Weathering and Erosion Flashcards

Weathering and Erosion

Introduction

Erosion is a dominant force shaping the Earth's surface. Although running water is typically considered the primary agent of natural erosion, human activities have surpassed it in recent years. These activities include:

• Road-building

• Livestock overgrazing

• Deforestation

• Construction projects

• Mining and urbanization

• Poor farming methods

The impact of human activity has led some scientists to propose a new geological epoch called the Anthropocene, marked by significant human alteration of the Earth's land surface (over 75%) and a rapid extinction of species. We should consider actions to reduce human contributions to erosion.

Vocabulary

• Abrasion: The process of wearing down by friction.

• Stream: A body of water flowing through a channel.

• Breaking Wave: A wave that collapses as it enters shallow water.

• Chemical Weathering: Breakdown of rocks through changes in chemical composition.

• Delta: A fan-shaped deposit of sediment at the mouth of a river.

• Erosion: The process by which sediments are transported.

• Finger Lake: A long, narrow lake in a U-shaped valley.

• Flood Plain: A flat area alongside a river that is subject to flooding.

• Glacial Groove: A groove in bedrock caused by glacial abrasion.

• Glacial Parallel Scratches: Parallel scratches on bedrock caused by glacial abrasion.

• Glacier: A large mass of ice moving downhill.

• Mass Movement: Downslope movement of rock and soil due to gravity.

• Meander: A bend in a river.

• Physical Weathering: Breakdown of rocks into smaller pieces without chemical change.

• Sandbar: A ridge of sand formed in a river or along a coast.

• Sandblasting: Abrasion by wind-blown sand.

• Sediment: Solid particles that have been transported and deposited.

• Stream Abrasion: Wearing down of rocks and sediments by the action of a stream.

• Stream Channel Shape: The shape of the bed that confines the stream.

• Tributary: A smaller stream that flows into a larger stream.

• U-shaped Valley: A valley with steep, straight sides and a flat bottom, formed by glacial erosion.

• V-shaped Valley: A valley with sloping sides that come to a point at the bottom, formed by stream erosion.

• Watershed: An area drained by a river and its tributaries.

• Weathering: The breakdown of rocks at or near the Earth's surface.

Weathering

Weathering is the chemical and physical breakdown of rocks at or near the Earth's surface. It occurs when rocks are exposed to air, water, pressure changes, and the actions of living things. Weathering is responsible for the origins of most of the earth's landscape features and soils and affects structures such as buildings and roads.

Types of Weathering

Weathering processes are classified into two types: chemical and physical.

Chemical Weathering

Chemical weathering involves the breakdown of rock through a change in mineral or chemical composition. One form of chemical weathering is oxidation (e.g., iron + oxygen = rust--iron oxide). Another form is the dissolving of rock by water, sometimes called the universal solvent, and acids from decaying organic matter.

CO_2 from air + water = carbonic acid, which dissolves limestone and marble, leading to:

• Sinkholes

• Caves

• Limestone terraces

• Limestone "icicles"

Physical Weathering

Physical weathering involves the breakdown of rock into smaller pieces without chemical change. It occurs due to:

• Frost action: Water expands when it freezes (by nearly 10%), widening cracks in rocks. This is similar to the formation of road potholes.

• Pressure changes: Rocks expand and crack when confining pressure is reduced (e.g., by removal of overlying rock due to erosion, mining, quarrying, or melting glaciers).

• Abrasion: Rock particles grind against each other when moved by ice, gravity, running water, or air. This causes the rounded shape of rocks in river beds and beaches.

Factors Affecting the Rate and Type of Weathering

Several factors determine the rate and type of weathering in a given location, including:

• Exposure to air, water, and living things: The closer a rock is to the Earth's surface, the faster it will weather. Differences in solar energy on opposite sides of mountains affect temperature and moisture, leading to different soil characteristics.

• Particle size: Smaller particles have a greater surface area per unit volume, leading to a faster rate of weathering. (e.g., granulated sugar dissolves faster than sugar cubes).

• Mineral composition: Different minerals have different physical and chemical properties. Calcite in limestone and marble weathers rapidly in slightly acidic water, while quartz is largely unaffected by chemical weathering.

• Climate: Chemical weathering is pronounced in warm, moist climates. The higher the average temperature and precipitation, the more rapid the chemical weathering. In cold climates, frost action is the most common form of weathering.

Soil Formation

Physical and chemical weathering are important in soil formation. Soil is a mixture of rock particles and organic matter that supports rooted plants. Living things (e.g., ants, burrowing animals, plant roots, decaying leaves) play a significant role. Soils form in place from parent material (e.g., bedrock) and develop horizontal layers with a distinctive profile.

• Soils formed in place: Have characteristics of the parent material. Climate is the most important factor in determining the type of soil.

• Soils transported into an area: Usually by wind, moving water, or glaciers. In New York State, most soils formed from sediments deposited by glaciers during the last ice age. These soils have a soil profile similar to immature soil.

Erosion

Erosion is the process by which weathering rock particles are transported as sediments. Over time, erosion shapes and lowers the Earth's surface.

Transporting Systems of Erosion

Erosion involves:

• An agent of erosion (stream, glacier, ocean wave/current, turbidity current, human activity).

• Sediments being moved.

• A driving force (usually gravity; sun's energy plays an indirect role by driving the water cycle and wind patterns).

Gravity Erosion

Gravity causes sediments to move downslope, either on its own (mass movement, landslide) or via another agent like running water or glaciers. Mass movement involves:

• Downward pull of gravity.

