1. External Forces
Shaping the Earth

2. 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 Weathering
a. 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.

3. 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.

4. 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.

5. 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).

6. 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?

7. 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.
C. 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.

8. Movement: At Chakachamna in Alaska, a glacier
moves down a mountain.
Question:
What effect
has the glacier
had on the
landform
shown here?

10. 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.

11. 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.