Chapter 7: Weathering, Erosion, and Soil

relate to weathering and How does this cartoon relate to weathering and erosion?

BIG Idea: Weathering and erosion are agents of change on Earth’s surface.

Weathering: the process by which rocks are broken down chemically and physically by the action of water, air, and organisms.

I. WEATHERING PROCESSES Mechanical Weathering Strictly a physical process Rocks and minerals break into smaller pieces No change to chemical composition

Agents of MECHANICAL Weathering: ice plants and animals gravity running water wind

1. Ice (Frost) Wedging water seeps into rock cracks and then freezes

What causes this to happen?

2. Pressure Exfoliation: outer rock layers are stripped away (like an onion’s layers can be peeled)

3. Organic Activity

Root Action: tree roots grow within cracks in rocks and cause the rocks to split

4. Abrasion The collision of rocks with one another, causing them to break down Agents of abrasion: gravity running water wind

wearing down or smoothing of a material due to constant collisions

Wind Abrasion

Water Abrasion

B. Chemical Weathering Process by which rocks and minerals undergo changes in their composition Can cause some substances to dissolve Some new minerals form New minerals have properties different than those of original rock

What items have you seen rusted? What could you do to such objects to prevent or reduce rusting?

Agents of CHEMICAL Weathering: Water Oxygen Carbon dioxide Acid precipitation

Hydrolysis: reacting with water Carbonation: carbon dioxide combines with water to produce carbonic acid (dissolves limestone) Oxidation: reacting with oxygen (rust)

Carbonic acid creates caves

Oxidation of the iron in the rock causes a red hue

Acid precipitation (rain): sulfur dioxide, carbon dioxide and nitrogen oxides released into atmosphere by human activities; gases combine with oxygen and water to form strong acid rains Plant acids: from decaying plants

II. RATES OF WEATHERING All rocks do not weather at the same rate (speed)… Some factors that affect the rate at which rocks weather are:

A. Rock Composition Igneous and metamorphic rocks break down more SLOWLY Hardest minerals resist weathering (i.e. quartz) Rocks containing calcite weather QUICKLY

Which layer is MORE resistant to weathering? LEAST resistant? D C B

B. Amount of Exposure time exposed surface area

Surface Area Which will weather faster, solid rock or crushed rock?

The crushed rock of course! the large rock is tightly packed and has less surface area exposed the crushed rock is loosely packed and has more surface area exposed

Increased surface area = increased weathering

C. Climate Rocks in tropical regions weather faster than rocks in dry desert areas More rain, humidity, wind, and temperature range = faster weathering

Physical Weathering: works better in a cool and dry climate Chemical Weathering: works better in a hot and moist environment (more water = more weathering)

Cleopatra’s Needle stood in Egypt (hot/dry) for 3,000 years…

It was only in New York City for 100 years Rain, ice, and pollution destroyed it

Compare the two tombstones in the next picture… How do they compare in appearance? Which appears more weathered?

marble slate

Which tombstone is older? marble slate

This means that rocks can weather at different rates! marble slate

D. Topography Higher elevation = faster weathering (colder, more ice wedging) Steep slopes - gravity pulls rocks down hill

III. Weathering and Soil Soil: loose rock fragments and clay from weathered rock mixed with organic material.

A. Soil Composition A = Humus - dark organic material B & C = Soil – mixture of minerals, gases, and dead organisms D = Regolith – layer of weathered rock fragments E = Bedrock – solid unweathered rock

B. Soil Profile Soil Profile: cross section of the soil layers Horizons: layers in the soil

Two Types of Soil: Residual: parent material is the local bedrock Transported: develops from parent material that has been moved far from original location

Residual Soils forms from the weathering of the underlying bedrock

Transported Soils Due to…

IV. EROSION Process by which the products of weathering are TRANSPORTED (moved) Agents of erosion: Running WATER wind glaciers (ice)

A. Gravity and Erosion Talus: rock fragments at the base of a slope

Mass Movements Rock fall: fall of rocks from a steep cliff

Landslide: loose soil and rocks that move down the slope of a hill

Mudflow: rapid movement of a large mass of mud

Creep: slow downhill movement of rocks and soil

B. Landforms and Erosion 3 major landforms: Mountains: steep, high elevations Plains: flat or gently sloped, not high above sea level Plateaus: high elevations, flat surfaces