Earth's Surface

Science, Grade 6


Table Of Contents: Earth's Surface

1. Weathering and Erosion Overview

2.1. Weathering and Erosion
Forces of weathering and erosion are constantly reshaping the Earth’s surface. Weathering is a group of natural processes that break rock into smaller pieces over time. Erosion occurs when rock and soil are transported.
2.2. Mechanical and Chemical Weathering
The two types of weathering are mechanical and chemical. Mechanical weathering is the physical decomposition of rocks. Chemical weathering is the decomposition of rocks by chemical reactions.
2.3. How Is Rock Eroded?
Rock can be eroded by many forces such as blowing wind, running water, ocean waves, flowing ice and gravity. These forces all contribute to sculpting the Earth’s landscape.

2. The Erosion and Deposition Cycle

3.1. Steps of the Erosion and Deposition Cycle
The cycle of erosion and deposition has many steps. First, weathering breaks down rock, and then erosion transports the material. Deposition occurs and sediments begin to accumulate. Over time these sediment layers lithify and become stone.
3.2. Sediment Deposited in the Ocean
Sometimes the erosion and deposition cycle is repeated. Sediment eventually makes it way from the high points of continents, down through rivers, and into oceans.

3. Mechanical and Chemical Weathering

4.1. Abrasion and Plant Growth
There are many examples of mechanical weathering. Abrasion occurs when one rock grinds against another. A rock can also break when plant roots grow into cracks on its surface.
4.2. Exfoliation and Frost Wedging
Exfoliation is a repeated cycle during which rocks expand in the daytime heat and contract at night, causing rocks to flake. Frost wedging occurs when cracks fill with water and undergo a repeated cycle of freezing and thawing, causing rocks to crack apart.
4.3. What is Chemical Weathering?
Chemical weathering involves the breaking down of rocks by chemical reactions. The three main chemical reactions that decompose rocks are acid reactions, oxidation and hydrolysis.
4.4. Natural Chemical Weathering
Acid-producing lichen and tree roots that eat through rock are natural sources of chemical weathering. Many minerals are also relatively unstable and deteriorate in the presence of water and natural chemicals. For example, the feldspar found in granite breaks down into clay.
4.5. Chemical Weathering and Pollution
Chemical weathering can be caused by pollution. Acid rain, created by the burning of fossil fuels, dissolves some types of rocks such as limestone.

4. Forces of Erosion and Deposition

5.1. Natural Forces of Erosion and Deposition
Many natural forces cause erosion and deposition, including gravity, moving water, glaciers, ocean waves and wind. These forces continuously wear down and build up material on the Earth’s surface.
5.2. Gravity and Mass Movements of Rock
Gravity can cause unstable rock material to move suddenly. Landslides and mudflows occur when loose soil and rocks slide down steep slopes. Slump is the sudden movement of a single large mass of rock material. Creep is caused by gravity, but is a slow downhill movement of sediment over time.
5.3. Moving Water Carries Sediment
Rivers and streams move across the Earth's surface and shape the landscape. Flowing water has enough energy to move large amounts of sediment, composed of soil, rock, clay and sand. The amount of sediment that is carried by a river or stream is called its load.
5.4. Deltas and Alluvial Fans
When a river empties into an ocean or lake, sediments are deposited and a triangular-shaped delta is formed. When water transports sediments from a hilly area to a flat area, the water slows down and sediment forms an alluvial fan.
5.5. Glaciers Shape the Land
Glaciers are massive sheets of ice that form over continents and in high altitude mountains.Glaciers move and shape the land by grinding, breaking and transporting rocks. Continental glaciers create flat landscapes while alpine glaciers create rugged, mountain features.
5.6. Glacier Landforms
Glacial erosion creates many landforms including mountain horns, cirques, aretes and U-shaped valleys. When a glacier melts, the deposited sediment left behind is called glacial drift. Moraines, drumlins and kettle lakes are landforms created by glacial drift.
5.7. Waves and Rock Formations
Ocean waves contain energy that breaks down rock and shapes coastlines. Rock formations created by wave erosion include headland cliffs, sea caves, sea arches and sea stacks.
5.8. Waves and Deposition
Waves also move and deposit rocks, sediment and sand. Beaches are composed of different sources of eroded rock. Barrier spits are created from longshore wave currents, and sandbars are built up offshore by incoming storm waves.
5.9. Wind Erosion and Deposition
Wind causes erosion through deflation, the blowing away of surface materials, and through abrasion, the grinding down of rock by blown particles. Wind also deposits sand into land formations such as sand dunes and loess deposits.

