Igneous and Metamorphic Flashcards
Effect of stretching continental crust up to 10%
Surrounding plates move and continental crust is stretched and thins. Mantle below is lifted upwards to replace it. The rate of uplift is at tectonic speeds (a few cm per year). Mantle doesn’t have time to cool by conduction. Hotter at a given depth than stable geotherm.
About 10% increase in length means solidus is intersected and you get melt e.g. East african rift valley.e
Effect of stretching continental crust more than 10%
Stretching can be so extensive that the continental crust breaks. This is where a new ocean starts to form e.g. how atlantic ocean formed.
Passive margins
Margins or continental crust adjacent to ocean floor. The result of continental crust thinning and then breaking. E.g. east coast of the americas and west cost of europe and africa
What you get from melting the upper mantle (peridotite)
Peridotite is composed of olivine, clinopyroxene, orthopyroxene and a very little bit of plagioclase.
Partial melting of peridotite creates picritic liquid and leaves a depleted rock (harzburgite) made of olivine and orthopyroxene.
Picritic liquid is less dense than harzburgite and peridotite and so moves upwards. Loses olivine as it ascends and becomes basalt.
Basalt is composed of clinopyroxene, plagioclase and very little olivine.
How thickness of oceanic crust affects the amount of melt and what this shows about the mantle
The oceanic crust is between 6.5 and 8km thick. The amount of melt depends on the depth at which melting begins. So, the composition of the mantle is constant and the mantle is at a constant temperature (1300 degrees celsius)
Evidence for the composition and structure of the oceanic crust
Magnetic stripes in oceanic crust are symmetrical about ridges. So, symmetrical spreading.
Seismic profiles:
4 distinct layers in the oceanic crust from 4 distinct P wave velocities. At ridge itself, LVZ from magma chamber.
Deep-sea drilling:
Can take samples directly from the uppermost few km of the oceanic crust. Core are then analysed.
Submersibles:
Volcanic activity at ridges can be observed directly from a submersible.
DIRECT OBSERVATION FROM Ophiolites (e.g. gulf of oman)
Ophiolites and internal layering of oceanic crust
Ophiolites are sections of oceanic crust uplifted onto continental crust. e.g. Gulf of Oman
- Sediments - deep water mudstones and cherts (from dead plankton skeletons)
- Extrusive sequence - pillow basalts. Pronounced marginal chilling with a thin layer of glass at the surface. 1.5km
- Sheeted dyke complex - parallel basaltic or doleritic dykes. Show conclusively that oceanic crust is created entirely by stretching. 1km
- Intrusive rocks - Mainly unlayered gabbros at the top and layered cumulates at the bottom. Frozen magma chamber beneath the spreading ridge. 4km
- Mantle
Boundary between the intrusive rocks and the mantle is the petrological moho
Earthquakes at ridges
Spreading causes extensional faults
Earthquakes are small and shallow because there’s only a thin layer of brittle, cold rocks over the hot, ductile layer of rock`
Black smoker
caused by hydrothermal circulation
Much more efficient cooling mechanism than conduction
Cool the crust up to 2km
Cause hydrothermal alteration of basalt and create hydrated minerals like hornblende
Hotspots
Isolated regions where there’s a lot of volcanic activity.
Upwelling of hot, convecting mantle that is still solid (diffusion creep)
Cause melting above them since the geotherm is higher than usual.
Move at much slower rates relative to each other (<10mm/year) than tectonic rates of motion (up to 150mm/year). So the movement of plates relative to hotspots can be deduced as as approximately absolute relation in space. Trails of extinct volcanoes
E.g. Hawaii –> hawaii-emperor chain of volcanic islands has a change in direction that shows that the pacific plate changed direction of motion
Magma will only rise to a point where it is equal density to the surrounding rocks. It won’t rise if it’s more dense than what’s above it.
