Rocks and Minerals
A mineral is a single inorganic substance - like salt, or coal, or gold, or silver. A rock is usually made up of several minerals that have been compressed together by geologic forces over thousands of years of constant pressure. In our California History section we will be learning more about the mad dash for minerals - gold and oil - that has shaped the State and impacted on the Channel Islands and the Backcountry. Here in Geology we will now look at the rocks that make up our landscape and ocean floors and how they got here.
The three main types of rock on the earth are:
Igneous Rocks are formed by the cooling and solidification of molten rock. The core of the earth is still very hot, much like the entire earth was in its early formation, before plants and animals could live here. As the earth's crust cooled, down deep in the core of the earth there still remains a huge mass of fiercely hot rock called magma that exists in a liquefied state. When that molten rock pushed us near the surface - or breeches the crust and expels onto the surface - it cools down as it comes in to contact with the cooler crust or when it reaches the earth's atmosphere. Rock cooled below the surface of the earth forms large particles of minerals that can be seen with the eye - like granite. Rock that cools when it reaches the actual atmosphere on the earth's surface cools much faster and is glassy, with no visible particles - like obsidian.
Sedimentary rock is composed of sediments from rocks that have undergone erosion from wind and water. The sediments broken off from the larger rock - hard igneous rock for example - are washed by rains and rivers down into lakes and oceans. As these particles of rock come to rest beneath a large body of water, the weight from that water body bears down on them, compressing them into sedimentary stone. Under such pressure the single sediments solidify - lithify - into sedimentary rock formations. This change of igneous rock to sediment and into sedimentary rock is part of what scientists call the Rock Cycle.
Sedimentary rock formations make up only 5% of the earth's crust, but loose sedimentary rocks on he earth's surface comprise 75% of all rocks we find today. When sedimentary rock formations are exposed to the earth's surface as the covering waters recede, this soft rock is easily sculpted and shaped by the wind, rain, streams and rivers. In fact, some of the most unusual rock formation in the world occur in sedimentary rock - arches, boulders teetering on finger-like spires, multi-colored layers cliffs and canyon walls, rounded rock faces carved with dozens of wind caves.
Another phase of the Rock cycle is called metamorphic rock. Metamorphic means change and transformation. As a third phase of geologic rock formation, metamorphic rocks can be sedimentary rock combined with igneous rock that undergoes massive pressure and form a new type of rock that can have chunks of igneous rock imbedded in tiny little grains of sedimentary rock. Sometimes this happens when the different rock types are buried under many feet of the earth's crust, which weights down on them and changes them. Other times it can also occur as a result of an igneous eruption that has enough heat to change the composition and combinations of rocks right on the earth's surface.
To complete the Rock Cycle, metamorphic rock that has been buried deep below the earth's crust once again becomes part of the molten core. It may then someday re-emerge on the earth's surface during a volcanic eruption and the cycle then begins to repeat itself.
Here is an example of the Rock Cycle:
Magma>reaches the earth's crust>becomes igneous rock>erodes>becomes sediments that wash out to sea>becomes sedimentary rock under the ocean>waters recede and rock is exposed>earthquake brings igneous rock to the surface amidst the sedimentary rock>both buried by more sediment>under pressure becomes metamorphic rock>under more pressure is pushed back down to molten core>becomes magma again> cycle continues.
Next time you are outside, pick up a sampling of rocks from your area. Using the descriptions above and any Internet or book resources you have access to, study the rocks and determine : are they igneous ? sedimentary ? or metamorphic ?
Next, write a short story about how that rock was formed and how it got to your location - imagine the life of that rock, which began deep in the core of the earth … how did it reach your community ? What plants and animals has it known along the way ? What has it witnessed in its lifetime ?