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Intro to Earth Science > Rocks From Melts - Lec 10 > Flashcards

Rocks From Melts - Lec 10 Flashcards

1
Q

What is magma?

A

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

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2
Q

How does magma form?

A

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)

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3
Q

What causes the wide variety of magma compositions?

A 
The composition of the source rocks
Partial melting
Fractional Crystallisation 
Mixing
Assimilation of solid rock into the molten magma
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4
Q

What do different types of magma have?

A

Different compositions, minerals, temperatures and viscosities

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5
Q

What magma forms at divergent plate boundaries?

A

Basaltic magma formed as mantle peridotite rises and partially melts

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6
Q

What magma forms at convergent plate boundaries?

A

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

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7
Q

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

A

Plate tectonic activity

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8
Q

What magma forms at magma plumes?

A

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

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9
Q

What magma is created when continental crust is melted?

A

Rhyolitic magmas

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10
Q

What are the major silicate rock-forming elements?

A

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

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11
Q

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

A

H20 (most abundant), CO2, SO2, H2S

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12
Q

What are the main controlling factors and processes for magma?

A 
Heating (not usually most important)
Pressure decrease (important at divergent boundaries and magma plumes)
Addition of volatiles (important at convergent boundaries
Original composition
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13
Q

Factors that effect more mafic rocks (Mg, Fe)

A

Higher melting temperatures
Higher pressure increase
Less water content

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14
Q

Factors that effect more felsic rocks (Si, Al)

A

Lower melting temperature
Lower pressure increase
More water content

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15
Q

What do pressure and temperature determine in a rock?

A

If it stays solid or begins to melt

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16
Q

What is the solidus?

A

P/T conditions at which rock starts to melt

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17
Q

What is the liquidus?

A

P/T conditions at which rock melts completely

18
Q

What is decompression melting?

A

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

19
Q

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

A

a) Rate of cooling

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

20
Q

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

A

Rhyolite, Andesite, Basalt

21
Q

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

A

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

22
Q

Where does decompression melting occur?

A

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

23
Q

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

A

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

24
Q

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

A

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

25
Q

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

A

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

26
Q

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

A

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

27
Q

Describe melting as a result of heat transfer from rising magmas

A

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

28
Q

What are the 4 major types of magma?

A

Felsic - Intermediate - Mafic - Ultramafic

29
Q

What are the major types of magma types based on?

A

Their SiO2 content

30
Q

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

A

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

31
Q

Why do magmas rise?

A

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

32
Q

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

A

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

33
Q

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

A

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

34
Q

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

A

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

35
Q

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

A

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

36
Q

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

A

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

37
Q

How does a higher viscosity affect melt movement and explosivity?

A 
Cooler
More SiO2 (felsic) eg. rhyolites and andesites
Volcanoes more explosive (smaller with steeper slopes) eg. Andes, Indonesia, Vesuvius
38
Q

How does a lower viscosity affect melt movement and explosivity?

A

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

39
Q

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

A

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

40
Q

How do large igneous provinces occur?

A

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

41
Q

What is the odd exception to igneous rocks and why?

A

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

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