G1 : rock forming processes KI1 (generation of magma) Flashcards
what is magmatic differentitation?
a number of processes that cause a parent magma to evolve into magmas of different compositions. this leads to different igneous rocks being produced from a single parent magma.
what is fractional crystallisation?
as olivine and pyroxene form at high temperatures, they use iron and magnesium from the magma in their crystal lattices. the high temperature plagioclase crystals are rich in calcium. the magma become depleted in iron, mg and ca. the remaining liquid becomes enriched in silica, potassium, na and h2o because the early formed minerals are poor in these elements. over time the composition of magma changes
what is gravity settling?
crystals are denser than the liquid and settle out. early formed minerals (olivine and pyroxene) have a greater percentage of iron and are denser to sink to form s cumulate layer at the base of the intrusion or magma chamber. crystals that remain suspended in magma will react with that magma over time, following either side of the bowens reaction series, gravity settling removes crystals from remaining liquid so they can no longer react with magma changing the composition
what is filter pressing?
during crystallisation of magma, there is a point where crystals and liquid exist together as a slushy mass. due to the weight of overlying crystals, the liquid gets squeezed out, forming a separate. layer above. this liquid is depleted in the elements which are incorporated into the early formed crystals and enriched in the elements which form felsic minerals.
what is the discontinuous series in bowens reaction?
the crystallisation of minerals that are rich in iron and magnesium with silica. in a mafic magma, olivine will be the first mafic mineral to form at a high temperature (>1500c). as the temperature lowers, pyroxene, then amphibole and finally biotite will form. if cooling takes place slowly then the early formed, high temperature minerals react with the magma to form the next mineral down the series.
whats a reaction rim?
one mineral surrounding another, as a result of a reaction between the inner mineral with the melt, to form the outer mineral
Incomplete reactions for the discontinuous series
Corona structure
what is the continuous series in bowens reaction
describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich (anorthite) to more sodium-rich (albite)
anorthite-bytownite-laradorite-andesine-oligolcase-albite
whats a zoned crystal?
a crystal starts to grow of plagioclase feldspar when the temperature is high. as the magma cools the crystal continues to grow with new layers forming, gradually becoming richer in Na.
Incomplete reaction for the continuous series
how is bowens reaction series significant in the weathering of crustal rocks
lower temperature of crystal formation is more resistant as it is more used to the conditions on the surface.
What are the largest and most significant geological deposits of PGE commonly rich in
Platinum and palladium together with igneous intrusions are associated with sulfide minerals in layered mafic or ultramafic igneous intrusions (cumulate deposit)
When do magmatic sulfide deposits form
During the fractional crystallisation process when mafic magma becomes saturated in sulfide
What happens when magma can no longer hold sulfer in solution
The sulfer exsolves producing an immiscible sulfide liquid. Droplets from this liquid are denser than the magma and so sink through it.
What are siderophile elements
(Iron loving) platinum group elements and gold and nickel
What do siderophile elements do
Preferentially form metallic bonds with iron and will concentrate in molten iron
The only place a liquid like this exists is in the Earth outer core
What happens if siderophile elements do not have iron to react with
They behave like the chalcophile (ore loving) elements silver and copper and will concentrate in molten sulfide, bonding more readily with sulfer than with oxygen
They are therefore gathered up by sinking sulfide droplets and removed from the magma, to form layers that range from a few mm to a few m thick
What is the most reasonable mechanism for the largest sulfide deposits
Assimilation of country rock that is rich in sulfer or silica contamination from assimilation of silica rich country rock, which is thought to decrease the solubility of sulfer in a mafic magma
The skaergaard intrusion in Greenland
One of the most studied in the world and contains magmatic sulfide in layers rich in PGE, gold and selenium. It was intruded during the tertiary igneous province when the North Atlantic was opening
What happens when molten rock becomes solid
As a magma crystallises the crystals that form with have a different composition from the melt. When the crystals separate from the rest of the magma as they grow, the chemistry of the magma changes. A single magma could form ultramafic, mafic, intermediate and solicit rocks if there is enough time for differentiation.
If a parent magma is mafic than in theory what will it produce
-peridotite at the base of the intrusion, rich in dense olivine
-gabbro forming the main part of the intrusion
-diorite may exist towards the top of the intrusion
-granite may form veins or a thin layer at the top of the intrusion
What are major layered intrusions
Large mafic or ultramafic igneous intrusions that cooled slowly below the surface so there was time for them to form distinct layers. Most of the world platinum and chromium comes from them, giving an added incentive to understanding how they formed. Gravity settling is the key process in the key formation of layers of magnetite, chromite and platinum
The palisades sill in New Jersey
Crops out along the Hudson River and is over 300m thick, it was intruded into Triassic sediments at depth of about 3km, so that it cooled slowly and has now been exposed by erosion. This sill can be divided into three sections, each of which shows a different igneous process
1st section of the palisades sill
Only parts to cool rapidly are the upper and lower edges, which were in contact with cold country rock. These chilled margins have fine crystals and most importantly have the same composition as the original magma as they cooled before and differentiation took place. The rock is a basalt.