If basaltic magma rises and then hits granite (density of c. 2.6 g/cm^3), it will stop as it has a lower density. But, basaltic magma has a greater temperature than the melting temperature of granite, so the granite above it may melt. e.g, yellowstone
Mantle plumes
Mantle that rises as part of a convection current to form a hotspot
Where mantle plumes come from
The core-mantle boundary
Large scale seismic studies have found Large Low Shear Velocity Provinces (LLSVPs) on this boundary.
Most currently active hotspots are found above the boundaries of the 2 LLSVPs
Plutonic rocks
rocks that solidified from igneous melt at great depth coarse grained (> 5mm)
What happens to magma that doesn’t reach the surface
Solidifies at depth to form plutonic rocks in the form of intrusions
Hypabyssal intrusions
Small intrusions at shallower depths.
Contain rocks with medium sized grains (1-5mm)
Batholith
Large bodies of magma (10-1000km) deep in the Earth’s crust
Xenolith
Fragments of rock found in magma from another source, commonly from the surroundings
Stoping
Mechanism by which country rock xenoliths end up in magma
For granite to rise:
Regional deformation squeezes rhyolitic magma out from partial melt. Upward movement of magma in dykes feeds other intrusions like sills. As the intrusions get bigger and more frequent, portions of the country rock breaks off into the batholith
Dyke
Steeply inclined bodies of magma filling vertical fractures.
Discordant as they cut across the bedding planes of the country rock
Either flow up to feed a batholith or up to the surface from a batholith
Sill
Sheet-like intrusion that is along bedding planes. Magma flows along fractures parallel to bedding planes.
Concordant
Lacolith
An intrusion that starts as a sill, but bulges and forces the overlying rocks to dome
Volatility of magma
Depends on the gas content of magma (steam and CO2) and thus the original composition of the rocks that melted to form it. There will be a lot of steam if the original rocks contained hydrated minerals like amphibole and micas. Gases are dissolved in magmas at high pressure.
As the magma rises, solubility decreases and gas bubbles form. At c. 75% volume, bubbles will touch, amalgamate and there is an explosion
Viscosity of the magma
Increases with the complexity of the molecules that form the magma. Silica-rich minerals contain long chains or frameworks of silicate tetrahedra. So, silica rich magma is viscous (dry granite has a viscosity of 10^16 Pas) as long molecules get intertwined.
Water in solution decreases viscosity because it depolymerises some of the long silica chains.
Melting temperature of magma
Melting temperature of rhyolitic magma (700 degrees celsius) is lower than the melting temperature of basaltic magma (1200 degrees celsius) as it’s a mixture with water
Pahoehoe
Fast-flowing basalt that doesn’t crystallise and has very low viscosity.
Flows quickly and has a wrinkled top
Aa
Slow-moving basalt
crystals nucleate and increase viscosity
Shield volcanoes
Form from basaltic lava
e.g. hawaii
Flood basalts
Form typical stepped terrain as layers weather
Putorana plateau
Tuffs
Form in shallow water when there is basaltic magma.
in water gets into the vent from which the magma is rising, it transforms to steam due to rapid increase in temperature.
Steam produced rapidly and there’s an explosion that shatters the magma and creates tuff
Phreatic
Explosion off the type that creates tuff underwater
Andesitic magma
1100 degrees celsius
Forms at destructive plate boundaries
Contains water from amphibole in melted oceanic crust
Magma stalls and cools slightly in the crust. It fractionates and becomes more silica rich.
Lava domes
Form in volcanoes at subduction zones.
Where andesitic magma stalls and fractionates
e.g. mount st. helens
Will explode when gas content reaches c. 75% volume.
Gas bubbles can’t escape due to high viscosity of magma
Pyroclastic deposit
Fragments created from the explosion of a lava dome
Volcanic bombs
Large fragments of pyroclastic deposits
Cinder cone
steep-sided cones of pyroclastic deposits
Tephra
Highly aerated rock that solidifies as it’s flung in the air by a volcanic explosion
Plinian eruptions
- Column of pyroclastic deposit shoots up
- Outer section of the column sucks in surrounding air which heats up.
