Earth Science Notes Weathering and Soil

Weathering Weathering : surface processes that work to breakdown rock into sediment Types of sediment – sand, silt, clay

Types of Weathering

Types of Weathering Mechanical Weathering - the physical break down of rocks The chemical make up of the rock remains unchanged. Ex: burrowing animals, growing plants, expanding ice Ice wedging – water will enter cracks in rock and then freeze pushing the rock apart. This increases surface area of rocks and allows them to be exposed more to chemical agents.

Mechanical Weathering increases Chemical Weathering

Mechanical Weathering

Types of Weathering Chemical Weathering – chemical reactions dissolve rocks Agents of chemical weathering Carbonic acid (H2CO3) – acid produced from carbon dioxide in air and soil. Dissolves limestone. Oxygen – reacts with iron containing minerals to form rust. Gives rocks and soil its reddish color. Sulfuric acid (H2SO4) – acid produced by sulfates from factories. ACID RAIN.

Chemical Weathering

BIG IDEA: Weathering creates SOIL!!!

SOIL Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air.

Weathering and Soil Formation Recall from the last Unit: weathering produces sediments. Below are the classifications of sediments Clay  silt  sand  pebble  Cobble  Boulder Sand, silt, and clay are the main sediments that make up soil. These sediments give soil its TEXTURE.

Weathering and Soil Formation The relative proportions of sand, silt, and clay allow us to classify soils. The USDA has a model it uses for classifying soil types based on the sediment composition (the triangle chart – next slide). These soil classifications allow tell us how a soil will accept and retain moisture. Sandy – doesn’t retain water Clay – doesn’t accept water

Weathering and Soil Formation

Soil Formation The weathering of Rock into soil occurs in stages (seen below)

Soil Profiles and Soil Horizons Over time (and much weathering) distinct layers form in the soil. These layers are called soil horizons.

Soil Profiles and Soil Horizons The first soil horizon to form is the C horizon. Again, weathering process convert the exposed bedrock to smaller sediments.

Soil Profiles and Soil Horizons After the C horizon, the A horizon is the next to form. Weathering continues. Plants begin to colonize the sediments and die. Their remains contribute to a growing layer rich in organic matter. This is the A horizon.

Soil Formation Plants and Animals play a part in soil formation

Soil Profiles and Soil Horizons The last layer to form is actually the B horizon. It forms as a result of minerals leaching downward from the A horizon to the C horizon.

Leaching Removal of minerals by water

Soil Profiles and Soil Horizons A soil profile is a cross-section of earth that shows the different soil horizons.

Soil Formation Soils vary greatly in composition. In addition to sediment size (i.e. sand, silt, clay), the parent materials from which these soils come are very diverse.

Soil Formation is influenced by vegetation and climate.

Topography and Soil Formation Soil Formation is influenced by Topography Steeper land  influenced by force of gravity Flatter land  water is more apt to settle

Ways to Reduce Soil Erosion Terracing / No-till / Contour farming

Factors Affecting Soil Erosion Exposure to elements (weathering) Wind, rain, freezing, frost, etc. Type of Parent material Weight of material, Density, etc. Farming Overgrazing / tilling Foresting and mining Climate Topography

Factors Affecting Soil Erosion

Factors Affecting Soil Erosion

Weathering and Soil Summary What is weathering? Types of Weathering Mechanical and Chemical Weathering Creates Soil Soil Formation Factors Affecting Soil Formation Soil Profiles Factors Affecting Soil Erosion