From Minerals to Rocks: How the Crust Works (Lectures 15-20) Flashcards
How are igneous rocks brought about?
Solidification from hot molten material (magma)
What evidence is there of bodies of magma at depth in the crust?
Active volcanoes
Hydrothermal activity
What is the major chemical differentiation that occurred inside the Earth?
Fe-S-Ni melts sank to the centre of the planet to form the core
Basaltic melts moved upwards to form the crust
For igneous rocks, which minerals are present at 40, 50, 60 and 70% silica content respectively?
40%: olivine + pyroxene
50%: low olivine, pyroxene, Ca-rich plag feldspar
60%: plagioclase feldspar, hornblende (amphibole)
70%: quartz, K-feldspar, Na-rich plag feldspar, biotite
What are the terms for the fine-grained and coarse-grained variants at 40, 50, 60 and 70% silica content?
40%: peridotite (coarse) and picrite (coarse)
50%: gabbro (coarse) and basalt (fine)
60%: diorite (coarse) and andesite (fine)
70%: granite (coarse) and rhyolite (fine)
How do Si-rich and Si-poor rocks differ in colour?
Si-rich are pale (leucocractic)
Si-poor are dark (melanocratic)
What are the three main ways to make the geotherm of the mantle intersect the solidus to produce melt?
Stretching
Hot spots
Add water
How does stretching induce melting of the mantle?
5 points
Movement of rigid plates on the surface sets up stresses within plates
Extensional stresses thin the plate
Reducing plate thickness pulls up the underlying mantle
Upwelling mantle too fast to cool conductively (behaves adiabatically)
Hotter than stable geotherm at any depth so melts
What happens if the stretching of a plate were to continue?
4 points
Continental crust ruptured
Formation of a new ocean basin
Mantle upwelling causes melting
The melt rises, solidifies and forms new oceanic crust
How do hot spots induce melting of the mantle?
2 points
Unsteady convection of the mantle causes narrow areas of hot upwelling material called plumes
Geotherm is hotter than usual and intersects the solidus at depth and causes extensive melting and volcano formation
How does adding water induce melting of the mantle?
6 points
The oceanic crust formed at mid-ocean ridges is in contact with seawater
Seawater penetrates upper few km of the hot crust
Hydration of pyroxene forms amphibole
Subducted ocean crust reverses hydration
Water released and flows into the overlying mantle
The melting point of the wet mantle is lower than that of dry mantle
What is peridotite made of?
Mostly olivine
Pyroxene
Minor spinel, plagioclase or garnet
How does basalt form from the mantle melt?
The mantle is modified during ascent and loses olivine
Why does melt migrate upwards towards the surface?
Buoyancy
Compaction of the hot and weak solid residue (squeezes out melt like a sponge)
When does rising melt stop?
At a level of natural buoyancy
OR a low-density material, i.e. granite, provides a barrier
What happens if mantle melts pond at the base of the crust?
3 points
Hottest magmas are basalts (1100-1300 degrees C)
Melting T of the crust is 700-900 degrees C
Hence the crust melts
Which processes take place in a magma chamber?
5 points
Magma stalled in the crust at its LNB forms a magma chamber
Cools by conduction and convection
Crystallisation occurs
Order minerals appear controlled by melt composition and pressure
Many crystallised first are denser than the magma and sink to the base to form cumulates
What does fractional crystallisation change?
The residual composition of the magma
Si % increases
What are plutonic rocks?
Large bodies (>100's m) that cooled well below the Earth's surface Coarse-grained (>5mm)
What does hypabyssal mean?
Smaller intrusions (cm to 10's m) at shallow depths Fine- to medium-grained (1-5mm)
What is a batholith?
A large body of magma (10-1000 km scale) emplaced deep in the Earth’s crust
When are batholiths revealed?
Substantial crustal uplift and erosion
What is a xenolith?
Fragments of rock from another source
Derived from the surroundings, different parts or earlier stages of igneous intrusion
How are country-rock xenoliths incorporates into the magma?
Process called stoping
What are dykes?
Steeply inclined or wall-like bodies formed by magma filling fractures
What does it mean for dykes to be discordant?
Dykes cut across bedding planes in the country rock
When are dyke swarms seen?
Stretching of the Earth’s crust produces many fractures
Large numbers of parallel or radiating intrusions occur
What are sills?
Sheet-like intrusions emplaced along bedding planes (concordant)
When do laccoliths form?
A concordant sheet of magma has bulged to dome the overlying country rocks
What does the behaviour of a volcano depend on?
Volatile content (gases)
The viscosity of the magma
Rate of lava emission
Environment (in water, under ice, on the surface)
What kind of volcanic eruption is seen in the intraplate setting?
Mainly basaltic
Low viscosity magma (fast-flowing)
Loses volatiles easily (not explosive)
What are the two types of basaltic lava flow?
