Weathering, Erosion, Deposition, and Landscapes © Lisa Michalek

Weathering The breakdown of rocks due to physical or chemical changes.

Physical Weathering Changes the size and/or shape of a rock without changing the chemical composition. In frost action, water seeps into cracks in rocks. The water expands as it freezes and makes cracks in the rock a little larger . When the ice melts and the liquid evaporates, the rock is left more porous. Over time, the freezing and melting in the cracks will cause the rock to crumble.

Physical Weathering Plant roots grow in rock crevices and gradually push the rock apart. Animals that burrow beneath the ground will expose new rock surfaces to weathering. Abrasion occurs when rock particles are carried in water, they bump and rub against other rocks.

Chemical Weathering When rocks are exposed to the atmosphere and hydrosphere, they often undergo changes in the chemical composition of rock, forming new substances. Usually requires heat and water to bring about chemical changes. Therefore, it takes place more rapidly in warm, moist climates.

Soil Formation Soil is a mixture of weathered rock and organic remains the usually covers bedrock. The weathering of soil produces horizon layers. The top layer is usually the best for growing crops because it is rich in organic remains called humus. The next layer is usually mineral enriched from groundwater transporting minerals. The lowest layer is composed of broken-up bedrock.

Soil Formation

Erosion of Sediments Sediments are rocks that have been broken into fragments. Erosion is the transporting of sediments away from their place of origin and depositing them elsewhere. Gravity, water, wind, and ice erode sediments.

Erosion by Water The smallest particles are carried in solution. The largest and most dense particles are rolled or bounced along the streambed. Particles of low density, such as organic remains, are carried along the surface by flotation.

Velocity of Streams The velocity of a stream is controlled by the slope and the amount of water flowing in the stream (discharge). As the stream gradient increases, so does the velocity of the water flowing in the stream. Velocity is also increased by an increase in the quantity of water in the stream.

Velocity of Streams There is a direct relationship between the velocity of a stream and the size of the particles it can transport. Large grains of sediment can only be eroded by fast-moving water.

Velocity of Streams Streams with broad, flat valleys often develop S-shaped curves called meanders. At the bends in the stream, the fastest-flowing water swings to the outside of the bends, causing erosion along the outer bank. The slowest moving water stays to the inside of the bends, causing deposition.

Erosion by Wind Wind can pick up loose rock materials, such as sand, silt, and clay, and carry them away. Wind erosion occurs mostly in dry areas, such as deserts and beaches. Sand blown by the wind can erode down material on a rock’s surface.

Erosion by Ice If more snow accumulates in the winter than melts in the summer, the snow on the bottom turns to ice. If it becomes thick enough, its weight will cause it to move under the pull of gravity. A glacier is a large mass of moving ice. As a glacier moves, it carries, pushes, and drags loose rock material.

Erosion by Ice The glacier, with pieces of rock, smoothes, striates (scratches), and grooves bedrock. As a glacier moves through a valley, it will make a U-shape instead of a V-shape caused by water. When the ice melts, unsorted rocks and boulders are left scattered around on hilltops and sides of valleys.

Deposition When an agent of erosion deposits, or lays down particles and fragments of earth materials (sediments). Also called sedimentation. Most deposition takes place in water.

Deposition Factors Particle Size Particle Shape Smaller particles (clay and silt) settle more slowly than cobbles and boulders. Particle Shape Friction between water and the surfaces of particles slows down settling. Smooth, rounded particles settle more slowly.

Deposition Factors Particle Density Settling Rate and Time Denser particles settle faster, less dense particles take more time to settle. Settling Rate and Time Sediments that settle at a faster rate require less time. As the rate of settling increases, the settling time decreases.

Sorting of Sediments Horizontal sorting occurs at the end of streams. The smaller sediments can be carried a greater distance from shore. Vertical sorting occurs when a landscape dumps a variety of particle sizes in to still water.

Deposition by Wind Usually sorts sediments by size. Cross bedding layers meet a different angles.

Deposition by Gravity At the base of a cliff, where pieces of weathered rock have fallen, you will find pieces of many different sizes. The sediments are not sorted.

Deposition by Glaciers Occurs when a glacier melts and sediments are released. Glacial erratics are large rocks that have been transported by glacial ice without being broken into small particles. They are often found high above stream valleys. Rounding and striations (scratches) indicate transport by glaciers.

Alpine Glaciers Also known as valley glaciers, occur in mountain regions, and may carve out U-shaped valleys with their moving ice.

Continental Glaciers Also known as ice sheets causes sculpting of the land as rocks are carried within the glacier or dragged under the flowing ice.

Glacial Features Glacial Polish – when glaciers act like sandpaper, grinding the jagged edges from the mountains and smoothing hard bedrock surfaces. Striation – parallel grooves and scratches in the bedrock left by the rocks in the glacier.

Glacial Features Drumlins – accumulation of rock and soil that builds up in front of the flowing ice. Moraines – where the ice front stops its southward advance, piles of unsorted soil and rock remain.

Glacial Features Kettles – Low spots in the glacial deposits and places where large, buried ice blocks melt leave these dry depressions and ponds called Kettle Lakes.

North American Continental Glaciation

The Oceans and Coasts Seawater covers almost 71% of our planet. The average depth is about 4 km (3 mi). One Liter of seawater contains about 3.5% of dissolved solids. Sodium chloride (table salt) is the most common. When evaporation of ocean water causes the salts to become too concentrated to stay in solution, they are precipitated as sediment.

The Oceans and Coasts The edges of the oceans are places of change caused by the action of waves and longshore currents. Beach sediments are rounded and reduced in size by abrasion as the energy in the breaking waves causes the particles to rub against one another.

The Oceans and Coasts Sand is often transported along the beach and just outside the breaking waves in the zone of longshore transport. This movement of sand builds the features of the ocean shorelines including sand bars, barrier islands, and sand spits.

Landscapes A region on Earth’s surface with physical features, such as hills, valleys, and streams. The shape (topography) and composition of the landscape is determined by the climate, bedrock, geologic structures, and human activities. Topographic relief is the change in elevation between the highest and the lowest places.

Landscape Regions Mountain landscapes have the greatest relief between the highest peaks and the deepest valleys. A great variety of rock types are common. Mountains are common where converging tectonic plates collide. Stream gradients are high, and the fast moving streams quickly erode deep valleys between the mountain peaks.

Landscape Regions Plateau landscapes are relatively flat or rolling uplands where streams have cut deep valleys. Commonly underlain by flat layers of sedimentary rock. Less topographic relief than mountains but more relief than the plains.

Landscape Regions Plains have the least topographic relief. They may contain a few small hills, but are generally flat and at low elevation. Commonly underlain by flat layers of sedimentary rock.

Climate Influence on Regions Moist, Humid climates usually have rounded landscapes. Slopes are not as steep because moist climates promote a protective cover of vegetation. Plants cover protects the soil from rapid runoff and erosion.

Climate Influence on Regions Arid (dry) climates usually produce thin soils with little humus. With little plant cover to protect the soil, sediment is carried away during rainfall. Large areas of exposed bedrock and steep rock faces are the result.

Landscape Regions of the US

Landscape Regions of NY

Drainage Patterns Harder rocks will form hills and ridges. Erosion of the weaker rock will make the major valleys. Streams will tend to follow zones of weaker rock and flow downhill.

Drainage Patterns

Human Activities Can Affect Landscapes Farming and construction projects can accelerate erosion and effect landscape development. Farmers and engineers must be guided in planning their projects by appropriate conservation practices.