• Friction (opposing force).

Mass movements occur when the forces holding materials in place become weaker than the downward pull of gravity. Conditions include:

• Heavy rainstorms (saturates the ground, lowering frictional forces).

• Earthquakes, wave erosion, stream side cutting, saturated soil, road-building.

Types of mass movements vary greatly (soil creep, debris flow, mudflow, rock fall).

Running Water Erosion and Streams

Running water erosion is the dominant natural agent of erosion and begins with raindrops. Thin sheets of water flow over the land and become confined to temporary stream valleys called rills or gullies. When confined to a channel, a stream exists.

A smaller stream that flows into a larger stream is called a tributary. Streams can be permanent (water flows year-round). Streams carry sediments:

*Dissolved minerals (in solution).
Solid sediments (suspended in the water).
Larger sediments (rolling, sliding, or bouncing along the stream bottom. Stream abrasion results in rounding of sediments).

Over time, streams carve deeper channels forming a V-shaped valleys (downcutting, runoff, and mass movement cause the sides of the valley to cave in). The area of land drained by a stream is called its watershed or drainage area. A region of higher land that separates watersheds is called a divide or drainage divide. The continental divide in the United States follows the crest of the Rocky Mountains.

Stream Velocity: Determined by:

• Gradient (slope) of the stream.

• Discharge (volume of water) of the stream.

• Stream channel shape (a semicircular channel results in less friction and greater stream velocity).

Maximum velocity occurs at the center of the stream or outside of a curve. Greater stream velocity allows stream to carry larger sediment and more total sediment.

Evolution of a Stream

Streams change their characteristics over time in response to:

• Elevation

• Gradient

• Volume of water

• Available sediments

Streams origin often begins at finger lakes (bodies of water in U-shaped valleys). In its early stages, most of stream's energy is used for downcutting a narrow V-shaped valley, resulting in waterfalls and rapids. Downstream, more water increases the size of the stream. As the stream grows larger, it shifts its course in a series of bends called meanders and carve a wider valley during times of flooding, which become floodplains. Near the end of the stream, discharge of water is great and there is little slope with a very wide valley. Deposition of sediments form sediment mounds called levees. A delta forms from sediments deposited over time from the ever-shifting channel at the stream's mouth.

Wind Erosion

Wind erosion occurs in arid regions and coastlines with loose sediments (sand/smaller). Two main aspects of wind erosion:

• Deflation: Winds blow away small, loose sediments (lowering the land surface).

• Sandblasting (abrasion): Winds blow sand or silt grains against rocks (eroding the rocks. The lower portions of exposed rocks become more eroded than the upper portions).

Colliding sand grains also result in abrasion and a frosted appearance.

Glacial Erosion

A glacier is a naturally formed mass of ice and snow that moves downhill. Mountain glaciers are found in mountain valleys; ice-sheet or continental glaciers cover huge landmass regions. Glacial boundaries are determined by:

• The accumulation of snow and ice.

• The wasting away of the glacier by melting, iceberg formation, and sublimation.

Glacial movement occurs by plastic flow. The glacial ice acts like a fluid. Glaciers are often called "rivers of ice". Valley Glacier moves fastest in the middle and slowest at the sides. Often, glacial motion is caused by sliding at the base of the glacier when liquid water helps to overcome friction.

Types and Features of Glacial Erosion: When a glacier moves over the land, loose rocks and other materials freeze into the ice and are dragged along. These rocks and sediments act like gougers and sandpaper on rock surfaces and create glacial grooves, glacial parallel scratches, and more loose sediments. Direction of glacial grooves and scratches show former glacial movement. Glacial sediments can be carried hundreds of miles. Erratics (Contrasting sediments) are often found in northern United States and most of New York State. Wide, thick ice erodes its confining valley walls creating U-shaped valleys. Continental and mountain glaciers have left erosional features (Finger Lakes, etc.) in New York State.

Wave and Current Erosion

Where lake or ocean waters meet land, waves and currents act as agents of erosion and create unique landscape features. Winds transfer energy to the surface of lakes and oceans, creating waves. Waves entering shallow water near the shore drag against the bottom and become unstable. Water rushes toward the shore as breaking waves, or surf. Breaking waves pound the shoreline. Waves often arrive at an angle to the shore and are refracted or bent. Wave energy is concentrated to the shore that extend out to the water.

If the coastline is straight, the wave energy is more evenly spread out. A sea cliff can be cut by waves when the shore has no depositions to protect it. Waters near shore are pushed in one direction along the shore (longshore current). Sediments on shore tend to move in a zig-zag pattern. Creates sand bar with mounds of sediment. Shores, over time, become more rounded, smoother, and smaller.

Sediment Features and Erosional Agents

Each agent of erosion produces distinctive characteristics in the sediment it transports Solid sediments in wind, beach, or stream erosion will become smooth, rounded, and smaller. Wind-blown sediments are more frosted than stream sediments. Glacial sediments are partly rounded (sub-rounded) and often have scratches. Sediments produced by physical weathering or moved directly by gravity are very angular in shape.

People and Erosion

Humans have become a major agent of erosion: construction projects, road-building, mining, deforestation, overgrazing, and poor farming methods expose soil and loose materials and increase erosion rates. Solutions: replanting logged forests, restoring coastal vegetation, farming techniques such as no-till farming and contour plowing, and highways designed to create gentler slopes that are less likely to produce mass movements.