Created by Mrs. Hutchins Rocks And the Rock Cycle Created by Mrs. Hutchins

Sedimentary Rocks There are three different types of sedimentary rocks: clastic, chemical, and organic. Clastic sedimentary rocks are the rocks that most people are familiar with. Forming clastic sedimentary rock is a four-step process.

Breaking Rocks Apart - Step 1 Feldspar chemically changes into clay minerals. Chemical Weathering Minerals from rock are dissolved into rainwater or changed from one type of mineral to another. Chemical weathering happens quickly in warm, moist environments. The water needed for chemical weathering is present, and the heat speeds up the process. Quartz does not weather easily.

Physical Weathering Physical weathering occurs from water flowing in rivers and streams. Physical weathering also occurs because of wave action on the shoreline. Tree and plant roots push rocks apart, especially in areas where there is little topsoil. Water freezing and expanding can crack rocks apart. Large temperature changes cracks rock.

Biological Weathering Weathered rock moved by water, wind, or plants and animals become part of the soil. Soil is formed when rocks are weathered at the surface and then combine with plant and animal remains. The ridges of rock here are more resistant to weathering than the surrounding rock.

Rocks on the Move – Step 2 Large pieces of rock are carried in strong currents. They settle quickly to the bottom when the current slows down. Small particles (silt, clay) are carried by slow currents. They settle very slowly onto the bottom of quiet waters like lakes, swamps, and lagoons.

Sediments Settle Down – Step 3 Larger pieces of rock settle more quickly than small pieces of rock. The settlement of bits of rock to the bottom of the water is called deposition.

Sediment to Solid Rock – Step 4 Cement holds the sediments together. Cement is a mineral that forms when seawater or groundwater travels through empty spaces between the sediments. If the water contains chemicals that are needed, the mineral crystals will form in between the sediments. The minerals hold the sediments together. The mineral crystals and the water form the “cement” to hold the sedimentary rock together. Common minerals include calcite, quartz, and hematite.

Chemical Sedimentary Rocks Chemical sedimentary rocks are not made from sediments. Instead, they are comprised of mineral crystals made from elements and dissolved in water. Through evaporation the dissolved elements form crystals. Calcite is typically found in limestone. Halite is rock salt and used to season your food.

This picture shows the formation of salt through evaporation in Death Valley, California.

Organic Sedimentary Rocks Sediments are made of fossils. Animal parts that become cemented together make limestone. Organic rock made from tiny silica skeletons is called chert. Plant remains that go through the lithification process becomes coal.

Igneous Rocks Igneous rocks form either above ground or underground. An igneous rock that forms above the ground is called an extrusive igneous rock. An igneous rock that forms under the ground is called an intrusive igneous rock.

Intrusive Igneous Rocks There are many cracks and crevices deep within the earth. Magma from volcanoes often flow through these cracks and crevices into pockets within the earth. The magma is under a tremendous amount of pressure and is very hot. The magma in the pockets cools very slowly. This gives the minerals in the rock time to form large crystals. These crystals are large and you do not need a microscope to see them.

A large pocket of magma underground is called a basolith.

Minerals in Igneous Rocks There are primarily 8 different types of minerals in rocks. The minerals are Olivine Pyroxene Amphibole Biotite (mica) Muscovite (mica) Orthoclase (feldspar) Plagioclase (feldspar) Quartz

Granite is the most commonly known intrusive igneous rock. Can you see the crystals that have formed in the rock? What is granite used for?

Extrusive Igneous Rock Extrusive rocks are formed when magma from the upper mantle of the earth (50 to 150 km below the surface) makes its way to the surface. The magma may explode from a volcano, or flow just under the surface in channels and tubes.

Extrusive Igneous Rocks If the magma (lava) cools very quickly there is very little, if any, crystal formation in the rock. Instead, the magma cools into glass. Magma that cools very quickly creates pumice, obsidian and scoria. Pumice has many air pockets and is very light. It can be used in polishes and soap. It floats! Scoria has some air holes in it and is fairly rough. It is a kind of glass. Obsidian is a very smooth black glass.

Pumice Obsidian Scoria

The magma from an exploding volcano tends to cool very quickly The magma from an exploding volcano tends to cool very quickly. Pumice forms with the air as the volcano explodes. Scoria and obsidian form more slowly as the magma cools. Magma that flows in large channels or tubes cools more slowly still. The rock from this slower cooling magma tends to have very small crystals formed in it. The rock formed is called basalt.

Metamorphic Rocks One of the places metamorphic rocks are formed is where tectonic plates come together. At that location there is very hot temperatures and very high pressure.

Metamorphic Grade The amount of metamorphism that takes place in a rock is called the metamorphic grade. Little metamorphism takes place at relatively low temperatures – about 200 ºC, a little higher than the oven temperature to bake chocolate chip cookies. Mica minerals crystallize. These are found in the igneous rock granite and the metamorphic rock schist.

Metamorphic grade, continued Intermediate grade metamorphic rocks form between 200ºC and 800ºC. These rocks contain mica and garnet. High grade metamorphic rocks form in places where most rocks almost melt. It is 800ºC or above.

Foliation Sometimes the minerals in the rock form crystals that are lined up in right angles to the direction of the pressure. This is called foliation. At low temperatures shale morphs into slate. At medium temperatures schist forms. At high temperatures gneiss forms.

Non-foliation Often the minerals do not form layers. Rocks that morph without layers are said to be non-foliated. Marble morphs from limestone at medium to high temperatures. Quartzite morphs from sandstone at medium to high temperatures. Metabasalt (greenstone) morphs from basalt at low temperatures. Sometimes greenstone is foliated. It is then called greenschist.

Contact Metamorphism Contact metamorphism occurs when rocks come into contact with hot magma. Underground the rock that surrounds the basolith area can be altered because of the high temperature of the magma and the length of time it takes to cool. Above ground the rock is altered on the surface from the lava that flows onto it and over it.

The Rock Cycle – Explained All rocks on the surface are exposed to weathering – mechanical, chemical, and biological The weathered rock is transported by water and wind, gradually settling out in a process called deposition. Deposition means the rock settles into layers. The layers are placed under pressure and go through a process of forming together called lithification. The new rock might be uplifted to the surface and be weathered once more. Or the new rock may become more buried and become exposed to additional pressure and heat, thereby becoming a metamorphic rock.

Metamorphic rock may be uplifted to the surface through plate movement or earthquakes. If it is uplifted to the surface it becomes weathered once more and starts into the rock cycle once again. Metamorphic rock may also be placed under additional heat and pressure, melting into magma.

If the magma is trapped under the earth and cools slowly, large crystals will form in the rock as the magma cools. This magma becomes an intrusive igneous rock. The rock may eventually come to the surface and become weathered, or it may be buried deeper and go through metamorphosis again. The magma may come to the surface in a volcanic eruption. When the magma comes to the surface and cools quickly it turns into extrusive igneous rock. There is very little crystallization of mineral crystals that occurs. Weathering occurs.

How long does the rock cycle take? The rock cycle is an on-going process. Weathering is always happening, volcanoes are always erupting, earthquakes occur on a regular basis, and the tectonic plates are always moving causing uplift and subduction.

Key Words to Know: Sedimentary Rocks Igneous Rocks Clastic Organic Chemical Igneous Rocks Intrusive Extrusive

Metamorphic Rocks Foliated Non-foliated Weathering Mechanical Chemical Biological

Rock cycle Transportation Deposition Lithification Crystallization Uplift

Credits University Corporation for Atmospheric Research (UCAR), University of Michigan Rock Hounds Encarta