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 1. 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. 1. 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 2. It occurs when weathered material is moved by action of wind, water, ice or gravity. For erosion to occur water must be present. 3. Erosion cuts, a V-shaped valley, along which sediment is deposited. Glaciers, waves, stream flow, or blowing winds cause erosion by grinding rock into smaller pieces.

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? 4. Parent material: the chemical of the rock before it decomposes affects it fertility. 4. Relief: higher mts. Erode easily & do not produce soil quickly. 4. Organisms: plants, small animals like worms, ants, & bacteria help to loosen soil & supply nutrients for plants. 4. Climate: it needs to have a moist and cool climate. 4. Time: it varies, but 2.5 cubic centimeters per century.

Water Erosion 1. moves sediment to new places, widens riverbeds; cuts into banks, reduces, increases, or changes the location of beaches, builds sandbars and islands Most streams erode both vertically & horizontally– When a river enters the ocean, the sediment is deposited in a fan-like landform called a delta.

Wind Erosion 2. moves sediment to new places, creates and changes landforms; may help create fertile soil It is similar to water erosion because the wind transports and deposits sediment in other locations. Dust storms are capable of carrying as much as 6,000 tons of sediment per cubic mile of air. Depending on speed of wind it can create new landform such as sand dunes or shape rock sculpted into new forms.

Glacial Erosion 3. Moves sediment to new places; slowly changes the land over time by creating new landforms such as valleys, ridges, and kettle lakes 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.