Chapter 7: Weathering and Erosion © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Learning Objectives Weathering—The first step in the rock cycle Contrast chemical and mechanical weathering and their place in the rock cycle. Products of weathering Describe the end products of weathering and the difference between soil horizons and soil profiles. Erosion and mass wasting Identify the sources of erosion and mass wasting. © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Figure 7.2 How rocks disintegrate © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle How Rocks Disintegrate The chemical and physical breakdown of rock exposed to air, moisture, and living organisms Regolith A loose layer of fragments that covers much of Earth’s surface Soil The uppermost layer of regolith, which can support rooted plants © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Mechanical weathering The breakdown of rock into solid fragments by physical processes Chemical composition of rock NOT altered Chemical weathering The decomposition of rocks and minerals by chemical and biochemical reactions Figure 7.3 From rock to soil © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Joint A fracture of rock, along which no appreciable movement has occurred Sheet jointing (exfoliation) Frost wedging Root wedging Abrasion Wearing down of bedrock by the constant battering of loose particles transported by wind, water or ice Joint Formation © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Joint Formation © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Figure 7.4b Frost wedging Figure 7.4a Sheet jointing © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Figure 7.4c Scree slope Figure 7.4d Root wedging © 2012 John Wiley & Sons, Inc. All rights reserved.

Weathering—The First Step in the Rock Cycle Chemical Weathering Dissolution Separation of materials into ions in solution by a solvent, such as water or acid Rainwater Acts as weak solution of carbonic acid Anthropogenic actions (acid rain) Figure 7.5 (a) marble, (b) granite © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Chemical Weathering Figure 7.6 Ion exchange © 2012 John Wiley & Sons, Inc. All rights reserved.

Chemical Weathering Figure 7.7 Hydrolysis and oxidation © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Tectonic setting Young, rising mountains weather quicker Mechanical weathering most common Figure 7.8 Tectonic setting © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Rock structure Distribution of joints influence rate of weathering Relatively close joints weather faster Figure 7.8 Rock structure © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Topography Weathering occurs faster on steeper slopes Rockslides Figure 7.8 Topography © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Biologic activity Presence of bacteria can increase breakdown of rock Figure 7.8 Biological activity © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Rock composition Minerals weather at different rates: Calcite weathers quickly through dissolution. Quartz is very resistant to chemical and mechanical weathering. Figure 7.8 Composition © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Vegetation Contributes to mechanical and chemical weathering. Promotes weathering due to increased water retention. Vegetation removal increases soil loss. Figure 7.8 Vegetation © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Climate Chemical weathering is more prevalent in warm, wet tropical climates. Mechanical weathering is less important. Mechanical weathering is more prevalent in cold, dry regions. Chemical weathering occurs slowly. Figure 7.9a Climate and weathering © 2012 John Wiley & Sons, Inc. All rights reserved.

Factors Affecting Weathering Figure 7.9 Color corresponds to climates in Figure 7.9b © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Clay Tiny mineral particles of any kind that have physical properties like those of the clay minerals A family of hydrous alumino-silicate minerals Figure 7.10 Kaolin: from (a) clay to (b) fine china © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Sand A sediment made of relatively coarse mineral grains Soil Mixture of minerals with different grain sizes, along with some materials of biologic origin Humus Partially decayed organic matter in soil Figure 7.11a Lunar regolith Figure 7.11b Earth soil © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Soil Profiles Soil horizons One of a succession of zones or layers within a soil profile. Each horizon has a distinct physical, chemical, and biologic characteristic. Soil profiles The sequence of soil horizons from the surface down to the underlying bedrock. Soil profiles vary, influenced by factors such as climate, topography, and rock type. © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Figure 7.12 Soil profile containing soil horizons (zones or layers) © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Figure 7.13 Climate and soils © 2012 John Wiley & Sons, Inc. All rights reserved.

Products of Weathering Figure 7.13 Comparison of soil from Kansas (l) and Connecticut (r) © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Erosion by Water The wearing away of bedrock and transport of loosened particles by a fluid, such as water Bed load Sediment moved along the bottom of a stream Saltation A transportation mechanism in which particles move forward in a series of short jumps along arc-shaped paths Suspended load Sediments carried in suspension by a flowing stream of water or wind © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Figure 7.14a Stream’s load © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Erosion by Wind Saltation is primary mechanism for wind erosion. Figure 7.15 Massive dust storm © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Erosion by Ice Glacier A semi-permanent or perennially frozen body of ice, consisting of recrystallized snow, that moves under the pull of gravity Figure 7.16b Polished and grooved rock from glaciers Figure 7.16a Debris from glaciers © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Figure 7.16c Two glaciers merge © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Gravity and Mass Wasting The downslope movement of regolith and/or bedrock masses due to the pull of gravity Slope failure Falling, slumping, or sliding of relatively coherent masses of rock Three basic types: Fall: vertical or near vertical drop of rock fragments Slide: rapid displacement of rock/regolith down steep or slippery slope Slump: rotational movement of material © 2012 John Wiley & Sons, Inc. All rights reserved.

Mass Wasting caused for very few reasons: *Weight, *Water, *Slope and *Geology © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Figure 7.17a Repeated rockfalls Figure 7.17b rockslide © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Figure 7.17c Slump © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Flows Any mass-wasting process that involves a flowing motion of regolith containing water and/or air within its pores Slurry flows Granular flow Creep The imperceptibly slow downslope granular flow of regolith © 2012 John Wiley & Sons, Inc. All rights reserved.

Erosion and Mass Wasting Figure 7.18 Flows © 2012 John Wiley & Sons, Inc. All rights reserved.

Factor of Safety and Landslide Prediction Factor of safety (FS) The balance between destabilizing forces (shear stress) and stabilizing forces (shear strength) Tectonics and mass wasting Major historic landslides clustered near converging lithospheric plates Mountains/earthquakes Figure 7.19 Earthquakes can cause mass wasting events; Alaska 1964 © 2012 John Wiley & Sons, Inc. All rights reserved.

© 2012 John Wiley & Sons, Inc. All rights reserved. Critical Thinking On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why? Why are some granite bodies extensively jointed, while others are essentially joint free? Youtube mass wasting video… © 2012 John Wiley & Sons, Inc. All rights reserved.