Lecture Seventeen Flashcards
What is an igneous rock?
An rock that has formed out of a molten source, I.e. magma (when inside the Earth) or Lava (when erupted). Many types, depending on: What they are made of Where they cooled How fast they cooled. 95% of the Earth’s crust is made of igneous rocks.
What is Magma?
Molten rock. Working backwards - the liquid form of whatever minerals make up that rock type.
How can you melt a rock to make magma?
One of: Heat it up. Reduce its pressure. Add water.
How can you freeze magma to make rock?
One of: Cool it down. Increase its pressure. Remove water.
Where does magma got made?
Increase temperature - Above a hotspot. In a subduction zone. Reduce its pressure - At rifts. Within rising pumas (at hotspots) (magmas pressure decreases as it rises). Add water - At subduction zones (in the ocean).
Why does magma form due to particular conditions?
Heating it up: Add enough heat, eventually everything will melt. Hot stuff reduces in density, becomes more buoyant. Rises though it denser surrounding rock - usually though cracks, but can ,let through if a long-lived heat source is present. Reducing the pressure: Decompression melting. Solids (minerals in rocks) are stable at certain temperatures and pressures. Change the pressure, and the minerals/rocks melt. Produces a magma, which is buoyant. Adding water: Hydration melting. Adding water (OH) changes the stability of minerals/rocks, even if the pressure does not change. The more (OH) you add, there lower the temperature they melt at. Subducting rocks lose water into the mantle above them, which melts the mantle and add water to the magma. Produces a magma which is buoyant.
What happens as the magma rises up through the crust?
Hot liquid magma melts surrounding rocks and they add to magma - may change magma composition if surrounding (country) rocks are chemically different to magma. Surrounding rocks are cooler - magma freezes (cools/chills) slowly within them to become rock made of minerals = intrusive rock. Some magma makes its watt to the surface - now called “laval - where it freezes rapidly = extrusive rock.
What factors determined what sort of rocks you can make?
1) What you started off with. 2) The material that has been asses to it (or subtracted from it). 3) How fast it cools - does not change the chemical structure, but the physical structure.
What are the naming conventions for rock nomenclature.
Phaneritic: Cools slowly within crust. Intrusive. Plutonic, Log time for minerals to grow = big crystals = course grained. Aphanitic: Cools quickly at surface. Extrusive. Volcanic. Short time for minerals to grow = small crystals = fine grained. Glass: If cools too fast for minerals to grow. Porphyritic: Cools slowly then fast - initially within crust then at surface. Two crystal sizes. Course grained phenocrysts in a find grained groundmass.
What are the two types of laval flow?
Pahoehoe - ropey lava Aa - angular jumbled blocks.
What are the eight minerals generally contained in igneous rocks?
What are the names of the fast and slow cooled intrinsic and extrinsic rock?
