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
