Lesson 7 Metamorphic and Sedimentary Rocks

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Geology

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Oct 30, 2023

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GEOL 1120 Lesson 7 Metamorphic and Sedimentary Rocks At which of the following plate boundary types does magma form due to lowering pressure? A. Divergent boundary B. Oceanic-oceanic convergent boundary C. Continental-continental convergent boundary D. Transform boundary E. None of the above Magma forms in three ways 1. Increase in temperature a. Subduction zones  the subducting plate descends into the Earth and is heated up… but it doesn’t melt 2. Decompression melting (decrease in pressure) a. Divergent boundaries  pulling apart plates reveals material below and lowers pressure, causing it to melt 3. Adding water/volatiles a. Subduction zones  water in the subducting plate is liberated, then causes the mantle above that plate to melt Rock Types – Metamorphic - Meta = change - Morph = shape - Changed by high pressure, high temperature, fluids, and time - Basically cooked! Metamorphism: mineralogical and textural changes in a rock caused by pressure, temperature, and fluid conditions different than those when the original rock formed. What causes metamorphism? - One or more of the following factors: 1. Increased temperature 2. Increased pressure 3. Fluids 4. Time - 1. Increased temperature o Average geothermal gradient 77 °F/mile depth o Deeper is hotter! - 2. Increased pressure o Lithostatic: stress equal in all directions o Compressive: stress is not equal in all directions  Produces a foliated rock

GEOL 1120 - 3. Fluid activity o Water in spaces in a rock carries dissolved ions in solution o Fluids increase rates at which new minerals form - 4. Time o Metamorphism is a chemical reaction o The changes take time to progress o Time alone can lead to changes Foliation - Due to compressive stress - Same root as foliage (leaves) - Layers in the rock - Pervasive – starts at the molecular level Which of these statements about a foliated metamorphic rock must be true? A. The rock used to be a sedimentary rock that contained distinct layers. B. The rock was subjected to high heat as it was forming. C. The rock was subjected to high compressive stress as it was forming. D. The rock was melted as it was forming. E. The rock was subjected to high lithostatic stress as it was forming. Two Main Types of Metamorphism 1. Contact metamorphism 2. Regional metamorphism Contact metamorphism - High temperature, low pressure - Caused by magma that “cooks” the rock - Localized around magma chamber - Results: recrystallization, new minerals, new textures - Non-foliated rocks (lithostatic stress only) Regional metamorphism - High temperature + high pressure - Caused by compressive stress over large areas of crust - Greater depth in crust than contact metamorphism (compressive stress is high enough) - Results: recrystallization, new minerals, new textures - Foliated rocks (lithostatic stress still exists, but compressive stress dominates)

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