parent rock. There are two main types of sedimentary rocks, based on the source of the sediment.

Sedimentary rock can form as solid particles from weathered rocks. These particles are called detritus, and detrital sedimentary rocks are accumulations of detritus. Therefore, the source of sediment for detrital sedimentary rocks is weathered rock from another form. For example, particles of sand from other rocks can form sandstone, and mud can form shale, which is the most common form of sedimentary rock.

Chemical sedimentary rocks are derived from material carried in solution to lakes and oceans. Under certain conditions, the dissolved material precipitates out of solution and, thus, settles to the bottom. Precipitation may occur due to physical processes such as evaporation or through living organisms. For example, limestone is the most abundant chemical sedimentary rock, and it's typically formed when calcite precipitates due to evaporation. Calcite is produced by living organisms to form shells which can, in turn, form sedimentary rocks. Now, interestingly, 90% of limestone is of this biological origin.

Metamorphic Rocks

Metamorphic rocks are created by the physical or chemical alteration by heat and pressure of an existing igneous or sedimentary material into a denser form. Due to the action of plate tectonics, compression, stress and shearing forces over long periods of time, rocks can be essentially warped and deformed, causing them to be compacted into a smaller volume of space. As a consequence, metamorphic rocks are always more dense than their original material, and also much less susceptible to erosional breakdown. As the Earth's plates move over geologic time, a plate containing igneous or sedimentary rock may become subducted under another plate. The sheer weight of the material above it can cause the rock to undergo metamorphism. In some cases, heat from the Earth's interior can melt the rock slightly, in a process termed "contact metamorphism." Examples of metamorphic rocks are schist (converted basalt), quartzite (compressed sandstone), and marble (compressed limestone or dolomite).