There are 3 main types of rock that make up this planet we call Earth. They are Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks.
Igneous Rocks
Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are).
Sedimentary Rocks
In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks:
Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation.
Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time.
Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock.
Metamorphic Rocks
The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.
Here along Rock Creek road there are tons of Shale deposits. I really like the way it forms and flakes away.
Shale is a detrital sedimentary rock composed of very fine clay-sized particles. Detrital sedimentary rocks are sedimentary rocks composed of the weathered and eroded particles of larger pieces of rock. Clay forms from the decomposition of the mineral feldspar. Other minerals present in shale are quartz, mica, pyrite, and organic matter. Shale forms in very deep ocean water, lagoons, lakes and swamps where the water is still enough to allow the extremely fine clay and silt particles to settle to the floor. Geologists estimate that shale represents almost ¾ of the sedimentary rock on the Earth’s crust. Geologists are specific about the definition of the rock called “shale.” Shale is composed of clay-sized particles that are less than 0.004 mm in size. Siltstone is composed of particles that are between 0.004 and 0.063 mm in size. When the sedimentary rock is a mixture of clay and silt, geologists call the rock mudstone.
Layers of other sediments eventually cover the silt and mud that collects on ocean and lake floors. The weight of these sediments compacts the mud leading to lithification (lithification literally means turning to stone). The lithification process creates very fine layering in the shale. This layering is called lamination. Shale splits easily into relatively thin sheets due to this lamination.
Shale can be red, green, grey or black. The different colors are due to different minerals in the shale. Black shale typically has a very high content of oily kerogen. Kerogen is organic matter trapped in the sediments that is the remains mostly of plants and some water-born microorganisms. Kerogen is not oil, but is thought to be the material that, through complex geological processes, becomes oil. Though still economically unfeasible, a process of heating (in an oxygen-depleted environment) can remove kerogen from shale in the form of liquid oil and natural gas.
Please email me with the answers to the following questions. Include the name of this cache and the number of people in your group.
#1. Based on your reading and what you see onsite, which kind of rock is the hillside made from?
#2. What color is the Shale?
#3. What direction does the grain run? Vertical, Horizontal or Diagonal?
#4. On average, how thick are the shale pieces that are on the ground at your feet?
#5. Is there evidence of shale on the downhill side of the road?
Go to N 38 51.516 W 120 41.821. This is another shale face.
#6. Which direction does the grain run here?
(optional) Please post a picture of yourself in the area.
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