Surface Processes and Landscapes Weathering

Weathering and Erosion Weathering is the chemical and physical breakdown of rocks into smaller fragments. Erosion is the movement of weathered material from one place to another.

Weathering Types The two general types of weathering are physical and chemical. Physical weathering occurs when rock is cracked, split, or broken into sediments with no change in the rock composition. Chemical weathering occurs when rock is broken down by chemical action resulting in a change in the composition of the rock.

Types of Physical Weathering Frost action – caused by alternate freezing and thawing of water. Abrasion – caused as rocks rub or bounce against each other. Exfoliation is the peeling away of large sheets of loosened surface material. Roots from large trees and shrubs as well as roots from lichens and mosses will split rocks as they grow.

Types of Chemical Weathering Oxidation occurs when oxygen combines chemically with minerals. Ex. is rust. Hydration occurs when water combines chemically with minerals. Ex. is when feldspar or hornblende combine with water and form clay. Carbonation occurs when carbon dioxide combines with minerals. Example is carbonic acid which dissolves limestone creating caves, sink holes, stalactites and stalagmites. Organic Decay occurs when acids from decaying organic matter dissolve rock minerals.

Soils Soil is one of the major products of weathering. Soil is the part of the weathered regolith in which rooted plants will grow. Soil is composed of rock minerals and organic matter and consists of all 3 states of matter.

Soil Formation Soil types depend upon their place of origin. Soil formed by the weathering of the bedrock found beneath it is called residual soil. Soil formed from rock that was moved to its present location is called transported soil.

Soil Profile As soils mature they grow richer and develop a series of layers called a profile. Topsoil is the uppermost layer containing the most organic matter and the most weathered rocks. Subsoil is below the topsoil and is rich in clay-sized particles but contains little organic matter. Below the subsoil is partly weathered bedrock on top of unweathered bedrock.

Soil Solution A soil solution is the end product of weathering; a solution of ground water containing ions of potassium, nitrate, phosphate, calcium and iron that the water picked up as it seeps through soil.

Soil Conservation Due to the large demands upon earth’s farmable land, use of soil must be carefully planned and managed. Depletion occurs when too many nutrients are removed form the soil for crops to grow. Crop rotation, contour plowing, strip cropping, terracing, and wind breaks are all farming practices designed to preserve soil.

Erosion Erosion is the transportation of loose sediments or rocks produced by weathering. The major mechanisms of erosion are: Gravity Running water Glaciers Wind and Waves

Types of Sediment Transported sediment is rock material that has been moved to another location. Transported materials will become abraded as they grind and bump against other rock material. These materials begin to appear rounded as they are abraded. Residual sediment is rock material that has weathered and remained in its place or origin.

Running Water and Erosion Any running water makes a stream. Gradient, discharge, and channel shape influence a stream’s velocity and the erosion and deposition of sediments. Stream features include: v-shaped valleys, deltas, flood plains and meanders. A watershed area is the area drained by a stream and it’s tributaries.

Stream Velocity Carrying power is the ability of a stream to move particles of different sizes and this depends upon velocity. Velocity (speed) of a stream depends upon stream discharge and gradient. Discharge is the volume of water. Gradient is the steepness of the slope.

Stream Velocity and Gradient

Velocity and Channel Shape

Velocity and Sediment Size Greater stream velocity will carry larger sediments. Stream load is the material that a stream carries.

Stream Load Load is the material that a stream carries. 3 ways to carry load: Solution = disssolved Suspension = floating Saltation = bouncing

Wind and Ice Erosion Wind: greater velocity carries larger particles. Ice: greater volume carries larger particles.

Glacial Erosion

Effects of Erosional Agents: Valley Shapes River erosion creates “V-Shaped” valleys. Glacial erosion creates “U-Shaped” valleys.

Deposition Deposition is the final step in an erosional-depositional system. Factors affecting deposition are: Particle size, shape, density Velocity gradient Rock particles are eventually deposited somewhere: by sedimentation = dropping of particles by precipitation = dissolved minerals “fall” out of solution.

Horizontal Sorting As a stream or river enters a larger body of water, the velocity decreases as distance from the mouth increases. As a result the biggest particles fall out first and the smallest travel the farthest.

Crossbedding

Glacial Deposition Glacial deposits are unsorted (all mixed with no layering or bedding) and deposits are mostly 2 types: Till is unsorted rock deposited directly by the glacier; Outwash is rock material deposited by the meltwater of a glacier. Outwash can show horizontal sorting. Erratics are large boulders deposited by glaciers.

Erosional – Depositional System An erosional-depositional system combines the erosion process, transportation agents, and deposition. A river is a combination of continual erosion and deposition.

River Terms Source = beginning w/most potential energy. Mouth = end, where it flows downstream to. Meander = curve or bend. Flood plain = land between the stream and steep walls of the valley. Delta = deposit of sediment at the mouth of a river. Alluvial fan = deposit of sediment formed where river slows as it flows out of mountains.

Flood Plain (above) and stream history, oxbow, (left)