Weathering Lectures 1.Weathering vs. Erosion 2.Joints: Setting the Stages 3.Physical (Mechanical) Weathering 4.Chemical Weathering 5.Products and Forms Made by Weathering 6.Weathering Landscapes
1. Weathering vs. Erosion Common Error Made in K-12 Earth Science Teaching: these are synonyms No! Weathering – chemically dissolving candy or physically crunching candy – breakdown in place Erosion – moving pieces (dissolved or as fragments)
1 st - Weathering = decay in place 2 nd - Detach = break off 3 rd Erode = move
2. Joints: Setting the Stage Joints – fractures (that allow water to penetrate and weather the rock) Not what you are thinking!
2. Joints: Setting the Stage Joints dependent on lithification (sedimentary & foliated metamorphic rocks) – where fractures occur along bedding planes or along foliations Joints independent of lithification (all rock types) – all other fractures
Joints dependent on Lithification Sedimentary Rocks – where fractures occur along bedding planes
Joints dependent on lithification Can also be Foliated Metamorphic Rocks – where fractures occur along bedding planes or along foliations
Joints dependent of lithification Rocks can be cracked by cooling, leading To columnar jointing
Columnar Jointing From contraction after lava flow cools
Joints independent of lithification Rocks can be cracked by Tectonic Stresses
Pressure Release Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage pressure release shells
Erosion removes overburden & shells pop off as pressure is released
Glacial Erosion Great Way to Generate Pressure Release
Salt Weathering Salt is common in deserts Salt is common along coasts
Mechanics of Salt Weathering Salt Crystal Growth: Extreme pressures in cracks and rock pores are caused by salt crystal growth from solution. There are varying causes of and extents to which salt growth occurs. Hydration: The hydration of various salts causes expansion & contraction, pushing apart the silicate host minerals Thermal Expansion: During temperature fluctuations, salts trapped in pores may expand to a greater degree than the surrounding rock minerals.
Caverns (tafoni) Base of rock notching (basal weathering) Wedging
More than roots can affect rock. Plants growing next to rock art can burn and wind movement can abrade
Imagery seen in this presentation is courtesy of ASU faculty, Paradise Valley and Mesa CC faculty, students and colleagues in other academic units, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, city, state governments, other countries government websites and U.S. government agencies such as NASA, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.