EAE 06 Rocks (10) Flashcards
How are Igneous rocks formed?
They are formed from the cooling of molten rock
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How are Sedimentary rocks formed?
They are formed when weathered fragments of other rocks are buried, compressed, and cemented together, or when minerals precipitate directly from solution
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How are Metamorphic rocks formed?
They are formed by alteration (e.g., heat, pressure) of a pre-existing igneous or sedimentary rock.
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What is a rock?
Consolidated mixture of minerals, or mineral like substances
- Not necessarily more than one type of mineral
- Naturally occurring
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Key points
Minerals
- Definite chemical composition
- Definite crystalline structure
- Formed naturally
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What are the 3 main types of rocks?
Igneous
Sedimentary
Metamorphic
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What is the difference between Magma and Lava?
Magma = molten rock still underground.
Lava = molten rock that has broken through the Earth’s surface.
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What is an Igneous Rock?
- Any rock that has formed out of a molten source (i.e. Magma or Lava)
- Many types, depending on:
* What they are made of * Where they cooled * How fast they cooled
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How common is igneous rock?
95% of the Earth’s crust is made of igneous rocks
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How do you melt a rock to make magma?
3 ways:
* Heat it up * Reduce its pressure * Add water
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Where does magma get made?
- Heat it up
- In subduction zones
- Reduce its pressure
- At rifts
- Within rising plumes
- Add Water
- At Subduction Zones
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Why does heating rocks up happen?
- Add enough heat, eventually everything will melt
- Hot stuff reduces in density, becomes more buoyant
- Rises through its denser surrounding rock usually through cracks, but can melt through if a long lived heat source is present.
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How does magma get made through reducing pressure?
“Decompression melting”
- Solids (minerals in rocks) are stable at certain temperatures and pressures
- Change the pressure, and you change how stable they are
- Reduce the pressure, and the minerals/rocks melt
- Produces a magma, which is buoyant
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How does magma get made through adding water?
Hydration Melting
Adding water (OH) changes the stability of minerals/rocks, even if the pressure does not change
The more water (OH) you add, the lower the temperature they melt
Subducting rocks lose water into the mantle above them, which melts the mantle and adds water to the magma
Produces a magma, which is buoyant…
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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 way to the surface now called “Lava” where it freezes RAPIDLY → EXTRUSIVE rock
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What sort of rocks can you make?
Depends on three things:
What you started off with
The material that has been added to it (or subtracted from it)
How fast it cools
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Core Term
Igneous rock that cools slowly within crust
Intrusive = Plutonic
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What is igneous rock with big crystals, due to a long time for minerals to grow?
Phaneritic
Coarse grained
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What is igneous rock that has cooled quickly at the surface?
Extrusive
= Volcanic
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What is rock with only a short time for minerals to grow?
small crystals
Aphanitic
fine grained
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What is rock that has cooled too fast for minerals to grow?
Glass
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What is rock that has cooled slowly, then quickly?
Initial slow cooling within crust, then quickly at surface.
Porphyritic = two crystal sizes
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What are phenocrysts?
Coarse grains in prophyritic rock
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What is groundmass?
Fine grained minerals in porphyritic rock.
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What type of rock is
Granite
Phaneritic (coars grained)
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What type of rock is
Basalt
Aphanitic (fine grained)
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What type of rock is
Obsidian
Glassy
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What rock forms at
Mid Ocean Ridges
At ridges, the mantle decompresses (decompression melting) and then solidifies at the surface
It is essentially mantle material that has simply moved to the surface
Mantle = olivine + pyroxene + plagioclase feldspar = Basalt or Gabbro
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What rock forms at
Rift Zones
- On Land Early stage rifts
- Basalt from mantle mixes with granite from continental crust
- All sorts of magmas, depending on how old the rift is
- Young = more granitic = rhyolites to dacites
- Old = more basaltic = andesites to basalts
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What rock forms at
Subduction Zones
- Subducting slab loses water (OH) into overlying mantle dehydrates
- Water lowers melting point of mantle rocks, AND allows new minerals to form from mantle hydration melting
- Amphibole, Biotite , Muscovite, Quartz
- Andesites (low water) to Granites (water rich)
- Plutonic (intrusive due to thick crust) and Volcanic (extrusive) rocks
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What rock forms at
Hot Spots
- At hot spots, mantle ‘melts’ through the crust, and may incorporate overlying rocks
- Basalts to granites, depending on what composition of rock overlies the hot spot
- Old hot spots = basalts
- New hot spots on continents = granites
- Middle aged hot spots = mixed (intermediate)
- Magmas = andesites
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What is a Plutonic rock?
Igneous rock that cools slowly within crust
Intrusive rock
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What is phaneritic rock?
Igneous rock with big crystals, due to a long time for minerals to grow
Coarse grained
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What is extrusive rock?
Igneous rock that has cooled quickly at the surface
= Volcanic
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What is an Aphanitic rock?
Rock with only a short time for minerals to grow.
→ small crystals or fine grained
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What is natural glass?
Rock that has cooled too fast for minerals to grow
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What is porphyritic rock?
Rock that has cooled slowly, then quickly → 2 crystal sizes
Initial slow cooling within crust, then quickly at surface.
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