Pahoehoe: fast-flowing, very low viscosity as it does not crystallise during flow, thin with wrinkled tops
aa: slower-moving, nucleates crystals throughout, viscosity increases, a mass of rubble
The crystallisation of the flow crust may result in what?
Formation of lava tunnels
What kind of volcano is formed in the intraplate setting?
Shield volcano
What are flood basalts?
Vast quantities of lava that spread out over enormous distances
Outline intraplate volcanism underwater
At mid-ocean ridges
Basalt generally forms pillow lavas
Can be highly explosive in shallow water as steam is produced
How is magma released at subduction zones?
Magma stalls and cools in the crust and fractionates towards Si-rich composition
Assimilation of melted country rock
Erupted magma is cooler and more Si-rich (andesite but rhyolites occur rarely)
What occurs when gas is built up in lava domes?
Dome collapses to form a flowing cloud of gas and rock
Outline the process of an explosion of a lava dome (including the build-up to)
(6 points)
Assimilation of continental crust and wet mantle source means high gas content
High viscosity so gases can’t escape
Decompression so bubbles grow
At 75~ % volume, bubbles touch, magma loses strength
Explosion triggered
The explosion shatters lava into rock fragments, mineral fragments and glass shards
How are cinder cones produced?
Successive layers of small pyroclastic fragments
What happens when explosive eruptions produce large amounts of fine ash, and surrounding air is entrained in the eruption column?
(4 points)
Powerful convection currents set up
Eruption columns high into the stratosphere
Widespread tephra dispersion occurs
Plinian eruption type
What are lahars?
What can trigger them?
Super-fast mudflows
Abundant rainfall or snowmelt
What can occur when the volcanic edifice collapses?
Tsunami from the collapse of volcanic islands
Magma upwelling triggers eruptions as volcano collapses from being too steep
What is a pyroclastic flow?
How are they formed?
The base of nascent column collapses to form a cloud of hot gas and ash, pumice and rock debris that travels very fast across the ground
If the energy of the eruption can’t sustain the eruption column
Define tephra
Collective term for all pyroclastic particles
Usually applied to airfall material
What is the melting process for the mantle at mid-ocean ridges that forms basalt?
( 4 points)
Melting of mantle leaves a depleted rock of olivine and pyroxene
Picritic liquid forms by partially melting peridotite during adiabatic decompression
Moves upwards as it less dense than peridotite
Loses olivine during ascent and becomes a basalt
What causes the partial melt of the mantle to move towards the mid-ocean ridges?
The flow of the mantle as the plates move apart creates a pressure gradient
What evidence is there for the composition and structure of the ocean floor from the ocean floor itself?
(4 points)
Magnetic stripes preserved in oceanic crust
Seismic profiles - 4 distinct layers within the crust
Deep-sea drilling penetrates upper few km
Submersibles - observe volcanic activity
What are ophiolites?
Slices of oceanic crust emplaced onto the margins of continents
What do ophiolites indicate the structure of the oceanic crust is?
Top-down: sediments, pillow lavas, sheeted dyke complex, gabbros
What processes take place at mid-ocean ridges?
3 points
Spreading causes extensional faults
Shallow, small earthquakes - thin brittle layer overlying hot, ductile layer
Hydrothermal circulation - cools crust to a depth of 2km, water removes metalliferous elements
What is an accretionary prism?
Stacked repeated sequences of sea-floor sediments separated by low-angle faults
Which metamorphic rocks were formed from basalt by intense deformation and high-pressure?
Eclogite - garnet and pyroxene
Blueschist - blue amphibole called glaucophane
For a subducting slab the depth at which rocks change into another is not the same as the surrounding mantle, what does this do and why does it happen?
Olivine changes to wadsleyite at a lower depth
Wadsleyite is denser so this pulls the slab down
Olivine to wadsleyite is exothermic so happens at shallower levels
How are ophiolites potentially formed?
Parts of young oceanic crust formed by back-arc spreading, thrust onto continents during continental collision as they were too hot and buoyant for subduction
What is back-arc spreading?
Extension caused by slab roll-back
What two possible outcomes are there for subducted slabs?
Penetrate down to the base of the mantle
Get bent and stuck at the upper/lower mantle boundary
Where does the water come from in subducted slabs?
Water added by hydrothermal alteration at a mid-ocean ridge
Hydrated basalt carried down where water is released as the temperature rises
What do volcanoes look like above subduction zones?
Steep sides and explosive eruptions
Andesite mostly, but basalt or basaltic andesite also common
Outline the properties of andesite
3 points
Fine-grained intermediate rocks containing amphibole and biotite
Viscous magmas due to high silica %
Viscous because wet magmas crystallise when depressurised
How does silicic magma form plutons and batholiths?
