Weathering, Erosion, and Soils. Weathering and Erosion. Weathering and Erosion

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1 Weathering, Erosion, and Soils 1 The Grand Canyon, a landscape shaped by weathering and erosion 2 Weathering vs. erosion Types of weathering Physical Chemical Rates of weathering and erosion Climate Rock composition Other factors Soils a product of weathering S. Kuehn 3 4 Weathering - includes all processes which break down rocks at the earth s surface Physical (mechanical) weathering - there are two general types of weathering: physical py (mechanical) weathering chemical weathering - breaks rocks into smaller pieces Examples: Erosion - includes processes which remove weathered material from its source - water, wind, and ice, for example, can erode weathered material abrasion by particles carried by wind, water, or ice fractures created or widened by changes in pressure or temperature or by root action 5 6 Abrasion by glaciers Glaciers typically have many fragments of rock embedded in the ice. When the ice slides along, these rock fragments grind against the bedrock beneath the glacier. This wears down underlying rock and creates smoothed and grooved surfaces. S. Kuehn Abrasion by flowing water Water typically carries sand and gravel. These particles bump into each other and larger rocks wearing them down over time. 1

2 Mechanical weathering caused by growing roots Plant roots can grow into tiny fractures in rocks. As the roots grow over time, the fractures become wider. Fig 6.12 Understanding Earth 7 Mechanical weathering by frost action Freezing and thawing widens fractures over time. Pieces can then fall off the cliff and move down the slope. Mount Rainier andesite lava flow at Burroughs Mountain, Mt. Rainier National Park S. Kuehn Animation: Frost Wedging 8 9 Part of the Sierra-Nevada batholith: Intrusive igneous rocks in Yosemite National Park 10 Mechanical weathering by exfoliation (sheeting) Exfoliation occurs as rocks expand when brought to the surface from deep in the crust where pressures are much higher than at the surface. Photo: exfoliating granite in Yosemite National Park - Fig 6.14 Understanding Earth Animation: Uplift and erosion of a volcanic arc Chemical weathering - breakdown of minerals by chemical reactions to form dissolved material and/or other minerals such as clay Examples: Dissolution - dissolves in water Oxidation - oxygen is added Hydration - water is added The brown stain on the surface of this sandstone is called desert varnish. It is produced by chemical weathering and evaporation. S. Kuehn 2

3 Rates of 13 Understanding Earth 14 type of mineral or rock surface area climate acids time Understanding Earth Time Different rates of weathering of slate and marble. Marble is composed of calcite which dissolves easily in acidic water. The faster weathering of the marble headstone is apparent in the blurring of the text. Fig 6.1 Understanding Earth Weathering along natural fractures in rocks Photos of weathered granite at Yosemite National Park Spheroidal weathering of a basaltic lava flow in Oregon Chemical weathering of these rocks began along the fractures Photos by S. Kuehn 3

4 Weathering to produce soils In the example to the left, the soil was produced by weathering of bedrock with little erosion. Soils also may be formed in sediment deposits such as silt on a river floodplain or dune sand. The type of soil formed depends on the source material, the climate, and how much time has passed since the soil began to form. Fig 6.12 Understanding Earth 19 Importance of Soil Soil directly or indirectly plays a critical role in supporting life on land. Soils serve as an interface between the biosphere, hydrosphere, atmosphere, and solid Earth. They affect and are effected by life, air, and water. Soils help regulate the flow of water and dissolved material on land and help filter groundwater. Soils store and cycle nutrients. Soils support trees, buildings, bridges, etc. Most weathering occurs in or near the soil zone. Soils are strongly affected by human activities over very large areas (e.g. farming, urbanization). People depend on soils for growing food, waste disposal, etc Factors in soil formation Factors in soil formation 1. Starting (parent) material 4. Topography 2. Climate/weathering 5. Time 3. Biological activity Soil Forming Factors: The Story of Rocks and Soil Describing and Classifying Soils 23 Some Soil Profiles 24 Soil scientists classify soils into 12 soil orders, 64 sub-orders, and other sub-categories on the basis of physical, chemical, and biological properties including: Color Texture (e.g. sand, silt) Structure (e.g. blocky, platy) Roots Pores Composition (organic material, clay, carbonates, salts, volcanic glass, etc.) Climate (temperature, moisture) Soils are composed primarily of solids (mostly minerals and organic matter) plus some liquid (mostly water) and gases a soil formed in siltstone an Entisol formed in sandy sediments Source and Oxisol 4

5 Generalized (very!) Global Soil Map 25 Online Resources 26 Soil Formation and Classification (USDA) Descriptions and U.S. distribution maps for major soil orders Understanding Soil Risks and Hazards Soil education section of NASA s GLOBE program Smithsonian Soils Exhibit 5