Magma

Question 1

magma basics

Answer 1

• 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

Question 2

chemical/mineral composition of magma

Answer 2

• 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

Question 3

intrusive vs. extrusive

Answer 3

• 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

Question 4

mafic magmas

Answer 4

• more ferromagesian minerals – Ca, Fe, Mg
• darker in colour
• Volcanic (extrusive): basalt
• Plutonic (intrusive): Gabbro

Question 5

felsic magmas

Answer 5

• more K, Na, and Silica
• lighter in colour
• Volcanic (extrusive): rhyolite
• Plutonic (intrusive): granite

Question 6

intermediate magmas

Answer 6

• Volcanic (extrusive): andesite

- Plutonic (intrusive): diorite

Question 7

Bowen’s reaction series

Answer 7

• 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)

Question 8

magma components

Answer 8

• Liquid components (the melt)
• Solid components (crystallized silicate minerals)
• Gaseous component (volatiles – either dissolved in the melt or exsolved as bubbles)

Question 9

how do magmas form/how does solid rock melt? (and 3 ways to melt them)

Answer 9

• 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)

Question 10

magma generation in a plate tectonic setting (3 ways)

Answer 10

1. 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
2. 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
3. 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

Question 11

different magma types: plate tectonic setting

Answer 11

• 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

Question 12

magma evolution (differences in composition over time) (3 ways)

Answer 12

1. 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
2. Assimilation of host rock: Magma will crack/partially melt its way through another rock
3. 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

Question 13

intrusive igneous rocks

Answer 13

• 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

Question 14

country rock

Answer 14

older rock that igneous body is intruding into

Question 15

contact metamorphism

Answer 15

alteration of surrounding country rock by increased T

Question 16

xenolith

Answer 16

fragment of surrounding country rock incorporated into the magma

Question 17

diapirs

Answer 17

rising bodies/blobs of magma

Question 18

pluton

Answer 18

large igneous bodies that crystallize at depth within the Earth’s crust

Question 19

batholiths

Answer 19

• large (>100km^2 igneous bodies formed by coalescence/joining together of plutons)
• Ex. Chief

Question 20

dykes/dikes

Answer 20

• thinner sheet of igenous rocks
• usually high angled, cross-cut existing strata (vertically)
• Ex. Found on Chief, Ship Rock

Question 21

sills

Answer 21

• thinner sheet of igenous rocks

- moves in between strata (concordant with it); unzips it rather than cross-cuts it (horozontally)