Weathering The breakdown of rocks due to physical or chemical changes

WeatheringThe 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 • 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

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 • 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 • 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 • The glacier, with pieces of rock, smooths, 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 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

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

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 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 • 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

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 • 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

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

Weathering The breakdown of rocks due to physical or chemical changes