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plate boundaries and igneous processes Flashcards

1
Q

6 places Magma forms

A

Divergent plate boundary

Hot spot

subduction zones

Oceanic-Oceanic boundary

Continetal-Oceanic boundary

Continental-continetal boundary

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2
Q

Formation of Magma at divergent plate boundary
(Finish later)

A

Pressure realse- decompression melting (asthenospere wells upwards to surface + melting point decreases)

Basalt- erupts pillow lavas on sea floor

MOR rift valley

Partial melting of upper mantle produces mafic Magma (Really UM left behind)

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3
Q

Formation of Magma at Hotspot

A

Evidence og mantle plumb- Gabbro to dolorite dykes to pillow lavas

Magma reaches surface -spreads after hitting lithosphere

Magma rises from the deep and undergoes partial melting in hot coloumns - decompression melting (reduces melting point) when breaks crust

Tends to be more mafic as comes from deep mantle

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4
Q

Magma at subduction zones

A

Only occurs is O plate is present at convergent boundary

Must be under ocean overlayed by water

Water dragged down into mantle - lower Mp of minerals - Flux melting

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5
Q

Magma formation at O-O boundary

A

Older colder subducts

subduction - melting which produces volcanos

Mafic Magma moves up through mafic crust

Melting mafic O crust moves up through UM mantle

tends to be mafic Magma but can be intermediate ad only lower mp minerals are melting when moving through the crust

Flux melting

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6
Q

Magma formation at C-O boundary

A

Subduction produces volcanos from melting

intermediate Magma because melting od Mafic O crust then partially melting cont crust as it moves through and mixes

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7
Q

Magma formation at C-C boundary

A

No subduction

Do get Magma here - due to increased pressure + increased tempreature

some melting deep within mountain chain

Silica Magma- viscous - cools slowly and can’t rise so produces granite batholiths

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8
Q

Density of magma

A

Dependent on:
composition- mafic is dense then inter the silicic . Because mafic contains dense metals e.g. Mg + Fe

temperature- temp increases, expansion, density decreases
ORA

pressure-increase pressure, pushes particles closer together, increases density
ORA

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9
Q

viscosity of magma

A

This is the thickness or stickiness of magma

silicic most viscous then inter then mafic

mafic flow more easily

silicic magma is more viscous as it contains more silica tetrahedra - join in chains and sheet using bridging O2 - more connections so harder to flow

temp also effect- decreases viscosity decreases

pressure - increases viscosity increases

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10
Q

magma recharge

A

after eruption - depleted magma chamber
magma rises into chamber - recharges

evidence = little earthquakes, release of gases, heating of more H2O, increased seismic activity

tells us new magma is hotter and may be diff comp - may cause eruption as it heats diff comp magma leading to exsolution of more gas

may get zone crystals - old magma inside and new layers are new magma

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11
Q

magma mixing

A

when 2 different magmas meet - no of factors prevent mixing

different comp = diff density - less dense just overlies more dense
different viscosity-harder for more viscous to mix as harder to move
diff temperature-mafic hotter then silicic produces a thermal barrier as colder magma causes hotter to cool and crystalise producing barrier

2 can mix if movement and stirring through convection currents

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12
Q

groundwater and magma

A

groundwater percolates downwards

meets magma and becomes superheated- hot springs and geysers

hot column of water rises quickly and explosively

water rises quickly pressure decreases and we get flash boiling

then water condenses and the cycle restarts - often on regular timing

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13
Q

how does exsolution of volatiles lead to eruption

A

magma rises
pressure decreases
gases exsolve (come out of solution)
bubbles form

gases take up more space as they have expanded

increased pressure in M chamber

which can trigger an eruption

silicic magmas contain more gases - more explosive

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14
Q

concordant

A

intrusions parallel to existing beds

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15
Q

discordant

A

intrusions cut across existing beds

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16
Q

Country rock

A

any rock which an igneous rock intrudes

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17
Q

What is a Dyke + formation

A

discordant sheet like intrusion

formed from magma intruding vertically into country rock and cooling slowly

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18
Q

What is a sill + formation

A

concordant (usually along bedding planes)sheet like intrusion

formed from magma intruding horizontally into country rock and cooling slowly

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19
Q

formation of ring dykes

A

rising magma pushes rock up - creates dome shape

magma moves - leaves void

dome collapses inward + fractures in a circle

magma oozes into fracture + forms dyke

later eroded - looks like polo mint

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20
Q

Formation of cone sheets

A

looks like ring dyke when exposed but gets smaller further down - ice cream cone

collection of magma- pressure rises-wants to explode

forces magma up and out

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21
Q

what is a volcanic plug

A

landform created when volcanic magma hardens inside vent of volcano

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22
Q

Diapir definition

A

a body of relatively low density that rises up through overlying higher density material

rising block of magma e.g. lava lamp

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23
Q

Contact definition

A

where igneous rock meets country rock

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24
Q

Baked margin def

A

area of country rock that is heated by an intrusion and altered (contact met) - undergoes recrystallisation