4 points
Wet granite melts are buoyant and gravitationally unstable
Granite plutons formed by magma rising along fractures and congregating in inflating chambers
Upwards movement of melt in dykes feeds large intrusions
Intrusions get bigger by assimilation and stoping
What is metamorphism?
The process whereby new minerals and/or textures form in pre-existing rocks in the solid state, generally at increased P and T
What does metamorphic assemblage depend on?
Bulk composition
Pressure
Temperature
Why do metamorphic minerals often not revert to the to assemblages stable at lower P and T as they reach the surface (retrogression)?
Loss of water on metamorphism - must be re-introduced and is very hard to do so
Speed reactions occur is controlled by T, at lower T reactions are much slower
If retrogression is very uncommon, what can be said about the mineral assemblages at the surface?
They are metastable
What is the start of metamorphism?
Diagenesis
During which sediment is turned into a rock (lithification)
What is contact metamorphism?
High T, low P metamorphism
Followed by rocks which are intruded by plutons
Where does high P, low T metamorphism take place?
How are these conditions obtained?
When are these rocks seen?
Only in subduction zones
Rock is pushed down too rapidly to reach thermal equilibrium
Only see rocks from this if brought up rapidly before heating up i.e. by continental collision
When does high T, high P metamorphism take place?
Where does it take place?
Heating rocks with burial
Occurs in zones 100’s of km wide and 1000’s of km long called orogenic belts
Called regional metamorphism
During continental collisions
What is orogeny?
What is orogenesis?
Orogeny = continental collision, the environment of regional metamorphism Orogenesis = creation of mountains
How are higher than normal temperatures generated for regional metamorphism?
Erosion: high mountains are rapidly eroded, brings deep, hot rocks nearer the surface and makes a shallower geotherm
Radioactive decay: continents contain abundant granite, which is enriched in U, Th and Rn. These decay and generate heat
The combination gets rocks away from the stable geotherm
How is a rock fabric formed?
4 points
Prograde metamorphism involves a T increase, and generally a P increase
Increasing P involves large rock movement, so shearing stresses occur
Squashing/folding/stretching a rock affects the shape and orientation of minerals that are thermodynamically stable during the deformation
Deviatoric stress applied during mineral growth
What are the two types of rock fabric?
Foliation: any planar feature in a rock (bedding usually not referred to as a foliation)
Lineation: any linear feature (e.g. intersection of two surfaces, or alignment of elongate minerals)
Define cleavage
Secondary foliation
Generally cutting bedding
What can be decoded from mineral shape and orientation?
Strength and orientation of the stress field
What can be decoded from the mineral assemblage?
Where in PT space the deformation took place
What are porphyroblasts?
Larger than average crystals in metamorphic rocks
The relationship of porphyroblasts to the fabric tells us what?
When they grew relative to the fabric-forming deformation event
What is the process of metamorphism for mudrock?
Mudrock -> slate -> phyllite -> schist -> gniess
With contact metamorphism, what happens to T with distance? and what is a contact aureole?
Maximum T reached decreases away from the intrusion
A series of zones develop with minerals characteristic of high T found near the contact and lower T minerals further away
When is a hornfels created?
In the hottest parts of the aureole
Recrystallisation is so intense it obliterates any pre-existing fabric
What is the equation for the period of time for a pluton to cool?
What can the equation be approximated to?
τ = (a^2)/(π^2 x κ)
a is diameter
κ is thermal diffusivity
τ (years) = (a^2)/300 (a in m)
When do these respective rocks form:
eclogite, amphibolite and blueschist?
Eclogite: high P, high T
Amphibolite: medium P, medium T
Blueschist: high P, low T
Why is blueschist formed in subducting slabs?
The thermal anomaly of down-going slabs means rocks are metamorphosed at high P and low T
Steady shearing makes them strongly foliated
What forms eclogite in a subducting slab?
Basalt of the oceanic crust at greater depths than blueschist forms
Outline the Wilson cycle
- Embryonic stage: involves uplift and crustal extension of continents with rift valley formation
- Young stage: evolution of rift valleys into spreading centres resulting in a narrow, parallel-sided sea
- Mature stage: Widening of growing basin and development into a major ocean with the production of new oceanic crust along the ridge system
- Subduction stage: Expanding system becomes unstable and away from the ridge, the oldest lithosphere sinks into the asthenosphere (subduction)
- Terminal stage: Subduction outpaces new crust formation, the ocean contracts, island arcs collide and create young mountain ranges
- End stage: all oceanic crust has subducted, the continents converge along a collision zone with an active fold mountain belt. Plate boundary becomes inactive and becomes a new zone of weakness, restarts
Give examples of each stage in the Wilson cycle
- Embryonic stage: East African Rift System
- Young stage: the Red Sea
- Mature stage: Atlantic Ocean
- Subduction stage: Pacific Ocean
- Terminal stage: the Mediterranean
- End stage: the Himalayas