2nd section of the palisades sill
As the main part of the intrusion began to early formed olivine crystals began to sink by gravity settling. Olivine has a density of 3.8gcm-3 compared to 3.0gcm-3 for a mafic magma. The olivine crystals form a layer 10m thick at the base of the intrusion, just above the lower chilled margin
3rd section of the palisades sill
Crystallisation was taking place from both the top and bottom as the crystals grew in cooler areas. The main rock forming the intrusion has medium sized crystals and is a dolerite. The last part of the magma to crystallised is about 200m above the base and is a gabbro with coarse crystals, indicating it cooled slowly, as a result of fractionation, the composition is lower in mafic minerals than the original as the magma is depleted in iron and magnesium by the time the rock formed. It is richer in plagioclase as the last part of the magma to crystallise is enriched in silica
Hekla volcano in Iceland
Should be mafic as it is on a divergent plate margin, yet it erupts intermediate lave and has erupted rholitic lava in the past. The longer the period between eruptions the more acidic the magma is. If there is a long interval between eruption fractionation in the magma chamber causes more mafic minerals, which crystallise at high temperatures, to be found at the bottom and Felicia minerals at the top. Each ruction takes magma from the top of the magma chamber and then from lower in the chamber
The bushveld igneous complex in South Africa
Large layered igneous intrusion 400km x 800km with a volume of at least 1,000,000km3. It varies in thickness by reaches 9km in places. Formed at least 2 billion years ago, the complex contains some of the richest ore deposits on earth and the largest reserves of platinum group elements
What magmas are Felicia
Granitic and rhyolitic
What magmas are mafic
Basaltic, (gabbro and dolerite)
Magma formation
Pressure release
- the base of the crust is hot enough to melt mantle rock
-due to high pressure the rock does not melt
-a drop in pressure indicates “decompressional melting”
-pressure drops when hot rock rises (partial melting) to shallower depths
Why to magmas vary chemically
-initial source rock composition
-partial melting
-assimilation
-fractional crystallisation
Why do plutonic intrusions and larger igneous provinces often display a range of different rock types
-different composition of original magma
-magma mixing
-contamination of the magma by country rock assimilation
-evolution of a parent magma to produce one or more daughter magma through magmatic differentiation
What is magma mixing
Factors like density and thermal contrast keep different magmas separated. A vigorous stirring mechanism such as convection within the chamber is required to produce a magma that has an intermediate composition. The result combines the characteristics of the two magmas, often magma mixing is incomplete, resulting in blobs of one rock suspended within the other.
Stoping (contamination/assimilation)
Blocks of country rock from conduits or the walls/roof of the magma chamber are broken off by rising magma and incorporated into the magma as xenoliths, if these xenoliths melt and become assimilated into the magma they can contaminate it and change its bulk composition.
Why does magma migrate
It has a lower density that surrounding rock so rises upwards in the crust and may breach the surface (volcano). This transfers mass from deep to shallow parts of the earth, a crucial process in the earths system. Provides the raw material for soil, atmosphere and ocean
How does magma move and what factors increase the migration
-injection into cracks
-melting overlying rocks
-squeezed by overburden
Lower viscosity eases movement. Lower viscosity from - higher temperature means more fractures, lower silica content, higher volatile content
Effect of temperature of viscosity
Hotter - lower viscosity
Cooler - higher viscosity
Effect of volatile content of viscosity
Less volatiles - lower viscosity
More volatiles - higher viscosity
Effect of silica content of viscosity
Less SiO2 (mafic) - lower viscosity
More SiO2 (Felisic) - higher viscosity
Volatile content in magmas
Magmas contain small amounts of dissolved gas which is released as pressure is removed. Magma formed by melting of mantle rock has low volatile content, but those formed by partial melting of crystal rocks are volatile rich. A high volatile content decreases viscosity, and is one of the main factors in enabling some highly viscous melts to reach the surface. The release of gas during eruption will be explosive if magma is both viscous and volatile rich
Diapirism
Intruded buoyantly upward along fractures or zones of structural weakness through overlying rocks.
