Rocks From Melts - Lec 10

Question 1

What is magma?

Answer 1

Molten rock material consisting of liquid, gas and crystals (mainly liquid solutions of silicate rock-forming elements and volatiles)

Question 2

How does magma form?

Answer 2

Locally by partial melting of the crust or mantle (pre-existing rock), usually at depths between 10 and 200 km below the surface (most <20km)

Question 3

What causes the wide variety of magma compositions?

Answer 3

The composition of the source rocks
Partial melting
Fractional Crystallisation 
Mixing
Assimilation of solid rock into the molten magma

Question 4

What do different types of magma have?

Answer 4

Different compositions, minerals, temperatures and viscosities

Question 5

What magma forms at divergent plate boundaries?

Answer 5

Basaltic magma formed as mantle peridotite rises and partially melts

Question 6

What magma forms at convergent plate boundaries?

Answer 6

Distinctive magmas generated as result of dehydration of the subducting slab (may differentiate to form andesite and other silicic magmas)

Question 7

What is the emplacement of igneous bodies and eruptions closely lnked to?

Answer 7

Plate tectonic activity

Question 8

What magma forms at magma plumes?

Answer 8

Basaltic magmas form because when hot solids rise from the deep mantle the pressure drops

Question 9

What magma is created when continental crust is melted?

Answer 9

Rhyolitic magmas

Question 10

What are the major silicate rock-forming elements?

Answer 10

O, Si, Al, Fe, Mg, K, Na, Ca

Question 11

What are volatile substances dissolved in magma released in the form of?

Answer 11

H20 (most abundant), CO2, SO2, H2S

Question 12

What are the main controlling factors and processes for magma?

Answer 12

Heating (not usually most important)
Pressure decrease (important at divergent boundaries and magma plumes)
Addition of volatiles (important at convergent boundaries
Original composition

Question 13

Factors that effect more mafic rocks (Mg, Fe)

Answer 13

Higher melting temperatures
Higher pressure increase
Less water content

Question 14

Factors that effect more felsic rocks (Si, Al)

Answer 14

Lower melting temperature
Lower pressure increase
More water content

Question 15

What do pressure and temperature determine in a rock?

Answer 15

If it stays solid or begins to melt

Question 16

What is the solidus?

Answer 16

P/T conditions at which rock starts to melt

Question 17

What is the liquidus?

Answer 17

P/T conditions at which rock melts completely

Question 18

What is decompression melting?

Answer 18

Rock changes from solid to molten by reducing pressure for the same temperature

Question 19

What is a) grain size and b) colour related to in igneous rocks?

Answer 19

a) Rate of cooling

b) Composition (mafic=dark, felsic=light)

Question 20

Name the volcanic (fine grained) rocks in order from felsic to intermediate to mafic

Answer 20

Rhyolite, Andesite, Basalt

Question 21

Name the plutonic (coarse grained) rocks in order from felsic to intermediate to mafic to ultramafic

Answer 21

Granite, Diorite, Gabbro, Peridotite (*<45% silica)

Question 22

Where does decompression melting occur?

Answer 22

Mantle plumes, Mid-ocean Ridges and Continental Rifts (divergent boundaries - importance of extension)

Question 23

Describe the sequence of events that occurs at mid-ocean ridges to form peridotites, gabbro, sheeted dykes and pillow lavas

Answer 23

1) Peridotite from asthenosphere. 2) Plates diverge. 3) Pressure reduced. 4) Peridotite rises to fill space. 5) Drop in pressure (decompression melting). 6) Preferential melting of certain materials. 7) Parent ultramafic but magma mafic (>Si and Fe) 7) Melt is buoyant and rises. 8) Peridotite layer at bottom, slow cooling in magma chamber creates coarse-grained gabbro, this propagates upwards and cools in fractures to form sheeted dykes, then erupts onto surface via fractures creating pillow lavas

Question 24

Describe the layers of deep marine sediments from the bottom to the top (oldest to newest)

Answer 24

Peridotite - Gabbro - Sheeted Dykes - Pillow Lavas - Ophiolites (deep marine sediments)

Question 25

Describe melting due to the addition of volatiles (flux melting)

