All rocks contain minerals. Rocks are classified into three groups according to their origin, or how they are formed. The three groups of rocks are Sedimentary, Igneous and Metamorphic.
Clastic Sedimentary Rocks are formed when Inorgranic land derived sediments classifies according to grain size. These are formed by the compaction and cementation of the fragments deposited usually be running water entering calm water. The fragments are deposited in horizontal layers called beds or strata.
Weathering is the process of wearing down the earth's surface by water, wind, glaciers and gravity. Weathering breaks down existing rocks and forms sediments. These sediments are compacted and cemented together to form a clastic sedimentary rock.
The above image, Shale, is composed of clay. The grain size is less than 0.0004 cm.
Siltstone, the above image, is composed of silt. The grain size has a range of 0.0004 to 0.006 cm.
Sandstone, the above image, is composed of sand. The grain size has a range of 0.006 to 0.2 cm.
Conglomerate and Breccia are composed of pebbles, cobbles, and/ or boulders, embedded in sand, silt, and/ or clay.
II. Chemical/ Crystalline
Precipitation (settling out) of minerals from a solution. They are referred to evaporates and precipitates. These are formed when the minerals in the water settle and then the water evaporates, leaving behind the rock. Sedimentary rocks formed by this process are usually called monominerallic. Monominerallic means composed of one mineral. Four precipitate/ evaporates are Rock Salt, Rock Gypsum, Dolostone and Chemical Limestone.
Dolostone, the above image, is composed of dolomite. The grain size is fine to coarse crystals.
Rock Salt, the above image, is composed of halite. The grain size is also fine to coarse crystals.
Formed from the remains of once living things. They are referred to as Bioclastic. Two organic sedimentary rocks are organic limestone and coal.
Coal, the above image, is composed primarily of Carbon. The grain size is microscopic to very coarse.
Limestone, the above image, is composed of calcite. The grain size is also microscopic to very coarse.
Igneous means "fire-formed". Rocks that form from molten rock that solidifies (hardens) are called Igneous. There are two types of molten rock. Lava is molten rock at, or above the earth's surface. Magma is molten rock below the earth's surface. Magma cools intrusively (deep in the ground), so it cools slowly. The crystals that form are large because the magma cools slowly, allowing time for large crystals to form. Lava cools extrusively (at, or near the earth's surface), so it cools quickly. The crystals that form have a fine texture because the lava cools so quickly, there is only time for small crystals to form.
Some Igneous rocks cool so quickly that crystals do not have time to grow. These rocks have a glassy texture. This happens when the lava flows into water or was ejected high in the air.
Granite, the above image, has a coarse texture and is formed intrusively.
Basalt, the above image, has a fine texture and is formed extrusively.
Obsidian, the above image, has a glassy texture and is formed extrusively.
Metamorphic rocks form from already existing rocks. They can form from Igneous, Sedimentary and existing Metamorphic rocks. They are formed by extreme heat and/or pressure. The rocks don't melt, they recrystallize. There are two ways metamorphic rocks are formed.
I. Regional Metamorphism
This happens over a large area deep inside the earth. It causes layers of rock to become distorted (layers become wavy) and minerals to align in bands (minerals separate out according to density, creating bands). Metamorphic rocks that are distorted and/or have bands are foliated. The rocks have recrystallized to form bands of minerals. The minerals were rearranged without melting.
You can tell that the above rock is metamorphic by the wavy layers.
II. Contact Metamorphism
This occurs when rock is altered by coming into contact with molten rock. Rocks formed in this way are considered nonfoliated metamorphic rocks. these rocks are composed of crystals.
Some changes that occur to a metamorphic rock are that minerals increase in size (recrystallization), distortion, become more dense and decrease pore space, and mineral banding. Metamorphic rocks are formed inside the earth. The process that make metamorphic rocks take place underground (heat and/or pressure).
Gneiss, the above image, shows mineral banding because of the apparent lines in the rocks.
Quartzite, the above image, forms from sandstone. The pore space in the sandstone gets smaller and denser.
Tools of Geology
Streak plates are used to determine the color of the streak of a rock. You scrape the rock against the plate and the color of the streak is one of the defining characteristics to help you find out the which rock it is.
Since glass has a known density of 5.5, glass plates are used to determine the hardness of a rock. After the rock is scraped against the glass, if a scratch is left, the rock is harder than glass (more than 5.5 density). If there is no scratch present, the rock is softer than glass (less than 5.5 density).