5. Pause and Interact

6.1. Review
Use the whiteboard tools to complete the activity.
6.2. Forces of Erosion and Deposition
Follow the onscreen instructions.

6. Soil

7.1. What Is Soil?
Soil is a combination of broken down rock and decomposed organic materials. Geologists study the soil profile, which is a cross section of the soil from the surface down to the bedrock. The soil profile is divided into layers called horizons.
7.2. Types of Soil
Soil is classified according to climate, plant vegetation and soil composition. Different types of soil are found in different climate biomes. Plant vegetation impacts the amount of organic material called humus that is found in the topsoil.
7.3. Climates with Thin Soil Layers
Tropical climates with lush vegetation often have a thin layer of topsoil because high rainfall washes away humus and minerals in the A horizon. Soil layers are also thin in harsh climates such as deserts and arctic regions.
7.4. Soil in Temperate Climates
Temperate climates have the most nutrient-rich, productive soils. Moderate rainfall results in abundant plant life, and the soil profile has a thick layer of topsoil with humus.
7.5. Life in the Soil
Many organisms live in the soil and contribute to its formation. Burrowing animals help create the soil profile by breaking up rock and other materials. Worms, fungi and bacteria decompose decaying matter.
7.6. Soil Conservation
Soil is a non-renewable resource that can be easily depleted or destroyed. Removal of cover crops like grasses and wildflowers leads to rapid soil erosion. Without cover plants, wind can blow away topsoil in clouds of dust, and significant flooding can occur.
7.7. Topsoil and Farming
Many modern farmers use techniques to conserve soil. Crop rotation prevents depletion of soil nutrients. Low-till plowing minimizes soil disturbance, and contour plowing reduces the erosion of topsoil from water runoff.

7. Pause and Interact

8.1. Review
Use the whiteboard tools to complete the activity.
8.2. Soil Profile
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

8. Landforms and Topographic Maps

9.1. Types of Landforms
Three major types of landforms are plains, plateaus and mountains. A plain is a large region of nearly flat or gently rolling land with little change in elevation. A plateau is a highly elevated flat region that often contains rivers and streams. Mountains are high elevation landforms with steep slopes.
9.2. What Is a Topographic Map?
A topographic map provides information about the surface features of a particular area. Topographic maps show natural features such as rivers and mountains as well as human-made features like buildings, roads and bridges. The symbols that represent map features are found in the legend.
9.3. Topographic Map Contour Lines
Contour lines on a topographic map connect points of equal elevation. An index contour is a darker, heavier line with a marked elevation. The contour interval is the difference in elevation between two contour lines. The relief is the vertical distance between the highest and lowest elevation points on the map.
9.4. Topographic Map Rules
There are some basic rules to remember when reading a topographic map. Contour lines never cross. When contour lines are spaced close together, the slope is steep. If they are spread apart, the slope is gentle. Contour lines that cross a valley or stream are V-shaped with the V pointing toward the higher elevation. Tops of hills or depressions are shown as closed circles.

9. Vocabulary Review

10.1. Earth's Surface Vocabulary Matching
In this virtual investigation you will practice reading topographic maps and making topographic profiles. Topographic maps represent a view of the landscape from above. You can use the information from a topographic map to create a cross-section view of a particular region of the map called a profile. A profile can help you better understand the details of a landform such as a mountain.

10. Virtual Investigation

11.1. Topographic Maps
In this virtual investigation you will practice reading topographic maps and making topographic profiles. Topographic maps represent a view of the landscape from above. You can use the information from a topographic map to create a cross-section view of a particular region of the map called a profile. A profile can help you better understand the details of a landform such as a mountain.

11. Assessment

12.1. Earth's Surface