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25
chilled margin def
part of intrusion that cools quicker due to contact with Country rock ( finer crystals than rest of intrusion)
26
Pluton definiton
A large igneous intrusive body <100km2
27
Batholith definiton
a large igneous intrusion , aggregate of plutons >100km2
28
Features of a batholith + explanations
usually granite (sometimes granodiorite or diorite) most common form of MII Most are discordant outcrop pattern (what's exposed at surface) is usually circular with steep sides- roughly spherical underground coarse crystals- slow cooling interior cools slower than outside- larger crystals
29
Case study- Cornubian batholith
Formed 280ma large mass of granite- 235km long formed in early Permian - late stage mountain building (variscan orogeny) convergent C C probably melted silicic crust
30
metamorphic aureole
area surrounding an intrusion which has undergone contact met (recryst) due to heat mainly
31
contact metamorphism of shale (using batholith as example)
metamorphic areole forms - can be 100s meters wide shale metamorphosed- close to intrusion: shale - hornfels medium: shale - andalusite furthest away: shale - spotted rock
32
Formation of granite magmas
form at convergent plate boundaries form deep in core of fold mountains at CC- no subduction but high pressure causes melting of C crust due to temp increase at OC- subduction causes partial melting of O crust which rises and melts C crust and leaves it behind (silicic too viscous to mix)
33
partial melting definition
occurs only when portion of rock is melted - when heated minerals with lower Mps (e.g.quartz) melt and higher Mps remain solid
34
stoping definition
process that accommodates magma, as it moves up into Country rock, by mech fracturing of surrounding Country rock (country rock is broken up + removed by up movement of Magma)
35
xenoliths definition
clasts or blocks of pre-existing rock contained within igneous rock
36
assimilation definition
melting process that incorporates blocks (freed by stoping) of Country rock into magma
37
Effects of assimilation
Causes change in composition - magma may assimilate for example a silicic country rock - makes this magma more silicic if it started mafic it may become intermediate this can then change properties e.g. more viscous
38
what are Short period earthquakes? + cause
short amount of ground movement <1s fracturing of brittle rock as magma forces its way upwards- stoping
39
what are long period earthquakes? + cause
Longer period of ground movement 1-5s increased gas pressure in magma chamber
40
what are harmonic tremors earthquakes? + cause
long period of ground movement e.g. rumble magma vibrating in vent of volcano as it moves upward
41
Magma chamber definition + explained in terms of molten to solid rock
area where crust and mantle are partially molten like a water filled sponge sponge-solid water-molten e.g. yellowstone is 90% solid, 10% molten
42
tiltmeter definition
measure very small changes in vertical level
43
GPS defintion
Global positioning system + radio navigation system that allows determination of an exact positions
44
fumaroles definition
opening in or near volcano, through which hot gases emerge
45
Evidence for location of magma chamber
orientation of dykes- seismic tomography- ground movement- fumaroles- changes in groundwater- so all this at Yellowstone but also monitor stream temperatures
46
How does orientation of dykes show pos of magma chamber
dykes parallel suggest divergence (regional stress field) radial dykes- due to stress around pluton/volc used to identify volcano centre
47
How does seismic tomography show pos of magma chamber
P + S waves decrease in velocity at magma chamber - partialy melted hotter + less dense- LVZ molten material = no S waves P+S wave velocity higher adjacent + beneath chamber
48
How does ground movement show pos of magma chamber
tiltmeter + GPS accurately measure vertical and horizontal movement to accuracy of 1 mm measure rate and amount of swelling
49
How does fumaroles show pos of magma chamber
as magma nears the surface pressure decreases gases exsolve CO2 and SO2 - main (75% actually H2O(g)) changes in emissions from fumaroles indicate magma movement more gas- nearer surface
50
How does changes in groundwater show pos of magma chamber
monitor water levels in wells increases due to gases exsolved sudden decrease means that magma + gas is moving before an eruption
51
similarities between lava flows and sills
both layers of rock layered between (usually) sed rocks both may have phenocrysts both may contain xenoliths both have chilled and baked margin on lower contact
52
differences between sills and lava flows
lava flows have reddened and uneven surfaces - sills dont sills are intrusive so have bigger crystals (med) and 2 baked + chilled margins lava flows are extrustive so smaller crystals (fine) and 1 chilled + baked margin on bottom sills have xenoliths from rocks above and below, lava flows only below lava flows have vesicles and sills dont lava flows may show flow banding/phenocrysts may align with direction of flow sills may show magma differentiation / gravity settling basalt most common for lava flow, dolerite most common for sill