What are enclaves
Aggregate of minerals or rock observed inside another larger rock body. Usually referred to such situations in plutonic rocks. Micro granular enclaves in Celsius plutons result from the introduction of mafic magma into the magma chamber an it’s subsequent cool following incomplete mixing
Caldera
Collapsed volcano
Magmatic granite magma theory
Granite is derived by the crystal fractionation of magma
Granitisation granite magma theory
Granite is formed “in situ” by ultra metamorphosis
How does the magmatic theory involve Bowles reaction series
If crystal fractionation of a magma of basalt composition were to occur, one of its end products would be granite. Emplacements of granite plutons is synchronous to volcanic eruptions. Chemically, there is similarity in the composition of many granite plutons to their extrusive associates, the andecite-dacite-rhyolite series of rocks. This suggests there is a relationship between the emplacements of granite plutons and volcanism = orogenies
How are upper crustal granitic bodies formed
Magma is formed by partial melting in lower crust, form upwelling of hot mantle material. Would occur above a subduction zone. Magma is segregated from the source rock and ascend along a fracture or other conduit, over time periods as short as year to hundreds of year. Magma is emplacements in the upper crust by lateral spread along planes of weakness, and then by vertical thickening. The time scale is hundreds to thousands of years
What temperature is granite formed at
650-700
Dry granite
Is rhyolite
The melting point is high and increases with depth. In normal circumstances continental rocks are never hot enough to melt when dry
Wet granite
Is granite
Lower melting point that dry granite and decreases with increasing pressure
How is wet granite magma formed
Deep burial of water bearing rocks
Area of high heat flow in the crust
Water bearing minerals
Amphibole
Micas
Why is rhyolite rare
Rises slowly as it is viscous. Cooled before it reaches the surface = granite. As it rises the pressure dropped this reduces the effect of water in reducing the melting point so the melting point rises and the magma solidifies when It crosses the solidus
Hydrous granite
-presence of water lowers melting point
-melting point of 650
-due to the geothermal gradient gives this temperature at 20km
-this melt forms at shallow depth
As it moves upward it solidifies and therefore is not extruded
Anhydrous granite
-absence of water rises the melting point to 1050
-the geothermal gradient is greater at a depth of 35km
Because the melting point is higher it stays in liquid form as it is extruded at the surface - rhyolite, ash, pumice, obsidian, ignimbrite
How to measure the gradient of a geothermal
Gradient = temperature change / increase in depth
What is the geothermal gradient in volcanic regions
30-50
What is the geothermal gradient in ocean trenches
5-10
What is the geothermal gradient in metamorphic rocks of the Canadian Shield
10
Granite emplacements deep in crust beneath orogenic belt
Water source = melting of crust and water bearing minerals during subduction. The crystallisation of hydro bearing minerals acts to contain part of the magmatic water within a solidified igneous rock. Primordial water brought up from the deep mantle
Pressure source = mountain building, orogenic forces/compression
Source of heat = heat inside the earth - geothermal gradient
Yosemite granite formations
1)Basic magma rises and ponds beneath the crust. Heating causes partial crystal melting liberating granitic magma
2)MASH - melting, assimilation, storage, homogenisation
3)mixing if crust and mantle derived melts produces volatile rich intermediate magmas with sufficiently low density to rise buoyantly through the crust
Ocean to ocean margin magma
Sometimes more mafic or felsic in composition - dependant of the depth of magma generation
Shallow = basaltic intermediate
Deep = felsic
Basaltic magma
Very fluid with low silica
-Hawaii and iceland
Andesistic magma
Intermediate and is a common volcano
Rhyolitic magma
Rare magma - Yellowstone and chaiten
The solid solution series for discontinuous series
High temperature Mg rich olivine (forsterite) and low temperature Fe rich olivine (fayalite)
Magnesium is substituted for iron as temperature decreases
Solid solution series for the continuous series
High temperature Ca rich anorthite and low temperature Na rich Albite
Calcium is substituted for potassium as temperature decreases
Low temperature minerals in phase diagrams
When the two reaction series converge at a low temperature, the minerals that remain will not react with the remaining liquid. This final group of minerals to crystallise are the felsic minerals rich in silica. These are potassium feldspar followed by Muscovite mica and quarts at a temperature of 700c
Where does pure forsterite occur
Metamorphosed mg rich limestones and dolomitic metamorphic rocks
Where is Fo90-95 found
Ultramafic igneous rocks particularly dunkers and peridotites
Where is Fo60-90 found
Mafic igneous rocks like basalts and gabbros and sometimes andesites where it occurs with plagioclase and pyroxene
Where is Fa100-40 found
In Fe rich silica rich igneous rocks like rhyolite and granites
How to calculate the spreading rates of MORs
Found by radiometrically dating ocean floor basalts a known distance from the MOR, and by using pelagic microfossils to find the age if sediments at known distances from the MOR
Why is each MOR different
They have their own style of faulting, volcanic activity and spreading rate depending on the strength of the ocean crust, and is dependent on how cold and brittle the upper part of the lithospheric plate is as this alters its properties.
