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Rocks exposed at the Earth’s surface are constantly being eliminated: physically and chemically destroyed and transported elsewhere. These are the processes of weathering and erosion.
Exfoliation is the natural formation of fractures in rocks (mainly igneous plutons) due to the release of pressure upon reaching the surface.
The classic Sierra Nevada (CA) granite mountains photographed by Ansel Adams are examples of exfoliation domes.
Exfoliation can operate on very large scales. It is most important in areas of exposed igneous rocks, even though these rocks generally lack a layering or similar planar features.
The volume change as liquid water transforms to ice is an important physical weathering force in temperate climates.
The action of growing plants also promotes physical weathering.
A birch tree exploited a crack in this boulder in Maine and, together with freeze-thaw, has split it in half.
Chemical weathering is the decomposition of rocks and minerals as a result of chemical and biochemical reactions. As in all reactions, unstable minerals react to form a set of products more stable at near-surface conditions. So, broadly speaking, chemical weathering is the process whereby unstable high temperature/ pressure minerals are transformed into their Earth-surface-stable relatives.
The order in Bowen’s reaction series of minerals to crystallize from cooling magma is the same sequence when considering mineral stability at Earth surface conditions: olivine Ca-rich plagioclase feldspar pyroxene amphibole Na-rich plagioclase feldspar biotite (mica) K-feldspar muscovite (mica) quartz
Rocks chemically weather according to the stability of their minerals, the severity of their environment and the time available. For example, a granite (quartz, feldspars and mica) typically chemically weathers to a mixture of quartz and clay minerals.
When weathering begins along a perpendicular network of fractures, it will frequently weather out resistant rock in spherical shapes.
If there is more exposed surface, then more chemical weathering can take place, since it is on surfaces that chemical weather concentrates.
Organisms are important agents of weathering, physical and chemical. Imagine how weathering on Earth would be different without life.
Insoluble materials remain in the A horizon: Fe-oxides, Al-rich minerals, and organic compounds.
In arid climates, groundwater flow may be principally from the bottom up, as water is drawn to the surface and evaporates. This reverses the profile we see in humid climates, such that hard, calcite-cemented material dominates the upper portion of the profile.
Why is weathering enhanced in the temperate regions, even though temperature is lower than the tropics?
Some images in this presentation come from: Hamblin and Christiansen, Earth’s Dynamic Systems, 8/e AJ Kaufman (Univ. Maryland) GS Solar (SUNY College Buffalo) Brodo, Sharnoff and Sharnoff, Lichens of North America Plummer, Mc. Geary and Carlson, Physical Geology (8 th ed. ) Carlsbad Cavern National Park Press and Siever, Understanding Earth (3 rd ed. ) Marshak, Earth: Portrait of a Planet (1 st ed. )