53
silicic magma vs mafic
s= more viscous s=lower Mp S=more explosive on eruption s=lighter in colour
54
why are silicic magmas more viscous than mafic
more silica= more silica tetrahedra silica tetrahedra join together by bridging oxygens can form chains, double chains, sheets and framework - form network more bridging oxygens more vicious as harder to flow
55
Effect of temperature on viscosity
temp increases viscosity decreases temp breaks some of the bonds between silica tetrahedra - due to increased energy depolymerisation occurs
56
effect of pressure on viscosity
pressure increases viscosity increases atoms closer together, more compacted bonds already formed remain and new bonds form
57
effect of water on viscosity
presence of water reduces viscosity water causes bonds/bridging O2 to break depolymerisation H ions react with O at the corners of silica tetrahedra stops bonds forming
58
effect of CO2 on viscosity
increases CO2 increased viscosity Co2 causes polymerisation CO2 bonds with O on corner of Silica tetrahedra and then bonds 1 O to the next -> bridge between them
59
Melt definition
name given to magma or lava in liquid phase
60
Aa definition
lava flows with rough, blocky , jagged surface
61
Pahoehoe definition
lava flows with smooth/ ropey surface
62
Scoria definition
a volcanic rock that is usually mafic but can be intermediate
63
Pyroclast definiton
an individual fragment ejected during an eruption . describes all fragment materials formed by explosive eruption
64
Nuee ardente definiton
'burning cloud' is an incandescent, gaseous, pyroclastic cloud or magma droplets and ash - when cools + solidifies the deposit= ignimbrite
65
Pyroclastic flow definiton
hot mixture of pyroclastic material + gas
66
Pyroclastic material
ash- smallest <2mm - form tuff when consolidated Lapillie- 2-64mm- form lapillie tuft when consolidated Bomb + block- >64mm - from pyroclastic rock agglomerate close to crater nuee ardentes- gaseous magma that forms fast moving pyroclastic flow - ingimbrite pumice - silicic igneous rock - very light travels long distance (vesicles)
67
Shield volcanos
gentle slope - <10 degrees composed nearly entirely of basalt low viscosity so magma travels down gentle slopes roughly circular/oval
68
composite volcano
60% of earths volcanos cones - made from alternating layers of lava and pyroclastic material conical shape but most irregular due to caldera collapse / explosive damage eruptions last days to years silicic - viscous - doesnt flow - piles at vent - domes + very steep sides high gas content- explosive + pyroclastic flows
69
fissure eruptions
iceland parallel to axial rift of MOR erupt huge quantities of liquid magma over large area lava builds to form plateaus vast quantities of volc gas released
70
cycle of composite volcanos
magma chamber fills -once pressure exceeds mass of overlying rock blasts away blockage in vent - pyroclastic flow - lava reaches surface - forms layer on top of pyroclast - gas pressure decreases - lava solidifies in neck/vent of volc Cycle repeats
71
Types of volcanic land forms
Columnar jointing pillow lavas calderas
72
Columnar jointing
lava flows more than 3m thick inside cools steadily + slowly - outside cools in hours / days hexagonal columns form in centre as contracts during cooling cooling originates at equally spaced centres - proceeds in all directions from centre contraction causes tension cracks - half way between each cooling centre
73
Submarine volcanos
lava erupted underwater outerskin cools very quickly inside still molten pressure from more lava beneath causes lava to break through skin produces pillow lavas - rounded top + sagging bottom (used as way up + youngest rock identifier) may have vesicles present at upper edge
74
Calderas
large circular depression - caused by violent explosions followed by collapse of top of volc 1) series of violent explosive eruptions , eruptions of pyroclastic flows and large volumes of magma 2)magma chamber starts to empty as magma erupted + top of volc starts to collapse - down into weakened area - compresses remaining magma - erupts more violently 3) entire cone of volc collapses - can cause tsunamis
75
Types of volcanic hazard
lava flows - least dangerous as slow pyroclastic flow - dense cloud of toxic gas + tephra e.g. ash tsunami (unusual) underwater volc nuee ardent jokuhlaup - glacial lake that bursts earthquakes- caused by moving magma or volcanic fluid lahars - hot mud flow - mix of water and rock fragment that flow down slope tephra e.g. ash
76
Volcanic explosivity index
VEI measure of the explosiveness of volcanic eruptions allowing comparisons 0-8 increasing explosivity
77
Hawaiian eruptions
large amounts of very fluid basaltic magma from which gases escape, few pyroclasts 0/1 on VEI
78
Strombolian eruptions
more explosive with less fluid basalt + andersite lavas. Regular explosions of gas + pyroclastic material 1/2 on VEI
79
vulcanian eruptions Not done finish with text book
Violent with viscous andersite lavas + large amounts of pyroclastic materials 2/3 on VEI

Geology As + A2 (18 decks)

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