Tran- Lecture Review 17

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Texas Tech University *

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1401

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Geology

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Oct 30, 2023

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Review 17 17 Rocks and Weathering 17.1 Rocks Mineral: - Mineral is a naturally occurring inorganic compound with a characteristic crystal structure. E.g: quartz, SiO 2; crystal structure is hexagon - Rock is a mix of minerals (E.g granite). Three main kinds of rocks: igneous, sedimentary, and metamorphic. Intrusive igneous: magma solidifies below the surface; slow cooling—bigger crystals (e.g., granite). Extrusive igneous: magna cools at the surface; fast cooling—small crystals (e.g., obsidian). Clastic Sed. Rocks: - Are made of particles of broken rock or remains of organisms. - Clastic rocks are categorized by the size of the particles making them up: + Shale- clay particles + Mudstone—mixed slit and clay + Siltstone—slit. + sandstone—sand + conglomerate—pebbles and larger - Clastic limestone: made of calcium carbonate--skeleton/shells of living thing that accumulate, fuse together and form a solid rock Chemical Sed. Rocks: material that was chemically dissolved in water becomes solid (precipitates), accumulates and forms rock. - Most common chemical sed. rock is limestone; it forms when calcium carbonate (same material as in skeletal remains), precipitates out of seawater. Chemical Precipitates: If more of a substance exits in seawater than the water can hold, some of it becomes solid again and falls to the bottom (precipitate out) Evaporites: In arid areas (deserts), evaporating water can leave salt deposits, like rock salt and gypsum. Organic Sed. Rocks: made of formerly living things. - E.g., Coal is formed when vegetation accumulates in swamps and modified over time; first it becomes peat (the plant material settles under water in a swamp, and it start to get a thick deposit of the plant matter)  coal (it gets compressed enough and altered a bit chemically)

Metamorphic rocks: form when one rock is changed into another by high heat and pressure, so we have new minerals. Regional Metamorphism: deeply buried rock under high pressure and heat. Contact Metamorphism: where the rock is changed by contact with magma. Rock Cycle change type, and can be drawn: - Magma  igneous rock  sedimentary or metamorphic; Sedimentary  be metamorphosed or become a new sedimentary rock; metamorphic  sedimentary. And all three can be subducted and become magma again. 17.2 Weathering Chemical Weathering: the rock material is dissolved and carried away. E.g. our saliva dissolves the candy Physical Weathering: the rock is broken into piece. E.g., biting candy crushing it into small pieces Chemical stability: is influenced by the strength of the bonds between the particles making up the rock. - The amount of chemical weathering that a mineral undergoes is strongly dependent on the chemical stability of that mineral or how easily a mineral changed. E.g: + Quartz is a very stable mineral—hard to chemically break down. + Olivine: low chemical stability—breaks down quickly. Factors affecting chem. weathering: - Availability of water: wetter environments typically have more weathering than dry ones; water is negative on one side, positive in the other, can attach to other atoms and pluck them out of minerals. + Cation: positively charged ion. + Anion: negatively charged ion. - Temperature: Chemical reactions work faster at higher temperature. + hot, wet environment (e.g tropical rainforest) have lots of chemical weathering. + cold, dry environments (e.g Arctic) have little chemical weathering. - Availability of certain chemical: water can become acidic by accumulating carbon from the atmosphere and from organic matter in the soil. The water then becomes a carbonic acid , which is effective at dissolving some rock types (for example, limestone) Solution: minerals get dissolved in acidic water - Example: Limestone—there are whole landscapes dominated by landforms created by dissolving limestone (sinkholes and streams that disappear underground)

Hydrolysis: occurs when water molecules are broken up, the hydrogen separates from the oxygen, and the hydrogen ions replace metals in the minerals. - This is effective on granitic rocks and leads to the formation of clay minerals (new mineral created and very stable) Oxidation: iron and aluminum compounds are modified and produce oxides (or rust)—new minerals and quite stable End products of Ch. W: - Stabled minerals (like quartz)—doesn’t get altered at all. - Oxides—rust can get left behind, quite stable. - Clay minerals—get left behind, very stable.  these more stables minerals become the parent matter of soil. - Soluble ions—leave with water. Physical weathering Internal expansion: the most common in intrusive igneous rocks that solidified at temperatures and pressure different from those at the surface; when they are exposes at the surface, they expand. Exfoliation: is one form of internal expansions and when the outside layers of a rock flake off due to the expansion. External Forces Frost action: If water gets into a rock and then freezes, it expands and exerts pressure on the rock that eventually can break the rock in two. Salt crystal growth: in deserts when salty water evaporates leaving the salts behind - If these crystals form in a crack in a rock, they exert force that may break the rock apart. 17.3 Relation Between Chemical and Physical Weathering Relation between chem. & phys. Weathering - Chemical weathering depends on the amount of surface area available to be attacked. - Physical weathering makes more surface area. - Therefore, physical weathering should increase the rate of chemical weathering

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