External Forces Shaping the Earth

Chapter 2: Section 4

A Human Perspective

In Egypt, a seasonal dry wind is called khamsin (“fifty”) for the number of days the season occurs. During Khamsin, wind-driven sandstorms kill and injure people, close businesses and airports, and strip topsoil and seed from the ground. Sandstorms are not limited to the desert areas of Africa and Southwest Asia. For instance, a five-hour storm recently blasted Jingehang, China, causing millions of dollars of damage and killing about 300 people. Sandstorms are among the external forces that change the shape of the earth and affect the lives of the people in their paths.

Weathering

External forces, such as weathering and erosion, also alter landscapes and in some instances create the soil that is needed for plant life over many years or centuries.

Weathering: refers to physical and chemical processes that change the characteristics of rock on or near the earth’s surface.

Sediment: broken rock which can be identifiable as either mud, sand, or silt.

Two types of Weathering

Mechanical weathering:

process of breaking rock into smaller pieces.

Examples:

Ice crystals: ice builts in a mts. It creates enough pressure to break the mt.

Plants roots: digs into the rocks and breaks it.

Human activities break

rock into smaller pieces: road construction, drilling & blasting in mining.

Chemical Weathering: it occurs when rock is changed into a new substance as a result of the elements in the air, water, & minerals in the rock.

Acid rain is believe to be speeding up the process.

Mechanical Weathering

Chemical Weathering

Erosion

It occurs when weathered material is moved by action of wind, water, ice or gravity.

For erosion to occur water must be present.

Glaciers, waves, stream flow, or blowing winds cause erosion by grinding rock into smaller pieces.

Water Erosion

As water flows in a stream or river, the motion picks up loose material and moves downstream & the force will also break landform.

Most streams erode both vertically & horizontally– it will cut a stream as it get deeper & wider, forming a V—shaped valley.

Wind Erosion

It is similar to water erosion because the wind transports and deposits sediment in other locations.

Depending on speed of wind it can create new landform such as sand dunes or shape rock sculpted into new forms.

It will eat always at a landform.

Glacial Erosion

Is a large, long-lasting mass of ice that moves because of gravity.

As a glacier moves it carries rocks and sediments underneath the snow forming new landforms.

Building Soil

Weathering & erosion help in forming soil.

Soil is the loose mixture of weathered rock, organic matter, air, & water that supports plant growth.

What makes good soil?

Parent material: the chemical of the rock before it decomposes affects it fertility.

Relief: higher mts. Erode easily & do not produce soil quickly.

Organisms: plants, small animals like worms, ants, & bacteria help to loosen soil & supply nutrients for plants.

Climate: it needs to have a moist and cool climate.

Time: it varies, but 2.5 cubic centimeters per century.

The plus side of Volcanoes- Fertile Soil Volcanoes can clearly cause much damage and destruction, but in the long term they also have

benefited people. Over thousands to millions of years, the physical breakdown and chemical weathering of volcanic rocks have formed some of the most fertile soils on Earth.