External Forces Shaping the Earth

I. Weathering A. Altering the Landscape

a. Weathering—processes that alter rock on or near the earth’s surface.

b. Can change landscapes over time and create soil for plant life.

c. Sediment—mud, sand, silt created by weathering processes .

B. Mechanical Weatheringa. Mechanical weathering—processes that break

rock into smaller pieces.b. Does not change rock’s composition, only size .c. Examples: frost, plant roots, road construction,

mining.

C. Chemical Weathering 1. Chemical weathering—interaction of elements

creates new substance.

a. Example: when iron rusts it reacts to oxygen in air and crumbles.

2. Warm, moist climates produce more chemical weathering than cool, dry.

Making Comparisons: Why would chemical weathering be rare in a desert area?

Chemical weathering usually occurs where water is present. Desert areas have little water.

II. Erosion A. Weathered Material Moves

1. Erosion—when weathered material moves by winds, water, ice, gravity.

a. movement grinds rock into smaller pieces, carries to new location.

b. Example: water carries topsoil from hill to river, river narrows.

B. Water Erosion 1. Most streams erode vertically and horizontally .

a. A valley cut by a stream gets deeper, wider; forms v-shaped valley

b. a river deposits sediment at ocean, creates delta—fan-like landform (Δ Greek letter delta).

Movement: A view of the Colorado River and the Grand Canyon in Arizona. The canyon’s depth was created by water erosion, and the width by rain and wind erosion.

Question:What has happened to the sediment created by weathering in the canyon?

C. Wind Erosion 1. Wind transports sediment from one place to another.2. Loess (LOH.uhs) wind-blown silt and clay

sediment; produces fertile soil.

D. Glacial Erosion 1. Glacier—large, long-lasting mass of ice; forms in

mountainous areas.2. Glaciation—changing of landforms by slowly

moving glaciers.

a. Example: cutting u-shaped valleys in land.3. Moraine—hill or ridge formed by rocks deposited

by glacier.

Movement: At Chakachamna in Alaska, a glacier moves down a mountain.

Question:What effect has the glacier had on the landform shown here?

III. Building Soil A. Soil Formation

1. Soil—loose mix of weathered rock, organic matter, air, water.

2. Soil supports plant growth; fertility is dependent on three factors:

a. Texture.

b. amount of humus, which is organic material in soil.

c. amount of air and water.

B. Soil Factors 1. When geographers study soil, they look at five

factors:

a. parent material—the chemical composition of the original rock.

b. relief—the steeper the slope, the greater erosion; less soil made.

c. organisms—plants, worms, ants, bacteria loosen soil; supply nutrients.

d. climate—hot, cold, wet, dry climates produce different soils.

e. time—about 2.5 cubic cm. of soil produced each century.