Magma Flashcards
1
Q
magma basics
A
- molten rock - a very hot viscous liquid (1100 = 800 degrees celcius)
- Found at depths of the earth
- Lava = magma that’s reached the surface of earth
2
Q
chemical/mineral composition of magma
A
- Typical magmas are high in Si and O as well as Al, Ca, Na, Mg, Fe, and K -> form silicate minerals
- Magmas cool and crystallize to form igneous rocks
- Can be felsic, mafic, or intermediate
3
Q
intrusive vs. extrusive
A
- Different names for exact same chemicals/composition depending on whether or not they made it to the surface
- extrusive = “volcanic”
- intrusive = “plutonic”
- Intrusive has large crystals because it cools quickly, extrusive has small crystals because it cools slowly
4
Q
mafic magmas
A
- more ferromagesian minerals – Ca, Fe, Mg
- darker in colour
- Volcanic (extrusive): basalt
- Plutonic (intrusive): Gabbro
5
Q
felsic magmas
A
- more K, Na, and Silica
- lighter in colour
- Volcanic (extrusive): rhyolite
- Plutonic (intrusive): granite
6
Q
intermediate magmas
A
- Volcanic (extrusive): andesite
- Plutonic (intrusive): diorite
7
Q
Bowen’s reaction series
A
- The order in which minerals crystallize out of a magma
- Mafic magmas are high temperature: ~1100 degrees Celsius
- Felsic magmas are low temperature: ~800 degrees Celsius
- 2 branches:
- Discontinuous branch: ferromagnesian minerals - olivine -> pyroxene -> amphibole -> biotite -> potassium feldspar (from hottest to coolest)
- Continuous branch: plagioclase feldspar – calcium rich -> sodium rich -> quartz (from hottest to coolest)
8
Q
magma components
A
- Liquid components (the melt)
- Solid components (crystallized silicate minerals)
- Gaseous component (volatiles – either dissolved in the melt or exsolved as bubbles)
9
Q
how do magmas form/how does solid rock melt? (and 3 ways to melt them)
A
- Geothermal gradient increases with depth and pressure (the further you go into the earth, the hotter it is, yet the harder it is to melt the rocks)
- The deeper you are in the earth, the harder it is to melt rocks because it requires a lot more energy
- Temperature always helps melt rocks, but pressure of the earth’s depths prevents melting
- 3 ways to melt rocks (how to get above the geothermal gradient):
- Increase temperature
- Reduce pressure (“decompression melting”)
- Add water/volatiles (decompresses/reduces the melting point -> like adding salt to ice)
10
Q
magma generation in a plate tectonic setting (3 ways)
A
- Introduction of volatiles at subduction zones
- Lowers the melting temperature of overlying mantle material
- Causes partial melting in the mantle generating magma with a new composition - Heat is added (eg. A magma body from a deeper source intrudes cyrstal rock and the additional heat melts a portion of the rock)
- Increases temperature by “underplating”
- Again, partial melting causes generation of new magma - Convective upwelling in the asthenosphere results in decrompression melting
- The asthenosphere rises up close to the surface of the earth, where the pressure is lower
- Like taking the lid off a Coke bottle -> decreases pressure
11
Q
different magma types: plate tectonic setting
A
- Mafic magmas:
- Mid-ocean ranges (divergent plate margins) and hot spots
- Partial melting of mantle peridotite (ultramafic rock) produces mafic magma
- Intermediate and felsic magmas:
- Subduction zones (convergent margins)
- Partial melting of lithosphere/crust generates magma that is richer in silica
12
Q
magma evolution (differences in composition over time) (3 ways)
A
- Crystal settling (magmatic differentiation): As minerals crystallize out of the melt while magma is cooling, they can settle and remove themselves from the melt, changing its chemical composition
- Assimilation of host rock: Magma will crack/partially melt its way through another rock
- Magma mixing: Silicic magma moves upward slowly, while the mafic magma moves rapidly upwards and connects with it -> some mafic material crystallizes in silicic magma, resulting in black blobs
13
Q
intrusive igneous rocks
A
- Ex. Granite, diorite, gabbro
- Form from slow cooling of molten rock (magma)
- This occurs deep below the earth’s surface
- May take 100’s-100 000’s years to cool
- Slow cooling = large crystal size. Coarse-grained (macroscopic)
- Terms associated with intrusive rocks: country rock, contact metamorphism, xenolith, diapirs, pluton, batholith, dykes/dikes, sills
- Intrusive bodies get exposed by uplift and erosion of both overlying and surrounding material
14
Q
country rock
A
older rock that igneous body is intruding into
15
Q
contact metamorphism
A
alteration of surrounding country rock by increased T
