Weathering and Erosion Overview

Weathering and Erosion

• Weathering breaks down rocks into smaller pieces, known as sediments, through physical and chemical processes.
• These sediments are transported by water, wind, glaciers, and gravity.
• Sedimentary rocks form when these sediments are glued together.

Weathering

• Physical weathering breaks rocks into smaller pieces without changing the minerals within them.
• Chemical weathering alters the minerals within rocks by dissolving them, or changing their chemical composition.
• Chemical weathering leaves behind those minerals that are resistant to change.

Erosion

• Erosion is the removal and transport of sediment by wind, moving water, glaciers, and gravity.
• Eroded material is usually carried downhill.

Deposition

• Deposition occurs when the transport of sediments stops, and they settle out.
• The largest particles settle first, forming layers with progressively smaller grains at the top.
• Flowing water, wind, and glaciers can transport different sizes of sediment.

Lithification

• Lithification transforms sediment into sedimentary rock through compaction and cementation.
• Compaction occurs when the weight of overlying sediments squeezes out water and forces sediment grains closer together.
• Cementation occurs when mineral growth occurs between sediment grains, gluing them together into solid rock.

Sedimentary Features

• Sedimentary rocks provide a history of their formation and can help geologists interpret how they formed.
• Key features include bedding, graded bedding, cross bedding, and ripple marks.

Bedding

• The most common sedimentary rock feature.
• Consists of horizontal layers of sediment.
• Layers can range in thickness from a few millimeters to several meters.

Graded Bedding

• The particle sizes become progressively finer and lighter toward the top layers.
• Often seen in marine sedimentary rocks.
• Larger and heavier material settles out first, followed by finer material.

Cross Bedding

• Forms when layers of sediment are deposited at an incline across a horizontal surface.
• Occurs in sandy beaches, sandbars in streams and rivers, and migrating sand dunes.

Ripple Marks

• Form when wind or waves move sediment into small ridges.
• Symmetrical ripple marks form when waves push sand into symmetrical ridges.
• Asymmetrical ripple marks form when a current flows in one direction.

Angular vs Rounded

• Sediment initially has angular pieces when a rock breaks apart.
• As sediment is transported, pieces knock into each other and become rounded.
• The amount of rounding depends on the length of transport and how far the sediment travels.

Clastic Sedimentary Rocks

• The most common type of sedimentary rock.
• Formed from loose sediments that accumulate on Earth's surface.
• Classified by particle size.

Coarse-grained Rocks

• Consist of gravel-sized rock and mineral fragments.
• Include conglomerates, which have rounded, gravel-sized particles.
• Transported by high-energy flows of water, such as mountain streams, flooding rivers, ocean waves, and glacial meltwater.

Fine-grained Rocks

• Consist of silt and clay-sized particles.
• Form in swamps, ponds, and deep oceans, where water is still or slow-moving.
• Tend to be more layered than coarse-grained rock.

Chemical and Biochemical Sedimentary Rocks

• Formed through the evaporation and precipitation of minerals.
• Dissolved minerals are carried into lakes and oceans during weathering.
• Minerals are left behind when water evaporates.

Chemical Sedimentary Rocks

• Form when dissolved minerals in a body of water reach saturation and precipitate out of solution.
• Mainly found in arid regions, drainage basins on continents with low water flow, and coastal settings.

Biochemical Sedimentary Rocks

• Formed from the remains of once-living organisms.
• The most abundant is limestone, which is primarily composed of calcite.
• Shells of marine organisms settle to the bottom of the ocean and form layers of carbonate sediment.
• Calcium carbonate precipitates out of the water, forming limestone.

Table 2: Classification of Sedimentary Rocks

Medium-grained Rocks

• Consist of sand-sized sediments, such as sandstone.
• Contain several interesting features, such as ripple marks and cross-bedding, which indicate the direction of current flow.
• Contain relatively high porosity, making sandstone layers valuable reservoirs for oil, natural gas, and groundwater.