Sedimentary and Metamorphic Rocks Ch. 6

Formation of Sedimentary Rocks 6-1

Intro. To Sedimentary Rocks Much of the Earth is covered with sediments. Sediments are pieces of solid material that have been deposited on Earth’s surface by wind, water, ice, gravity, or chemical precipitation. Sediments join up to make sedimentary rocks.

Weathering Wherever Earth’s crust is exposed at the surface, it is continuously being worn into smaller pieces by physical and chemical weathering. Rock and mineral fragments produced by weathering are called clastic sediments. These range in size from boulders to microscopic particles. <.0039 mm Clay 0.062-0.0039 mm Silt 2-0.062 mm Sand Gravel 64-2 mm Pebble 256-64 mm Cobble >256 mm Boulder

Erosion and Transport Once rock fragments are weathered out, they are transported elsewhere in a process called erosion. These materials are almost always carried downhill.

Deposition Deposition occurs when sediments are laid down on the ground or sink to the bottoms of bodies of water. Sediments are deposited when transportation stops. Largest grains settle on the bottom, smallest on top.

New layers of sediment deposit on top of older ones, burying them Burial New layers of sediment deposit on top of older ones, burying them Burial results in an increase of temperature and pressure. These conditions lead to lithification. Lithification occurs when chemical and physical processes transform sediments into sedimentary rocks This literally means to turn into “stone.”

Lithification Lithification begins with compaction. Weight from the addition of layers above forces sediment grains together. Water is pressed out of layers of clay. Sand resists additional compaction. This creates open spaces in the sedimentary rock, allowing for storage of water, oil, and natural gas. At 3-4 km of depth, temps. reach approx. 100oC. This starts mineral changes that lead to cementation. Cementation occurs when mineral growth cements sediment grain together. Occurs when calcite or iron crystals form in the empty spaces. Can occur when minerals grow larger as more of the same mineral precipitates out of the water and crystallizes around them.

Features of Sedimentary Rocks Primary feature is horizontal layering called bedding. The type of bedding depends on the method of transport. Bedding where particle sizes become heavier and coarser towards the bottom is graded bedding. Often observed in marine sedimentary rocks deposited by landslides.

Cross-Bedding is formed as inclined layers of sediment move forward across a horizontal surface. Can show sand dune movement Can show water movements and type of environment. Back and forth movement of waves creates symmetrical ripples. Currents flowing in one direction make asymmetrical ripples.

One of the best features of sedimentary rocks are fossils. Evidence of Past Life One of the best features of sedimentary rocks are fossils. Fossils are the preserved remains of once-living organisms. If remains are buried quickly by deposition and cementation occurs, a fossil may form.

Types of Sedimentary Rocks Classification is based on how the rock forms. 3 types: Clastic Organic Chemical

Clastic Sedimentary Rocks Clastic sedimentary rocks are the most common types. Formed from the deposits of loose sediments on Earth’s surface. Classified according to particle size.

Coarse-Grained Clastics Consist of gravel-sized rock fragments. Conglomerates have rounded particles Breccia has angular fragments. Why the difference? Amount of time it took for the rock to cement.

Medium-Grained Clastics Consist of rock and mineral fragments that are sand-sized. (Sandstones) Sandstone may contain ripples, which tells us about past conditions on the land there. Ripple marks and cross-bedding indicates the direction of current flow. Sandstone has a high porosity. Porosity is the percentage of open spaces between rock grains. Sandstones can serve as underground reservoirs for oil, natural gas, and groundwater.

Fine-Grained Clastics Consist of particles that are smaller than sand grains. Composed of silt? = siltstone Composed of mud? = mudstone Shale is made of silt and clay. These have very low porosity. Typically form barriers to groundwater and oil.

Chemical Sedimentary Rocks Dissolved substances in water can get left behind to form rock if the water evaporates away. Layers of sedimentary rock formed in this fashion are called evaporites. These form mostly in arid regions. Most common evaporites are halite and calcite.

Organic Sedimentary Rocks Formed from the remains of once-living things. Most abundant form is limestone. Coal is also common.

Importance of Sedimentary Rocks Sedimentary rocks give us a snapshot of Earth’s ancient surface conditions. Energy! Coal, oil, and natural gas are stored in sedimentary rocks Uranium is often mined from sandstone.

Metamorphic Rocks 6-3

Causes of Metamorphism We all know that pressure and temperature increase with depth. When these become high enough, rocks melt and form magma. Sometimes, high temperatures and pressures combine to alter the texture, minerology, or chemical composition of a rock without melting it. Metamorphic means that the rock changed form, while remaining solid.

So where does the heat come from? Heat created by deep burial pressure. Heat from nearby igneous rocks.

Where does the pressure come from? Vertical pressure caused by the weight of overlying rock. Compressive forces generated as rocks are deformed during mountain building.

Types of Metamorphism Various amounts of temperature and pressure results in different types of metamorphism (See fig. 6-12 on pg. 133)

When high temperature and pressure affect large regions of Earth’s crust, they produce large belts of regional metamorphism. Can fall into low, intermediate, and high grades. These reflect the intensity of temperature and pressure.

Knowing the temperatures that certain areas experienced when rocks were forming can help geologists locate economically valuable metamorphic minerals like garnet and talc.

When molten rocks come in contact with solid rock, contact metamorphism occurs. Results from high temperatures and moderate-to-low pressure.

When hot water reacts with rock and alters its chemistry and mineralogy, hydrothermal metamorphism occurs. Common around igneous intrusions and near active volcanoes.

Metamorphic Textures There are 2 textural groups: nonfoliated and foliated. Foliated: Wavy layers and bands of minerals. Long thin bands form in the rock perpendicular to the pressure. Most common examples: Schist and Gneiss.

Nonfoliated metamorphic rocks lack mineral grains with long axes in one directions. Composed mainly of minerals that form with blocky crystal shapes. Examples: Quartzite and Marble

Porphyroblasts are minerals that grow quite large while surrounding minerals remain small. Found in areas of both contact and regional metamorphism.

Mineral Changes Minerals change in metamorphism just as fractional crystallization occurs. Minerals are stable at certain temperatures and crystallize from magma at different temperatures. These stability ranges apply to minerals in solid rock. During metamorphism, the minerals in a rock change into new minerals that are stable under the new temperature and pressure conditions. This is called a solid-state alteration. Scientists can use minerals found in the rocks to interpret the conditions inside the crust during the rocks’ metamorphism.

Compositional Changes Most metamorphic rocks reflect the original chemical composition of the parent rock. Ex: Gneiss has the same general chemical composition as granite. Sometimes, hot fluids migrate in and out of the rock during metamorphism, which can change the chemical composition of the rock. This is especially common during contact metamorphism near igneous intrusions. Valuable ore deposits of gold, copper, zinc, tungsten, and lead are formed in this manner.

Rock Cycle Any rock can be changed into any other type of rock. This continuous changing and remaking of rocks is known as the rock cycle.

Rock Cycle