Weathering and Erosion Chapter 11

Introduction Materials on the earth are constantly changing. Examples: Rock and Soil. Sometimes we notice these changes. Most of these changes go unnoticed because they happen gradually. Example: It took millions of years for wind and water to shape the grand canyon.

Weathering Lesson 11.1

Weathering The breaking down of rocks on the earth’s surface is known as weathering. Weathering occurs when rocks are exposed to air, water, or living things. Example: Think about the different colors, shapes, etc. of various gravestones in a graveyard.

Mechanical Weathering Mechanical weathering is when rocks break into smaller pieces but their chemical makeup stays the same. Water can cause mechanical weathering. Water expands as it freezes. As water freezes in the cracks in a rock it pushes the rock apart. This process may continue repeatedly until the rock breaks apart. A tree may causes mechanical weathering. The tree grows in bits of soil in the rock. Over time the roots of the tree extend/grow and break the rock apart.

Mechanical Weathering

Chemical Weathering In chemical weathering changes occur in the chemical makeup of rocks. New minerals might be added or taken away from the rock. The minerals might be changed into new substances. Example: Oxidation—process where oxygen from the air or water combines with the iron in rocks. This process results in a new softer substance called iron oxide (rust) Iron oxide stains rocks various shades of yellow, orange, red, or brown.

Chemical Weathering

Chemical Weathering Chemical weathering also occurs when water changes minerals in the rocks. The mineral feldspar is part of many rocks. Water changes feldspar to clay and washes it away. Without the feldspar to hold other minerals together the rock falls apart. Limestone caves are the result of chemical weathering. Rain and groundwater combine with carbon dioxide in the air to form carbonic acid. As carbonic acid trickles through the ground it dissolves calcite. As more and more limestone is dissolved small holes become large caves.

Chemical Weathering

How Soil Forms When a rock has weathered for a long time, soil may develop. Soil is a mixture of tiny pieces of weathered rock and the remains of plants and animals. The makeup of soil depends on the types of rock particles and remains that are found in it.

How Soil Forms As Soil develops, it forms layers. Fully developed soil has three layers. Topsoil—this layer has the greatest amount of oxygen and decayed organic matter. The organic matter helps the soil hold moisture. Subsoil—contains minerals that were washed down from the topsoil. Many of these minerals are forms of iron oxide giving the soil a yellowish or reddish color. Plants grow into the subsoil to get minerals and water. The last layer contains chunks of partially layered rock. Underneath this layer is solid rock

Soil Layers

Erosion caused by Water Lesson 11.2

Erosion After a rock is loosened by weathering, it is worn away and moved to another place. The wearing away and moving of weathered rock and soil is called erosion. The main causes of erosion are rivers, waves, glaciers, wind, and gravity.

River Erosion Water running downhill is a powerful force. Rivers change more of the landscape than any other agent of erosion. After rain falls to the earth, the water flows downhill. The water pushes soil and rock fragments as it moves (These solid particles are sediment). The water and sediment flow into small gullies, which lead to rivers.

River Erosion As water flows into a river, it erodes the banks and the riverbed (Think of a hose). Sand and stone in the river scrape against the banks and the riverbed, causing more erosion.

River Deposits Sediment carried by the agents of erosion is eventually dropped in a process called deposition. When a river slows down it may drop (deposit) its sediment. Heavy particles, such as stones, drop out first. As the river flows down further, lighter sediment, such as sand and clay, drop out.

River Erosion

River Deposits A river slows down considerably as it empties into a lake or ocean. The place where a river flows into a larger body of water is called the mouth of the river. Sediment settles out of the river’s mouth. Eventually , the sediment builds up above the water level and forms a fan-shaped area of land called a delta. Rivers usually branch off as winds through the delta and empties. Deltas provide rich farmland.

River Delta

Wave Erosion Waves in an ocean or a large lake change the shoreline through erosion and deposition. As waves pound the shoreline, they hurl not only water but also bits of rock and sand against the coast. These materials chip away at the rocky shore. Waves also force water into cracks in rocks along the shoreline. With each wave, water presses against the sides if the cracks. The cracks get bigger, and pieces of rock split off.

Wave Erosion This type of erosion forms the cliffs, towers, and other rocky shapes. During storms, waves reach higher on cliffs and carve steep sides. Arches form when waves erode through a cliff. If the top of an arch collapses, a tower of rock called a sea stack is left standing.

Wave Erosion

Wave Deposits Beaches are areas where waves have deposited sand, pebbles, or shells. Some of this beach material is sediment from nearby eroded rocks. Other beach material is sediment carried to the lake or ocean by rivers. Currents near the shore carry sediment to different parts of the shoreline. As waves break on shore, the sediment is pushed onto the beach.

Wave Deposits Currents along the shore can change the shape of the beach. One result is a spit, or curved finger of sand, sticking out into the water. Waves and currents can also carry sand away from the beach and deposit a long, underwater sandbar offshore.

Erosion caused by Glaciers Lesson 11.3

Erosion caused by Glaciers In cold climates water falls as snow. This snow builds up into thick layers. If the snow does not melt, increasing pressure causes the snow to form solid ice. Year after year, more ice builds up. This may form a glacier. A glacier is a thick mass of ice that covers a large area. Glaciers may be as small as a football field or hundreds of miles long. The weight of snow and ice and the pull of gravity cause a glacier to move slowly downhill.

Erosion caused by Glaciers As glaciers move, they pick up loose sediment. Because of their size, glaciers move huge boulders and soil. These materials freeze onto the bottom and sides of the glacier. They act like grinding and cutting tools as the glacier continues to move. Small rocks in a glacier act like sandpaper, smoothing and shaping the land beneath.

Deposition by Glaciers Glaciers continue to move downhill until they reach temperatures warm enough to melt. As the ice melts, it deposits sediment. The sediment forms ridges called moraines. Moraines are the “footprints” or evidence of glaciers.

Erosion caused by Wind and Gravity Lesson 11.4

Wind Erosion and Deposits Wind is another cause of erosion. Wind picks up and carries materials from one place to another. Wind also erodes by blowing sand against rock (think sandblaster). Much rock in desert areas are pitted with tiny holes from windblown sands.

Wind Erosion and Deposits Sand Dunes are examples of wind deposits. These mounds are formed when wind blows sand from one place to another. Wind may bounce sand along the ground until it hits an obstacle. A small sand pile forms behind the rock. The pile blocks other sand grains, and a larger mound forms. The mound continues to grow, forming a sand dune. The dune moves as wind blows sand up the gentle slope and deposits it on the steeper back slope.

Sand Dunes

The Role of Gravity in Erosion Gravity plays a part in all erosion. Rivers and glaciers flow because of gravity. Gravity can only move material that has been loosened in some way. Rocks and soil are loosened by freezing, thawing, and heavy rains. A great deal of water makes soil smooth and slippery. Soil in this condition flows easily.

The Role of Gravity in Erosion When material on a hillside is loosened, gravity can cause rapid erosion. Mudflows and landslides are examples of this. Gravity works slowly. We often only witness the final descent of a long process.

Landslide