Answer 25

Volatiles help break chemical bonds Adding volatiles decreases the rock's melting temperature Water is the most important volatile then CO2 It happens in the mantle above the subducting curst

Question 26

Describe the sequence of events at subduction zones (oceanic and continental)

Answer 26

1) Crust begins to subduct. 2) Water sourced from intergranular spaces in rock and by hydrous minerals is released. 3) Water rises. 4) Causes melting of descending plate and overlying plate (composition broadly mafic, but variable). 5) Magma accumulates in magma chambers (fractional crystallisation can occur here producing more felsic magmas). 6) Line of volcanoes parallel to subduction zone - eruptions at surface as volcanoes

Question 27

Describe melting as a result of heat transfer from rising magmas

Answer 27

Rising magma heats surrounding rock Surrounding crustal rock may melt Very hot magma from mantle stalls at base of crust or in crust and transfers heat into the crust

Question 28

What are the 4 major types of magma?

Answer 28

Felsic - Intermediate - Mafic - Ultramafic

Question 29

What are the major types of magma types based on?

Answer 29

Their SiO2 content

Question 30

List the 4 major magma types from the highest silica content to the lowest silica content and give an example of each (coarse grained)

Answer 30

Felsic (granite) - Intermediate (diorite) - Mafic (gabbro) - Ultramafic (peridotite)

Question 31

Why do magmas rise?

Answer 31

Less dense: buoyant Weight of overlying rock creates a pressure at depth: magma squeezed upward Less viscous magmas flow more easily Viscosity depends on temperature, volatile content and silica content

Question 32

How does variation in source rock composition help the variation in the composition of magmas?

Answer 32

Mantle (ultramafic) Oceanic crust (mafic) Continental crust (variable: mafic to intermediate to felsic) IE. PROCESSES MAKE MORE SILICA RICH (FELSIC) MAGMAS

Question 33

How does partial melting help the variation in the composition of magmas?

Answer 33

Most rocks contain variety of minerals Melting of each is not identical Initial magma to form will be more silica rich than 'parent rock' As melting progresses, progressively less silica remains IE. PARTIAL MELTING OF MORE MAFIC ROCKS MAKES MORE SILICA RICH MAGMAS

Question 34

How does fractional crystallisation help the variation in the composition of magmas?

Answer 34

Different minerals crystallise at different temperatures (Mg-Fe rich and Si poor minerals crystallise first at high T) Separation of crystal fraction from remaining liquid (eg. gravitational settling of crystals) Residual melt will be different from parent melt (and always more silica rich) IE. FRACTIONAL CRYSTALLISATION CREATES MORE SILICA RICH MAGMAS

Question 35

How does magma mixing help the variation in the composition of magmas?

Answer 35

Two or more different magmas mixed (often produce andesites) Rhyolite formed via melting 'country rock' in-situ Basalt formed via magma intruding from below

Question 36

How does assimilation help the variation in the composition of magmas?

Answer 36

Incorporation of wall rock through which magma passes as lumps that dissolve Elements may migrate out of the wall into magma without wholesale melting of wall IE. ASSIMILATION USUALLY MAKES MORE SILICA RICH MAGMAS

Question 37

How does a higher viscosity affect melt movement and explosivity?

Answer 37

``` Cooler More SiO2 (felsic) eg. rhyolites and andesites Volcanoes more explosive (smaller with steeper slopes) eg. Andes, Indonesia, Vesuvius ```

Question 38

How does a lower viscosity affect melt movement and explosivity?

Answer 38

Hotter Less Sio2 (Mafic) eg. basalts Volcanoes less explosive (bigger with lower slopes) eg. Iceland, Hawaii

Question 39

What factors dictate how quickly a rock cools? (4)

Answer 39

Depth of intrusion Shape of magma body Size of magma body Presence of circulating groundwater

Question 40

How do large igneous provinces occur?

Answer 40

Magma plumes at the base of the lithosphere Huge volumes of low-viscosity mafic magmas Persist for an extended interval of time Emitted as flood basalts Associated with gas emissions (eg SO2): possible role in mass extinction events

Question 41

What is the odd exception to igneous rocks and why?

Answer 41

Pegmatite- coarse grained, typically occurs as veins/dykes intruded into cool/solid rock Coarse grained because: water-rich melt-crystals can grow very rapidly