What dies the rate of spreading govern
How much heat is carried upwards by magma and how quickly it is lost from the top of the magma chamber mostly by hydrothermal activity
Slow spreading ridges e.g. the mid Atlantic ridge
South Atlantic - 4cm per year
North Atlantic - 2cm per year
Slow spreading ridges attributes
Median rift valleys up to 20km wide and 3km deep with very rugged ‘mountains’ at the crest. Some parts of the ridge are covered by young lava flows and pillow eruptions. Other parts show faulting instead. Sonar images show vast areas of the ocean floors covered with rough abyssal hills on either side of the ridge
Slow spreading ridges magma
Often insufficient partial melting to maintain a magma chamber although there may be mush zones where small volumes of melt exist within softened mantle. Magma chambers are short lived and discontinuous in extent along the axial rift. Each eruption is a distinct event. Some sections of the mid Atlantic ridge has small volcanic cones formed this way
Fast spreading ridges e.g. the east pacific rise
Up to 16cm per year between the Anzac and pacific plates
Fast spreading ridges attributed
High elevation, dome like topography resulting from many layers of lava built up from great supply of magma. Deep, hot rock rises quickly and more heat passes into the plate. Rock is a poor thermal conductor so heat cannot escape equally quick so the lithosphere becomes hotter, weaker and more ductile. The ridge crest cannot subside due to rising magma. There is no rift valley just a crack a long the smooth crest
MOR effects on global sea level
Increased activity at an MOR making it bigger taking up more space in the oceans this causes a rise in sea level and the displacement of water onto the continents. Calculations of the volume of MORs correspond well with the vail sea level curve. Long term high sea levels correspond with times of high plate velocities when lots of new material was being formed by sea floor spreading.
A high rate of sea floor spreading leads to an increase in subduction causing subsidence and flooding if continental margins
Decompression melting
Rising magma experiences a decrease in pressure. Expansion causes a reduction in temperature with the loss of heat as molecules use heat the move apart. Melting occurs as the melting point also decreases as the pressure decreases
What occurs at depths
At depths of 100 km to 200 km the mantle is at temperatures if about 1300c close the the melting point of peridotite. It is very hot and soft but still solid because the pressure is high.
What changes take place as mantle material rises below the MOR
1) the pressure drops and the mantle material expands, it experiences a cooling effect due to solely the increase in volume. This is an a diabetic change
2) as the pressure drops the molecules in the rock move more freely and therefore the melting point falls
What happens when rising mantle rock is hot enough and rises enough
It starts to melt by decompression melting. Once me,ting starts, about 3% of the rock melts for each 10km that the rock rises. More melt forms if the mantle beneath the MOR is hotter than normal, as it will start to melt deeper down, as happens with mantle plumes
What does the amount of magma produced depend on
The latent heat of fusion (melting). It takes a lot of heat to change silicates from the solid to the liquid state, which helps to explain why melting of mantle material is usually partial
What does seismic tomography suggest
Some magma chambers are made of a thin layer of molten rock lying on top of a partly molten mush zone the melted zone is monkey a few metres thick and about a km wide. Magma chambers with erupt magma all the time are said to be continuous, as at fast spreading MORs
What happens as the melt forms as thin films between the crystals
The liquid moves upwards joining other films of melt, growing in size and accumulating in a shallow magma chamber where it is cooled by the ocean above, including by infiltration of sea water, as more melt percolated upwards, the pressure inside the chamber rise until the tool of the shallow chamber splits apart to give a narrow crack, about a metre wide. The pressurised melt flows up the cracks and erupts onto the sea floor as basalt lava. The melt in the crack solidified to form a dolerite dyke as the system cools, the magma chamber solidified to form the gabbro of the lower ocean crust below the sheeted dykes and pillow lavas
How does the thickness of the oceanic crust reflect the amount of melting.
