Weathering and Erosion Chapter 17 Section 4 GoalsObjectives

Weathering and Erosion Chapter 17: Section 4

Goals/Objectives • After completing the lesson, students will be able to. . . – Identify the causes of rock shaping due to weathering and erosion; – Explain how chemical weathering can form underground caves; – Describe how acid rain affects the landscape

Introduction • Typically, weathered rock materials do not stay in place, especially where the ground slopes. • Gravity can cause soil and rock fragments to fall, slide, or move at slow speeds to lower levels.

Introduction • Rain or wind may remove sand dust from the side of a hill and rivers can transport these materials downstream.

Introduction • Mass Movement—The downward transportation of weathered materials by gravity. • Erosion—The removal and transport of materials by natural agents such as wind and running water.

Physical Weathering • Physical Weathering: – Also known as mechanical weathering – Breaks rocks into smaller pieces but does not alter the rocks’ chemical composition – Example: • • Ice Plants Rivers Wind • Chemical Weathering – Breaks down rock by changing its chemical composition – Example: • Oxidation • CO 2 dissolving into water • Acid Rain

Mass Movement • Soil protects bedrock beneath the soil, and when the soil and loose rock are removed, the bedrock is more susceptible to weathering. • Talus—Cliff rock fragments that have been weathered loose and pulled down by gravity.

Mass Movement • Landslide—The movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff.

Mass Movement • Landslides are most likely to occur on steep slopes, and after heavy rainfall or during the spring when large amounts of snow is melting. • Rain and melted snow add weight to the soil, which makes it easier for gravity to pull the soil downwards. • Geologists use terms such as creep, slump, earthflow, and mudflow to describe landslides. – Buildings built in these areas are at risk of being damaged or even destroyed.

Landslides

Creep • Creep—A slow, imperceptible movement of soil down a slope. • Even though it is hard to see move, the effects are very noticeable. – Fences leaning – Poles unleveled

Slump • Slump—A mass movement in which a block of land tilts and moves downhill along a surface that curves into the slope. • Slumps tend to occur because a slope has become too steep for the bottom of the slope to support the soil at the top of the slope.

Earthflows • Earthflow—The down-slope movement of a mass of earth materials that have been saturated with water. • The downhill movement is slower and less fluid than a mudflow. • Factors affecting the velocity of earthflow include the amount of water present, the composition of the soil, and the steepness of the slope.

Mudflows • Mudflow—The rapid movement of water that contains large amounts of suspended clay and silt. • Mudflows contain more water than earthflows and have been known to travel at up to 100 kilometers per hour down steep mountains.

Mudflows

Mudflow

Erosion and Landforms • Volcanic Neck—The solidified lava filling the central vent of an extinct volcano.

Deposition • Of all the agents of erosion, running water is the most effective. • Gravity pulls water downhill, eroding the soil and rock materials along the way. • Deposition—The process by which materials are deposited.

How Streams and Weather Erode Material • Running water in streams and rivers wears down Earth’s surface by breaking up bedrock and by removing eroded rock and soil materials—this is done primarily by mechanical means. • Most erosion occurs when running water abrades (abrasion) and hammers away at its bed by using sand, pebbles, and even boulders as cutting tools. • Potholes—Deep oval or circular basins cut into a stream or river bed by abrasion from swirling sand, pebbles, and small boulders. • Potholes occur in many different types of rock and range in size.

How Streams Transport Materials • Bed Load—Sand, pebbles, and boulders that are moved along the bed of a stream and that are too heavy to be carried in suspension. • These materials are moved along streambed, especially during floods.

How Streams Transport Materials • The relative amounts of a river’s load that are carried in solution, in suspension, and in the bed load depend on the following factors: – – The nature of the river The climate The type of bedrock The season of the year.

How Streams Transport Materials • Two measures are used to describe the ability of a stream to transport materials. – Competence—A measure that describes the maximum size of the particles a stream can carry. – Capacity—A measure of the total amount of sediment a stream can carry.

Depositional Features • Delta—A fan-shaped deposit that forms when a river flows into a quiet or large body of water, such as a lake, an ocean, or an inland sea. • River water comes almost to a standstill at a delta where most of a river’s sediment is dropped. • As long as the amount of sediment supplied by the river is larger than the amount that can be taken away by currents, the deposit grows. • A river flowing over its delta splits into branches called distributaries, which are responsible for the delta’s shape.

Depositional Features • A fan-shaped deposit called an alluvial fan may form when a steep mountain stream meets dry, level land at the base of the mountain. • The velocity of the stream decreases greatly when it reaches the land.

Depositional Features • An alluvial fan differs from a delta in several ways: – The deposit is formed on land, not in water. – The sediments are coarse sands and gravels rather than fine silt and clay. – Its surface is sloping, not flat like that of a delta.