Plate Boundaries And Igneous Processes Flashcards
What creates the rheid asthenosphere?
Partial melting within the mantle. Not enough to classify it as a liquid. Pressure allows material to remain solid due to particle compaction
What is geothermal gradient?
The change in temperature with depth (C/Km)
What are convection currents?
They help with rock movement within the mantle. Happens because of heat transfer from the core to the lower mantle.
Rock gets hotter, expands (less dense) and rises. Here it gets cooler, contracts (more dense) then sinks.
What is adiabatic heating?
Areas become hotter, despite no change in energy.
Contraction = heat energy shared over smaller volume (no gained energy). So the rock appears hotter
What is adiabatic cooling?
Expansion = heat energy shared over larger volume (no energy loss). Rock appears to cool
What is an adiabatic system?
No heat energy enters or leaves the system.
Happens in the mantle.
How does partial melting happen?
Rising magma experiences a decrease in pressure = expansion = reduction in temp with the loss of heat as energy used to move particles apart. Melting occurs because melting point decreases as pressure decreases
What are the 6 causes for magma formation at Earth’s surface?
Divergent boundary
Hotspots
Subduction zones
Oceanic-oceanic boundary
Continental-oceanic boundary
Continental-Continental boundary
How is magma formed at divergent plates boundaries?
Mainly seen as MORs (e.g. MAR).
Mainly due to decompression melting by pressure release. As it stretches and thins, asthenosphere upwell and pressure reduced (lower MP).
Partial melting of ultramafic peridotite produces mafic magma.
Basalts are produced, usually as pillow lava. Dolerite sills and dykes below
How is magma formed at hotspots?
Evidence of mantle plumes.
Magma rises from deep - decompression melting. Plumes carry heat upward in narrow rising columns of hot material, which spreads out when plume head reaches lithosphere.
Lower pressure = decompression + partial melting of mantle peridotite. Forming enormous volcanoes of basaltic lava
How is magma formed at subduction zones
Oceanic crust present.
Magma production here must occur under ocean. Some water dragged down during subduction. Presence of water lowers melting point of minerals. This called flux melting
How is magma formed at oceanic-oceanic boundaries?
Subduction causes volcanoes with usually mafic magma.
Partial melting of oceanic crust through ultramafic mantle
How is magma formed at a continental-oceanic boundary?
subduction causes volcanoes with intermediate magma.
Partial melting of oceanic crust through ultramafic mantle through silicic continental crust
How does magma form at continental-continental boundaries?
No subduction
Increased pressure = increased temps.
Some melting deep in mountains.
Silicic magma (viscous) in the base, so cools slowly (batholith).
Thickening on crust –> fold mountains
What controls the density of magma?
Composition. E.g. mafic > silicic (due to metals).
Temperatures. High temps = expansion if possible = reduction in density.
Pressure. Increase in pressure = increase density
What controls the viscosity of magma?
Composition.
Bonds. Silicic magma = more tetrahedra bonds = more viscous.
Temperature increase = reduced viscosity.
Increased pressure = increased viscosity.
What is magma recharging?
After an eruption, depleted magma chamber. More magma rises from deeper and ‘recharges’ the chamber
What is the evidence of magma recharging?
Seismic tomography, release of gases.
New magma is hotter than any left over magma (deeper) and different composition.
Different composition could trigger eruption.
Old magma may have started to crystallise (floating crystals).
Sometimes zoned crystals.
What factors prevent magma mixing?
Different compositions: different densities. Tend to have one overlying the other (buoyant).
Different viscosities: more viscous = harder to mix
Different temps: where they meet, we get a thermal barrier
What is a thermal barrier?
Where two magmas meet (different temps)
What does groundwater do for magma?
Groundwater percolates downwards –> becomes superheated if meets magma. Forms geysers and hot springs.
Flash boiling as pressure drops when water rises (release gases).
This then repeats
What does exsolve mean?