At normal mantle temperatures, enough melt forms to create ocean crust that is 7km thick, under Iceland, the mantle is much hotter due to a mantle plume. There is much more melting and the crust is up to 20km thick, as determined by gravity surveys
Why is it only possible to locate magma chambers that are molten
If the chamber solidifies it is hidden from seismic surveys. This applies to 90% of volcanoes. And by the time the new magma is formed perhaps 20000 years later, the old chamber will have been added to the lithosphere and spread away from the ridge
What are ocean core complexes
Large domes hills on the sea floor about the size of Ben Nevis with striations (grooves) clearly visible from top to bottom. Most scientists agree that these grooved surfaces are faults (detachment faults), where lots of small amounts of motion over time has built up these hills
What happens in the symmetrical mode of spreading
Ductile asthenosphere wells up to fill the gap and partially melts (10%). The melt sues and solidifies to form gabbro plutons, dolerite dykes and basaltic kavas at the crust. The melt may accumulate in one of more small mid crystal magma chambers on the way up. Once the crust has formed, the stresses pulling the plates apart form normal faults, with displacements of 100m, which adds some 5-10% of strain. The structure if both plates is similar as are their accretion or spreading rates; new material is added equally to both sides
How is the newly discovered asymmetric mode of spreading different
This mechanisms is triggered when less melt is delivered from the mantle, so a thinner crust is formed, and some of the normal faults may reach as far as the mantle. If seawater can percolate down then to reach peridotite it will react to produce the mineral serpentine, and the rock serpentinite (very weak) and lubricates the fault, making it easy to continue slipping. Such faults may penetrate right through the lithosphere (detachment fault) and accumulate displacements 10s of Kms over millions of years. The hanging wall place receives less melt than normal and so is thinner. The football plate is formed by pulling mantle material up. However, in some places sampling suggests their may be significant amounts of gabbro intruded into it. If all the plate separation is taken up by slip on the detachment fault, then no new material will be added to the hanging wall plate, and spreading will be 100% asymmetrical
What will autosub measure during the research if sea floor spreading
-Variations in the earths magnetic field - infer the spreading rate of each plate and compare this between areas if symmetric and asymmetric spreading.
-also measure high resolution topography (shape of sea floor), showing details of volcanoes and faults and will measure quantity’s such as the cloudiness of the seawater which may show the presence of hydrothermal plumes
Quartz (stability)
Stable at the earths surface as a result of it being resistant to weathering and erosion and resistant to being altered
Olivine (stability)
Stable in the mantle but easily altered, weathered, eroded and reacts to change its surface at the earths surface
Peridotite composition
Mainly consists of olivine. The crystals are bright green and usually large (5mm) clearly visible
What does the top layer of basalt have at the mid Atlantic ridge
Many cracks, fissures and faults
What happens when sea water sinks into cracks in the top layer of basalt at mid ocean ridges
It is warmed by the heat of the mantle below and as it is warmed it begins to rise again this then reacts with olivine and forms mineral serpentine which makes up the rock serpentinite.
How does serpentinite form
Sea water sinks into cracks in the top layer of basalt at a MOR and is heated which causes it to rise and react with mineral olivine to form mineral serpentine which forms serpentinite rock
What is the texture of serpentinite
Soap like and weak texture, which allows the rock to slip
Feels smooth and slippery
What does serpentinite do the faults
It lubricates them which exacerbates the rate of slippage of the fault. So deeper peridotite is brought nearer to the ocean surface, thus peridotite reacts with the warm seawater in the fault and more serpentine is formed (cycle continues)
What happens with serpentinite at subduction zones
Old, cold, dense, thick oceanic lithosphere starts to descend beneath the less dense continental lithosphere. The oceanic crust still contains the hydrous minerals such as serpentine, when the oceanic crust descends about 100km deep the water from the hydrous minerals is released due to heat and pressure of the environment. The water rises and causes overlying mantle to melt and magma is formed the chemistry of this magma is complex
What type of magma is formed at subduction zones
Andesitic with high silica content, with trapped gases where the only way to escape is through violent explosive eruptions. E.g. the Andes and other similar chain volcanoes behind subduction zones such and Indonesia and japan
What is magma at ocean ridges like
E.g. Hawaii
Formed of oceanic basalt so this magma is lower in silica and resulting eruptions are mainly runny lava, gas can escape easily and eruptions are rare
What state is the mantle
It is solid not liquid
Why do rocks partially melt
As they are a mixture of minerals there is not single melting point so it will begin to partially melt when individual minerals exceed their melting temperatures
How does the mantle source rock remain completely solid while still producing liquid
Miner,as with the lowest melting temperatures do so first along the edges of crystals, and that all the pockets of melt produced are linked (like a sponge) with the enormous pressure and the plastic nature of the remaining rock at depth the solid crystals are squeezed into the pockets of liquid melt which is forced to flow out (squeezing a sponge dry) this liquid flows out and collects in spherical regions around a km in diameter these absorb melt from the surrounding zones and rise quickly to the surface is waves.
