Weathering Erosion and Soil Chapter 7 Section 1

Weathering, Erosion and Soil Chapter 7 Section 1

Mechanical Weathering: process in which materials on Earth’s surface break down and change. n Mechanical weathering: type of weathering n ¨ Rocks and minerals break down to smaller pieces ¨ Also called physical weathering

Effects of Temperature Water freezes and expands n Frost wedging: water seeps into cracks in rocks, freezes and expands n ¨ Makes the crack larger ¨ Causes holes in roads

Effect of Pressure n Roots of plants and trees exert pressure on rocks ¨ Roots grow and expand, making holes and cracks in rocks larger n Bedrock is formed under great pressure ¨ When pressure is reduced due to erosion, the exposed cracks in bedrock expand n Exfoliation: process where outer layers of rocks are stripped away

Chemical Weathering Chemical weathering: rocks and minerals undergo changes in their composition n Agents of chemical weathering n ¨ Water ¨ Oxygen ¨ Carbon dioxide ¨ Acid precipitation

Chemical Weathering (cont) n Composition of rock determines effect ¨ Calcite (made of calcium carbonate) decompose completely in acidic water ¨ Marble and Limestone (made of calcite) greatly effected by chemical weathering

Effect of Water n Important agent in chemical weathering ¨ Can dissolve many kinds of minerals and rocks ¨ Acts as a medium in which reactions occur ¨ Can react directly with minerals ¨ Hydrolysis: potassium feldspar reacts with carbonic acid and water to produce kaolinite and a solution of minerals

Effect of Oxygen Important element in chemical weathering n Oxidation: chemical reaction with oxygen and another substance n Oxidation: ferrous oxide and oxygen produce hematite (iron and water = oxidized iron) n

Effect of Carbon Dioxide n n n Occurs naturally in the atmosphere and is a product of living organisms Combines with water in the atmosphere to form a weak acid – carbonic acid Precipitation with carbonic acid- acid precipitation (acid rain) ¨ Rain ¨ Snow ¨ Sleet ¨ fog

Effect of Carbon Dioxide (cont) Carbonic acid slowly reacts with minerals such as calcite in limestone and marble to dissolve rocks. n Long term effects- cracks n Carbonic acid: H 2 O + CO 2 = H 2 OCO 3 n

Effect of Acid Precipitation Acid precipitation caused by sulfur dioxide, carbon dioxide, nitrogen oxides n Released into atmosphere by human actions n ¨ Burning n fossil fuels Strong acids harmful to plants

Rate of Weathering n Chemical ¨ Rapid in warm wet climates with lush vegetation ¨ Greatest effects along equator ¨ Very slow in dry hot climates- little rain for acid precipitation n Physical/Mechanical ¨ Breaks down rocks faster in colder climates ¨ More frost wedges

Rate of Weathering (cont) n Rock type and composition ¨ Not all rocks weather at same rate ¨ Calcite rocks (limestone, marble) easily weathered ¨ Quartz rocks (granite, quartzite) less easily weathered

Rate of Weathering n Surface Area ¨ Mechanical weathering breaks rocks to smaller pieces ¨ Have more surface area ¨ More area to weather n Topography ¨ Slope determines rate ¨ Steep slope weathers faster ¨ Due to gravity