- Hot air rises due to lower density than surroundings
- Air reaches surrounding air of the same density and spreads out sideways
- Cold ash falls out
- Remaining gas is hotter than the surroundings and convects to form a mushroom cloud.
- Inner column doesn’t react with external air and doesn’t rise as high.
- Inner column collapses and forms a pyroclastic flow
- The pyroclastic flow traps air between it and the side of the volcano. It moves very quickly and wipes out anything in its path
Lahars
Super fast and devastating mud flows that start after heavy rainfall or snow melt due to the instability of the steep slopes of these volcanoes
Ultrabasic rock
Olivine, pyroxene, very little plagioclase
Peridotite
Picritic liquid
Basic
Pyroxene, plagioclase (Ca rich)
Gabbro (slow cooling)
Basalt
Intermediate (Not basic or acidic)
Amphibole, plagioclase (Ca and Na rich)
Diorite (slow cooling)
Andesite
Acidic
Quartz, Mica, K Feldspar
Granite (slow cooling)
Rhyolite
Melanocratic rock
Dark rocks, typically basic
Leucrocratic
Light rocks, typically silica rich
Fractionation in a magma chamber
As magma in a magma chamber cools, crystallisation occurs.
Order of crystallisation:
1. Olivine
2. Pyroxene
3. Amphibole
4. Micas
5. Quartz
As the order of crystallisation goes up in silica content, the remaining magma becomes more and more silica rich as other elements are removed.
Plagioclase feldspars follow the same trend:
1. Ca rich
2. Na rich
Benioff zone
Where earthquakes occur at subduction zones
Around the 600 degrees celsius isotherm
At the limit of the cold, brittle section of the subducting plate
Trench
Forms at the place where a plate subducts under another.
Not in isostatic equilibrium
Accretionary prism
Sea-floor sediments that are scraped off the subducting plate and form a heap in the trench
Olivine –> wadsleyite
Olivine isn’t stable at large depths as there’s too much pressure
Exothermic reaction
Happens at shallow places in the subducting slab than in the surrounding mantle. The increase in density drives subduction
Basalt –> eclogite
Eclogite is more dense and pulls the slab down. `
Where subducted slabs go
Either to upper/lower mantle boundary (670km) or to core/mantle boundary
Why wet magmas crystallise as they rise
The shape of the wet solidus means wet magmas can recrystallise as they are depressurised.
Water comes out of solution as the pressure decreases.
Why rhyolitic magma doesn’t rise by diapirism
Rhyolitic magma is too viscous for compaction to force the liquid portion out of a partial melt.
Metamorphism
Process which changes pre-existing rocks (in the solid state) into new minerals and/or gives them new textures.
New minerals form when the existing ones become unstable
Why metamorphosed rocks don’t return to their origin state
Water is lost during metamorphism
With decreasing pressure and temperature, the kinetic barrier for the reactions becomes too great
Basalt based metamorphic rocks
Regional metamorphism:
Amphibolite - Hornblende and plagioclase
Eclogite - Garnet and Pyroxene (red and green)
Subducting slab:
Blueschist - contains glaucophane.
Protolith
rock before metamorphism
What common rocks metamorphose to
Mudrocks –> pelites
Carbonates –> marbles
Sandstones –> Quartzites
Basalts –> Metabasites
Granites don’t change much
Burial metamorphism
Occurs in sedimentary basins when more sediments form on top of the existing rocks. Start of any metamorphic path
Diagenesis
Processes that change the sediment between deposition and lithification.
