Ch. 6

6-1

• 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.

• 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
Gravel64-2 mm
256-64 mm
>256 mm
Pebble
Cobble
Boulder

• 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.

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 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. 100 o C.
• 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.

• 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.

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.

• Classification is based on how the rock forms.
– 3 types:
• Clastic
• Organic
• Chemical

• 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.

• 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.

• Formed from the remains of once-living things.
– Most abundant form is limestone.
– Coal is also common.

• 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.

6-3

• 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.

• 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.

• 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.

• 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.

• 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.

• 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