To come out of solution
What are volatiles?
Substances which are unstable at surface conditions
What is the exsolution of volatiles in magma?
Magma rises –> pressure drops –> gases exsolve.
Formation of bubbles = more space (pressure).
Can trigger eruption
What is an igneous intrusion?
Magma that has come up but never reached the surface, cooling below
What is concordant?
Intrusions parallel to existing beds
What is discordant?
Intrusions perpendicular to existing beds (cuts across)
What is country rock?
Rock that was already present before an intrusion
What is a dyke? (basic description)
Discordant minor sheet-like intrusion
What is a sill? (basic description)
Concordant minor sheet-like intrusion
Sills and dykes are examples of what type of intrusion?
Minor igneous intrusions
What are the two types of dyke which appear the same at first?
Ring dykes and cone sheets
How are ring dykes formed?
Magma rises and pushes overlying rock into dome shape.
Magma retreats (pressure decrease), leaving void below the dome.
This then collapses and fractures the rock around it. Magma enters these circular fractures.
Then eroded to form a ring shape
How are cone sheets formed?
Magma rises and pushes upwards, meeting a barrier –> accumulates and pressure builds. This means it moves upwards and outwards.
Forces its way upwards and out, forming a cone shape –> looking like a ring from the surface once eroded
Ring dykes and cone sheets are often confused, why?
Both look circular in map view
What is a volcanic plug?
A crystallised magma that fills the vent of a volcano.
What is a diapir?
Any blob of rising magma (no matter the size).
a.k.a
A body of low density material that pierces and rises up through overlying material of higher density
What is meant by ‘contact’?
Where the igneous rock meets the country rock
What is a baked margin?
Area of country rock that has been heated by the intrusion. It has undergone contact metamorphism as it has been altered by heat
What is a chilled margin?
Part of an intrusion that cools rapidly due to contact with cold country rock. Has a finer crystal structure that remaining intrusion
Plutons and batholiths are examples of what type of igneous intrusion?
Major igneous intrusions
What is a pluton?
A large igneous intrusion. Smaller than batholiths
What is a batholith?
A large igneous intrusion.
Bigger than plutons, may be an aggregate of plutons.
>100 km^2
What are the features of a batholith?
Most are discordant.
Usually circular at surface and spherical underground.
Coarse crystals.
Usually granite, sometimes diorite
What is the Cornubian batholith?
Batholith under Devon and Cornwall. Formed in Variscan orogen in late Paleozoic
235km long
What is a metamorphic aureole?
A large area around a batholith where country rock has been metamorphosed.
Primary force is heat (contact)
How does a batholith affect shale country rock?
Large baked margin.
Close to intrusion = hornfels
Little further = Andalusite
Farthest = spotted rock
Is hornfels a rock or a mineral?
A rock
Is andalusite a rock or a mineral?
A mineral
How do granite magmas form?
Form at convergent plate boundaries. Deep in the core of mountains.
How do granitic magmas form at continental-continental boundaries?
No subduction, high pressure causes some melting
How do granitic magmas form at oceanic-continental boundaries?
Oceanic subduction melts. Melt rises.
Silicic minerals have lower melting point so rise first
What is partial melting?
When only a portion of a rock is melted. Lower temperature minerals melt, e.g. quartz
What is stoping?
The process that accommodates the magma as it moves upwards. Mechanical fracturing of the surrounding rock.
(Kinda like plucking in geography)
What are xenoliths?
Clasts or blocks of pre-exiting rock contained within an igneous rock if it doesn’t melt
What is assimilation?
The melting process that incorporates blocks of country rock, fed by stoping, into the magma
What can assimilation do to a magma?
Change the overall composition of the magma.
What are the three types of earthquakes caused by moving magma?
Short-period earthquakes.
Long-period earthquakes.
Harmonic tremors.
All small in comparison to large tectonic earthquakes
What are short period earthquakes?
