Reading: Boggs Chapter 1
We identify two main types of weathering: chemical and physical. Today we focus on chemical weathering processes, reactions, and products. Minerals in igneous and metamorphic rocks form at high P and T deep in the crust (or mantle), and therefore are unstable at the Earth's surface. The relative stability of minerals at the Earth's surface is the inverse of Bowen's Reaction series. Virtually all sand and clay is produced by chemical weathering. Most chemical weathering takes place in soils. Thus, we need to keep in mind that most clastic sediment on Earth (which later becomes sedimentary rock) is the by-product of chemical weathering that attacks bedrock as it passes through the filter we call "soil". As Sam Boggs says, soils - and the sediments that come from them - are the "survival assemblages" of chemical weathering.
1. The Major Chemical Weathering Processes are:
A. Hydrolysis
B. Dissolution
C. Oxidation
You should know how each of these works, what are the reactants and products, and be able to cite one or two examples of each. Good examples are provided in Tables in the text, and the handout.
2. Overall Rate and Intensity of Chemical Weathering depends on:
A. Climate (rainfall and temperature)
B. Mineral composition of the rock
C. Availability and supply of components
D. Grain Size (ratio of surface area:volume increases with smaller grain size)
E. Tectonics: Rapid tectonic uplift of the crust results in rapid erosion and
limited chemical weathering - this is true even in a humid tropical climate
if the uplift and erosion are very rapid. Slow uplift results in slow erosion
and allows time for chemical weathering to take place.
Understand: (1) how varying each of these would affect the rate, intensity, and "degree of completion" we would expect for chemical weathering in different settings and conditions, and (2) consider the relative stability of different minerals and use this to predict the composition of soils and sands in different types of weathering environments.
3. Products of Chemical Weathering:
Soils contain the "survival assemblages" of weathering: solid materials that are not destroyed by chemical weathering. We did not discuss different soil types in any detail, just mentioned that they exist.
4. Example from the Pennsylvanian:
A classic study of a Pennsylvanian paleosol by Wahlstrom (1948). The handout shows vertical changes in mineral and chemical composition within the ancient soil profile. These trends reflect the variable chemical stability of different minerals in the weathering environment, and degree of alteration as a function of depth in the soil profile, as discussed in class.
Reading: Boggs Chapter 5
1. Definition of Siliciclastic Sediments
2. Classification of Siliclastic Sediments and Sedimentary Rocks by: (1) Texture, and (2) Composition.
3. Some comments on Cement and Matrix
4. Compositional Classification of Sandstones using Ternary Plots
5. Compositional Maturity, definition and significance
6. Controls on Sandstone Composition (bedrock composition, climate, tectonics)
7. One Example: First-cycle quartz arenite modern sands in the Orinoco River (Johnsson et al., 1988), produced by extreme chemical weathering of sediments in a humid tropical climate. This finding contradicted the existing paradigm at the time, which is still popular and asserts that many cycles of deposition and erosion (taking 10's or 100's of millions of years) are required to produce compositionally mature quartz arenites.
Below are two photos that illustrate the above concepts:
Philippines. Lush tropical jungle with thick soil produced by extensive chemical weathering on a low-relief landscape. Tectonically not very active. Modern sands might be quartz-rich if quartz is present in the bedrock.
Taiwan. Steep incised landscape sculpted by rapid uplift and erosion in a tectonically active setting. In Taiwan, physical weathering dominates over chemical weathering despite the subtropical humid climate. This is because rapid rock uplift causes deep river incision, oversteepened slopes, and landslides that strip off thin soils and transport sediment directly out to the ocean. As a result, although physical and chemical weathering processes are both enhanced in this setting, the rapid erosion produces compositionally immature sandstones (lithic arenites) rich in unstable lithic fragments.
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