5: Igneous Rocks & Volcanic Activity Flashcards
How do igneous rocks form?
From the cooling and crystallization of magma
What is igneous rock texture?
Difference between intrusive and extrusive?
Texture: refers to size, shape, and arrangement of crystals within a rock
Intrusive: forms underground
- cooled relatively slowly, large crystals develop
- Phaneritic texture (large visible crystals)
Extrusive: forms above ground
- Cooled quickly, larger crystals develop
- Aphanitic texture (fine-grain crystals, non visible to eye
Aside from phaneritic and aphanitic texture, what is porphyritic texture?
2 stage cooling process
- minerals crystallize in the magma
- It is then brough to the surface where the rest cools quickly
We get large chunks in a more-uniform base
(think chocolate chip mint)
Where does magma come from?
Comes from fairly close to the surface (asthenosphere)
- Divergent Plate boundaries:
- Decompression melting: mostly-solid mantle moves upwards due to convection, but begins melting as pressure drops
- Partial melting occurs, preferential melting of minerals with lower melting point
- Ultra MAFIC MAGMA PRODUCED - Convergent Subduction Plate boundaries:
- Flux melting as oceanic plate subducts under continental plate
- Water lowers the melting point of rock, acting as the FLUX
- A range of magma compositions are produced, slightly leaning towards FELSIC MAGMA - Mantle Plumes (Hot Spots)
- Hot plastic rock rising from core-mantle boundry
- Mafic magma
- Decompression as it rises leads to melting
How do rocks melt? (3)
- Add Heat
Less common, but friction and hot rock can melt rock - Reduce pressure
At lower pressures, melting can occur at lower temp - Add volatiles (flux)
Water and other volatiles lower the melting point
In More detail, compare the composition of Mafic and Felsic rocks:
Felsic:
- More Silica
- More Sodium
- More potassium
- Lighter in color
- Rich in K, Na, Al, Si
- Comes from high viscosity
Mafic:
- More Calcium
- More Magnesium
- More Iron
- Darker in color
- Rick in Mg, Fe
- Comes from low viscosity
What is Bowen’s Reaction Series?
Minerals crystallize at different temperatures, and the “Order of Crystallization” is the Bowen’s Reaction Series.
Minerals crystallize in one of 2 sequences:
- Continous
- Discontinous
Describe Shallow Intrusive Igneous Rocks:
- depth < 2km
- Cool more rapidly
- Fine Grain
- Follow fractures or other planes of weakness
Dike/dyke:
Rock intrusions that cross-cut existing rocks
Sill:
Sheet structure parallel to existing layers of rocks
Describe deep intrusive Igneous rocks:
Depth > 2km
Pluton is generic name
stock = small pluton (<100km^2)
batholith = large pluton (>100km^2)
How do Intrusive Igneous rocks interfere with construction?
Generally strong:
- Good foundation
- allows for steep, unsupported slopes
- almost impermeable to water
BUT
- Can form extensive sets of fractures
- very difficult to remove
- prone to altercations
How are extrusive igneous rocks formed?
Through volcanic activity.
2 types:
- Effusive eruptions (nice, liquid, flowing, Hawaii)
- Explosive eruptions (all lava becomes fragments of ash)
What determines if magma is explsovie of effusive?
Explosiveness = viscocity + Gas content
Higher viscocity: FELSIC MAGMA
- increasing silica
- increasing sodium
- increasing potassium
Lower viscocity: MAFIC MAGMA
- increasing calcium
- increasing magnesium
- increasing iron
What are the 2 types of volcanos?
Shield Volcanos:
- Lava erupts from fissures, runs down slopes gently and cools
- erupts often
- Mafic lava flow
- not explosive
- short and wide
Stratovolcano”
- Interbedded Lava flows, pyroclastic flow
- erupts less often
- Felsic lava low
- Explosive
- Tall but narrow
Explain why felsic magma is more explosive compared to mafic magma?
Gas is dissolved in magma due to high pressures
Gas bubbles begin to form when pressure lowers as magma rises
Viscous magma (felsic) trap gas and explode when they errupt
Runny magma (mafic) allow gas to escape, non-explosive
Where are each type of volcano found?
Shield volcanoes are found at hot spots
Stratovolcanoes are found at convergent subduction zones
