Weathering and erosion

Goals To understand how rocks are broken down at the Earth’s surface and how the broken-down materials are removed (weathering and erosion).

Weathering: Rocks are weakened and broken down into smaller peaces Erosion: Weathered pieces of rock are carried away to be deposited somewhere else.

Weathering and erosion help shape our landscape and provide the basic inorganic components of soil from which we get most of our food.

Weathering Two end-member types, physical weathering and chemical weathering Physical weathering: Rocks are broken down by mechanical processes Chemical weathering: Rocks are broken down by chemical reactions at the Earth’s surface

Physical weathering Frost wedging: Water works its way into cracks in rocks, freezes and expands –physically wedges the cracks apart— analogous to freezing a can of soda pop

Physical weathering Thermal expansion: Rocks expand and contract due to daily and seasonal temperature fluctuations or fires –Differential expansion and contraction gradually breaks them apart No puppies were harmed in this fire

Physical weathering Mechanical exfoliation: Rocks equilibrated deep in the crust expand as they approach the Earth’s surface –Pop apart along discrete planes called joints Devil’s Tower, Wyo. Half Dome, Cal.

Physical weathering Abrasion: Water and wind-blown particles scour away bedrock

Chemical weathering Dissolution: Ionic bonds are broken in water Dissolving calcite is chemical dissolution Water’s dipolar shape helps pull ionic bonds apart Weathering of Calcite H 2 O + CO 2 ↔ H 2 CO 3 CaCO 3 + H 2 CO 3 ↔ Ca HCO 3 - Water molecule

Chemical weathering Hydrolysis: H + or OH - ions replace cations or anions in a mineral’s crystal structure Weathering of potassium feldspar to kaolinite is common example.

Climate and chemical weathering Chemical weathering favored by: 1.High rainfall and humidity 2.Warm temperatures

Erosion Puppies can be important agents of erosion

Think-pair-share activity Q—Which do you think is more important in shaping the Earth’s surface, wind erosion or water erosion? Can you think of an example of a feature of the Earth’s surface shaped by each? 1.Answer the question 2.Compare, discuss, and refine your answer with one or two of your neighbors. 3.Write up your results and pass them in.

The Grand Canyon

Big Hollow, Wyo.

Water erosion Water erosion: Particles are washed away –Does most of the work of shaping the Earth’s surface... even in arid areas. –Glaciers are more effective, but are less common

Wind erosion Wind erosion: Particles are blown away. –Most important in arid environments –Led to the “dust bowl” of the 30’s.

Soil formation Weathering produces layer of loose rock over the Earth called regolith Organic-rich upper few centimeters to few meters of regolith that supports plant life is called soil Bedrock Regolith Soil

Factors that influence soil formation Topography: Steep topography hastens erosion Soil formation is a competition between weathering and erosion.

Factors that influence soil formation Climate: Warm, moist climates speed regolith formation and accumulation of organic material Hot, wet = thick soil Cold, dry = thin soil

Factors that influence soil formation Composition of material from which regolith is derived Plants don’t like soil from ultramafic rocks