Rock Cycle & Sedimentary Rocks
I. The Rock Cycle
A. What is a rock?
- A mixture of minerals, rock fragments, volcanic glass,
organic matter, or other natural materials.
- Granite is an example.
B. The rock cycle
- A model that illustrates the processes that create & change rocks.
- Sedimentary rock can change by heat & pressure to form
metamorphic rock.
- Metamorphic rock can melt & cool to form igneous rock .
- Igneous rock can be broken into fragments and then cemented or
compacted to form sedimentary rock.
- Any rock can change into any of the 3 rock types.
C. Matter & the rock cycle
- The chemical elements that make up minerals & rocks are
not destroyed.
- The rock cycle follows the law of conservation of matter.
II. Sedimentary Rock
A. Formation of Sedimentary Rock
- Sediments are loose materials such as rock fragments, mineral
grains, and bits of shell that have been moved by wind, water,
ice, or gravity.
- Sedimentary rock forms when sediments are pressed and cemented
together, or when minerals form from solutions.
- Sedimentary rocks often form in layers, with he oldest layers on
the bottom.
B. Classifying Sedimentary Rocks
- Classified by their composition & by the manner in which
they formed.
1. Detrital Sedimentary Rocks
- Form from broken fragments of other rocks.
- S mall s ediments will stick together and form solid rock.
(Compaction)
- Larger sediments have to be cemented together.
- Minerals such as quartz, calcite, & hematite are deposited
between pieces of sediment and “glue” them together.
- Granular textures
- Conglomerate, Breccia, Sandstone, Siltstone, & Shale
2. Chemical Sedimentary Rocks
- Form when dissolved minerals come out of solution.
- They are NOT made from preexisting rocks.
- Limestone is formed when Calcium carbonate comes out of
solution as calcite.
- Rock salt is a chemical sedimentary rock.
3. Organic Sedimentary Rocks
- Rocks made of once-living things.
- Fossil-rich limestone is the most common.
- Chalk is made of microscopic shells.
- Coal forms when pieces of dead plants are buried under other
sediments and then compacted.
Weathering and Soil
I. Weathering
A. Weathering & its effects
- Surface processes that break down rock.
- Sediments: sand, silt, & clay
- Weathering changes the Earth’s surface.
B . Mechanical Weathering
- Occurs when rocks are broken apart by physical processes.
- Growing plants, burrowing animals, and expanding ice can
mechanically weather rock.
- Ice wedging occurs in temperate & cold climates where water
enters cracks in rocks, freezes, & expands.
- As the amount of surface area increases, more rock is exposed
to be weathered.
C . Chemical Weathering
- Occurs when chemical reactions dissolve the minerals in
rocks or change them into different minerals.
- Naturally formed acids can weather rocks chemically
- Carbonic acid can form when Carbon dioxide mixes with
water. The acid reacts with calcite in limestone to form
caves.
- Some roots & decaying plants give off acids that dissolve
minerals in rock.
- Oxidation occurs when some materials are exposed to oxygen
and water. (Rust)
D. Effects of Climate
- Mechanical weathering occurs more frequently in cold climates.
- Chemical weathering occurs more frequently in warm, wet climates.
- Rock type can affect the rate of weathering in a particular climate.
II. Soil
A. Formation of Soil
? Weathering gradually breaks rocks into smaller and smaller
fragments.
? Plants and animals add organic matter to the rock fragments.
? Soil is a mixture of
? Weathered rock
? Decayed organic matter
? Mineral fragments
? Water
? Air
? Causes for soil formation include:
? Climate
? Slope
? Types of rock
? Types of vegetation
? Length of time that rock has been weathering
B. Composition of Soil
? Decayed organic matter turns into a dark-colored material
called humus.
? Humus provides nutrients for plants.
? Burrowing creatures mix humus with the other rock fragments.
C. Soil Profile
1. A Horizon (topsoil)
? Can be covered by litter, organic material that will eventually become humus and helps prevent erosion.
2. B Horizon
? Li ghter in color & contains less humus.
? Leeching is the removal of minerals that have been dissolved by water.
? Water reacts with humus & carbon dioxide to form acid.
3. C Horizon
? Made of partially weathered rock and is found at the bottom of a soil profile.
D. Soil Types
? D ifferent types of soil vary in color, depth, texture, & fertility.
? T he type of soil depends on the region’s climate.
? Deserts are dry, prairies are semi-dry, & temperate forests are mild & moist.
III. Soil Erosion
A. Soil – An Important Resource
? Soil is eroded when it is moved to a new location by wind or water.
? Soi l erosion removes topsoil that is important for plant growth.
B. Causes and Effects of Soil Erosion
? Human activities can increase the rate of soil erosion.
? Soil erosion occurs rapidly on s teep slopes and areas that are not covered by vegetation.
? Forest harvesting & overgrazing contribute to the rate at which
erosion can occur.
C. Preventing Soil Erosion
? Farmers reduce erosion by pl anting shelter belts, using no- ti ll farming, and planting cover crops after harvesting.
? Contour farming and terracing are used to control erosion on slopes.
? Rows of trees (windbreaks) are planted on the plains to reduce
erosion caused by wind.