Short amount of ground movement. <1s.
Fracturing of brittle rocks as magma rises upwards (stoping)
What are long period earthquakes?
Longer amount of ground movement. 1-5s
Caused by increased gas pressure in a magma chamber
What are harmonic tremors?
Magma vibrating in the vent of the volcano as it moves upwards.
Longer rumblings, lasts a while
What is a tiltmeter?
Designed to measure very small changes in vertical level
What is GPS?
Global Positioning System.
The radio navigation system that allows determination of exact position
What are fumaroles?
Openings in or near a volcano, through which hot gases emerge
What is the evidence for the location of a magma chamber? (5)
Seismic tomography.
Ground movement.
Fumaroles.
Changes in water table.
Orientation of dykes
How does seismic tomography provide evidence for the location of a magma chamber?
Both P+S waves. Intrusions result in low velocity zones.
Molten materials, no S waves.
P+S waves will travel faster around the chamber
How does ground movement provide evidence for the location of a magma chamber?
Measures using tiltmeters and GPS that accurately measure both vertical and horizontal movements to the accuracy of 1mm.
How do fumaroles provide evidence for the location of a magma chamber?
Fumaroles are gas emissions near the volcano. CO2 and SO2 produced
How do changes in the water table provide evidence for the location of a magma chamber?
Water levels can be monitored using wells. If levels rise, it means more gases exsolved.
Sudden drop = imminent eruption
How does the orientation of dykes provide evidence for the location of a magma chamber?
Dykes may change distance and position due to increased gas levels below the surface.
This can be used as an indicator of the stress field in volcanic areas and to identify volcanic centres
What are rose diagrams also known as?
Circular graphs or stereonets
For orientations on rose diagrams, how do we plot the data?
Tally up the data. Add up opposite tallies. Plot.
For directions on rose diagrams, how do we plot the data?
Tally up the data. Plot.
No addition required
What are the similarities between sills and lava flows? (3)
Igneous layers of rock between earlier rock.
May have xenoliths and phenocrysts.
Baked and chilled margin on lower contact
What are the differences between sills and lava flows? (6)
Sill = 2 bakes margin. Lava flow = 1.
Lava flow may be uneven at top.
Sill = xenoliths top+bottom. Lava flow = xenoliths only bottom.
Lava flow = fine or glassy. Sill = fine or medium.
Lava flow = may have vesicles. Sill none.
Lava flow = reddened surface (oxidation of iron). Sill none.
At a molecular level, why are silicic magmas more viscous than basaltic magmas?
More silica tetrahedra.
More bridging oxygens (bonds).
Form chains, double chains, sheets, and frameworks
How does the temperature of lava impact its viscosity?
Increase temps = lower viscosity.
Energy breaks some of the bonds (depolymerisation).
Also the same the other way around.
How does the pressure of lavas impact their viscosity?
Increase pressure = increase viscosity.
Atoms more able to form bonds (polymerisation)
How does lava composition impact their viscosity?
Higher amount of silica tetrahedra bonds. More silica available = more bonds likely.
hence silicic more viscous
How do volatiles (water) impact lava viscosity?
Presence of water = lower viscosity.
Water breaking bridging oxygen bonds (depolymerisation).
Instead hydrogen ions are reacting with bridging oxygens
How do volatiles (CO2) impact lava viscosity?
Presence of CO2 = increased viscosity.
CO2 is a polymer, increases polymerisation.
CO2 connects to non-bridged oxygens and bridges to the next.
CO2 bridges them
What is meant by the term ‘melt’?
The name given to magma or lava in the liquid phase
What is aa?
Lava flows with a rough, blocky, jagged surface
What is pahoehoe?
Lava flows with a smooth or ropy surface
Which of aa or pahoehoe is more viscous?
Aa
What is scoria?
A volcanic rock that is usually mafic but can be intermediate in composition
What is a pyroclast?