Lower pressure and temperature than metamorphism
Lithification
Sediment turns to rock
occurs during burial metamorphosis
Metamorphism in subduction zones
Low temperature & high pressure within slab
rock pushed down quicker than it can reach thermal equilibrium
Makes blueshist
Below volcanic arc, there is HP/HT making eclogite and amphibolite
Next to ponded magmas, those that stall and fractionate, there’s contact metamorphism
Regional metamorphism
High pressure & high temperature Rocks and heated and buried simultaneously During continental collisions Makes eclogite at HP/HT Makes amphibolite at MP/MT
When two continents collide, the thickness of the total crust effectively doubles. A plot of temperature against depth is a saw-tooth profile. Over time, thermal relaxation occurs.
Continental crust contains many radioactive elements. Heating occurs by radioactive decay. Upper layers remain at higher temperatures as they’re heated from below by this and lower layers remain at higher temperatures because there’s excess rock above, like a blanket. Heat loss by conduction is slower as there’s a larger distance to the surface.
High mountains are rapidly eroded, bringing hot rocks to shallower depths. Thus the geotherm is is also shallower than usual.
Prograde metamorphism
Involves an increase in temperature. Common during regional metamorphism
Lineation
Any linear feature in a rock (e.g. intersection of surfaces)
Foliation
Any planar feature in a rock (usually excludes bedding)
Deviatoric stress during (re)crystallisation
Aligns grains of crystalising minerals forming a fabric
This happens because then dislocations in the lattice can move an release strain
Stress on preformed minerals
Affects shape and orientation
How we learn about the conditions of metamorphism
Stress and orientation of the stress field from the mineral shape and orientation
PT space the deformation took place in from the minerals present
The order in which deformations occur show us stages in the metamorphism. E.g. a folded fabric meant first stress field created fabric and 2nd folded it
Porphyroblasts
Larger-than-average crystals in metamorphic rocks
What relationship between the porphyroblasts and the fabric can tell us
Porphyroblasts can be pre-, syn- or post-tectonic
Pre-tectonic: fabric grows around the porphyroblast
Syn-tectonic: the porphyroblast is altered in shape and/or orientation to match the fabric
Post-tectonic: the porphyroblast overgrows the fabric
Where granites can be found
Above subduction zones - Basic melts melt the continental crust above to form rhyolotic magma which is too viscous to reach the surface
At continental collisions:
e.g. apalachians through Scotland and US formed 500Ma from the closure of the lapetus Ocean. Now eroded away, exposing the granite at the core of these old mountains. Iceland hotspot has uplifted the region.
When continents collide and the crust overthickens, radioactive decay causes heating. Half-lives of the unstable elements in the crust are very long, so heating takes a long time. Thus, granites are post-tectonic i.e. much younger than surrounding rocks
Contact metamorphism
High temperature & low pressure
Contact aureole forms around the pluton as temperature decreases with distance from it. Minerals formed at highest temperatures are found on the edges of the pluton.
Does not create a fabric because there’s no shearing. This is passive process with only heating.
New minerals overlay existing fabric and if enough grow, the existing fabric disappears
Smaller grains that regional metamorphism as occurs on shorter timescales.
Contact aureole
Region within which metamorphism occurs around a hot intrusion. Edge defined by the appearance of these metamorphic minerals
As granite is at c. 700 degrees, in mid-deep crust, it has no aureole
Basalt always has a granite as it’s hotter
Wilson Cycle
Embryonic stage:
Uplift and crustal extension (e.g. East African Rift Valley)
Young Stage:
Rift valleys extend enough to start creating oceans (e.g. Red Sea)
Mature Stage:
Growing basin widens and oceans develop (Atlantic ocean)
Subduction stage:
Expanding system becomes unstable and oldest section of oceanic crust subducts back into the asthenosphere, forming an oceanic trench subduction system (e.g. Pacific Ocean)
Terminal stage:
Subduction outpaces the rate of creation of new oceanic crust (e.g. Mediterranean)
End Stage:
All oceanic crust between continental masses has subducted and the continents collide with each other to form an active fold mountain range (e.g. Himalayas)
Textural changes in mudrock in regional metamorphism
Mudrock –> slate –> phyllite –> schist –> gneiss
Phyllite –> schist –> gneiss is increasing in grain size