An individual fragment ejected during an eruption
What is a nuee ardente?
Gaseous pyroclastic cloud of magma droplets and ash
When a nuee ardente deposit cools and solidifies, what rock does it become?
Ignimbrite rock
What is a pyroclastic flow?
Hot mixture of pyroclastic material and has
What is included in pyroclastic material?
bombs and blocks, lapilli, tuff, pumice, ash
What is lapilli?
Particles between 2mm and 64 mm in size
What rock does lapilli form?
Lapilli tuff
What is ash?
Smallest particles less than 2 mm in size. form tuff
What are volcanic bombs and blocks?
produced by the coarsest particles over 64 mm in size
What pyroclastic rock is formed from volcanic bombs and blocks?
Agglomerate
What are the features of shield volcanoes?
Gentle slope <10°
Almost entirely thin basalt.
Low viscosity.
Roughly circular
Little pyroclastic material
Frequent or constant eruption
How many of Earth’s individual volcanoes are composite volcanoes?
over 60%
What are the features of a composite volcano?
Alternating strata (lava & pyroclastic deposits).
Typically conical.
Eruption can last hours, days or years.
Viscous.
Wide range of compositions (typically higher silica)
What is the eruptive cycle of volcanoes?
Period of no activity. Magma builds, gas pressures.
Pressure builds, explodes. Gas rich pyroclastic material escapes.
Lava reaches surface and forms layer. gas pressure decreases
What are the features of fissure eruptions?
Huge quantities of fluid lava spreads out over large area.
Lots of volcanic gas (SO2).
Elongated breaks in the crust
What is columnar jointing?
Hexagonal columns formed from large lava flows cooling
How does columnar jointing form?
Lava flows >3mm thick.
Inside cools slowly (weeks, months)
Outside cools in hours or days.
Cooling at centres in all directions.
Contraction causes tension cracks (hexagonal)
What is the other name for submarine eruptions?
Pillow lavas
How do pillow lavas form?
Lava erupts underwater, outer skin cools rapidly while inside is molten.
Pressure of more lava causes breaks to form new pillows.
How can pillow lavas be identified?
Rounded shape.
Vesicles may be present towards outer upper edge of pillows.
Spaces between pillows may be filled with fine, glassy materials
What are the features of a caldera?
Large 1-20km crater circular. caused by violent eruptions, followed by the collapse of the top of the volcanic cone
What are the three stages of formation of calderas?
1) series of violent eruptions of pyro flows, ash, pumice. Removes large volumes of magma.
2) Magma chamber starts to empty. top starts to collapse down. This compresses magma, making eruptions more violent.
3) Entire cone collapses.
What other hazard might the collapse of the cone of a caldera cause?
Tsunamis if the volcano is coastal, e.g. Krakatoa
What kind of gases do volcanoes release/exsolve?
CO2 (12%)
SO2 (7%)
N (7%)
^add to 25%
H2O vapour (70%)
What are the potential hazards caused by eruptions?
Pyroclastic flow,
lahars (mudflows)
Tephra (ash,bombs)
Lava
Earthquakes, tsunamis
Floods
What is meant by ‘effusive’?
Describes the fluid, non-explosive, basaltic lava. Often constant eruptions
What is the Volcanic explosivity index?
A measure of the explosiveness of eruptions. Allows volcanoes to be compared. 0-8 scale
What are hawaiian eruptions?
Large amounts of fluid basaltic lava from which gases escape, but few pyroclasts
What are strombolian eruptions?
More explosive with less fluid lava. Regular explosions of gas and pyroclastic material
What are vulcanian eruptions?
Violent with viscous andesitic lava and large quantities of pyroclastic material from large explosions
What are plinian eruptions?
Extremely explosive with viscous gas filled andesitic and rhyolitic lava and high volumes of pyroclastic material blasted out
What is isopachyte?
A line joining points of equal thickness of a deposit such as ash. The maps may be called isopach maps
