Week 3: DYNAMIC PLANET Flashcards
What are igneous rocks?
-crystalline
-all crystallise from magma
-magma is molten rock in the subsurface and usually consists of…
liquid rock (predominantly silicate)
± crystals
± gas bubbles
Variation in the composition of that liquid and the proportion of crystals and gas bubbles will generate a magma that has different chemical and physical properties.
Why do magmas form?
Magmas form because the interior of the Earth is hot
What are the heat sources for magma formation?
When Earth formed from accretion of colliding planetesimals, kinetic energy from the colliding planetesimals led to heat energy. The Earth was so hot that most of it probably melted.
Another heat source was gravitational energy from iron sinking to form the core
Throughout the Earth’s history heat is being added from radioactive decay
(The hot Earth loses its heat through magmatism)
What is the composition of the Earth’s surface?
Continental crust
Andesitic – “intermediate”
52-63% SiO2 (high in silica) , high in K2O and Na2O
Dominant minerals= Quartz, Feldspars, Fe and Mg minerals
Oceanic crust
Basaltic – “basic”
45-52% SiO2 (less enriched in silica) , high in FeO, MgO, and CaO
Dominant minerals= Olivine, pyroxenes and plagioclase
Upper Mantle
Peridotite – “ultrabasic”
<45% SiO2, high in FeO and MgO
Dominant minerals= Olivine and pyroxenes
What are the main locations of melting/where do magmas come from?
upper mantle/aesthenosphere (ultrabasic in composition)
- hot spots
-decompression
…and lower continental crust (intermediate in composition)
-subduction zones
-orogenic belts
are the two main location where melting happens
lead to two major groups of igneous rocks (give or take):
-ultrabasic mantle melts to form a basic melt
-intermediate continental crust melts to give an acidic melt
If the Earth’s interior is mainly solid why is any melting happening?
volcanoes exist so we know that there must be molten rock- magma- at depth
but we also know from seismology that the Earth’s interior is mostly solid to base of mantle
production:
1. localised
2. require special conditions
melting caused by:
-increasing temp of rock to above melting point
What is the solidus?
temperature at which rocks start to melt
melting point of rock increases with pressure & depth= solidus profile
-melting can only occur if the geotherm (Tvs depth curve) intersects the solidus (which it normally doesn’t so in general rock does NOT melt as you go downwards)
What does geotherm mean?
relationship of temperature to depth in the Earth
what are the 3 causes of melting?
- heating
- decompression
3 addition of water
4 orogenic belts
How does heating lead to melting?
Introducing heat:
-through convection bring heat up from the hotter interior of the Earth up towards the Earth’s surface
-this very often happens through upwelling of warm, convective mantle plume
-by incteasing the temperature of depth can cause geotherm to intercept the solidus
-so melting occurs relatively deeply (around 100km) through partial melting of mantle peridotite
-basaltic melt rises due to its lesser density of the new phase
-this is how Ocean Islands/sea mounts form
Example= Hawaiian Chain
-Mauna Kea= an Ocean Island Volcano generated because of heating beneath the crust through mantle convection
What is decompression melting?
-most common cause of melting
-no heat is introduced
1. ocean crust is extended and thinned (lithosphere)
2. this removes the “lid” over the mantle and the shallow geothermal gradient is steepened
-by completely thinning the lithosphere to nearly 0 able to lift the geothermal gradient through the solidus
3. basaltic melt is produced through partial melting of the peridotite mantle (this melting occurs very very close to the surface unlike heating melting) at a few km depth
4. this is how ocean crust is produced at mid ocean ridges
all oceanic crust in the world generated by decompression melting. 2/3 (70%) of world covered in oceanic crust
What does water saturated solidus show?
the temperature at which the rock will melt if that rock is saturated with water.
How does melting occur through the addition of water?
addition of water to crystal structure of a mineral (usually creating a new mineral)= changes fundamental properties of rocks including decreasing its melting temperature.
How do you add water to rock to cause melting?
via subduction (ocean-ocean subduction and ocean-continent subduction)
-occurs to a lesser degree in other settings
during subduction as we take this oceanic crust that has been saturated in water downwards we recombine the water into the molecular structure of the rock lowering the melting point of the rock to below the geothermal gradient
-this generates melting at usually great depths of 100-250km
once generate melting that molten material is less dense than the solid rock so it will rise upwards.
Basalts, Andesites and Granites produced (more complex)
Example= Japanese Arc, Andes
How does melting occur in orogenic belts?
Rather more complex, but is fundamentally by heating.
1.Continental-continental collision effectively results in doubling of crustal thickness
2. The cold underriding slab is warmed
3. Rock is a terrible conductor of heat so the thick continental crust cannot lose radiogenic heat fast enough
4. Resulting in heating of the crust above the granite solidus.
Example= Himalayas
Normal situation:
-rock is not molten beneath the Earth’s crust
Unusual situations:
1. Hotspot/orogenic (mantle plume)= can increase geothermal gradient to above the solidus
-change temp
- Mid-ocean ridges= can increase geothermal gradient to above the solidus
-change pressure - Island arc (subduction zone)= reduce solidus so moves before the geothermal gradient
-change H2O
Where can magmas be derived from?
Mantle or crust
What determines magma composition?
- Composition of the source rocks
- Degree of partial melting
How does composition of the source rocks affect magma composition?
Mantle peridotite = ultrabasic
Olivine, Pyroxene and Garnet
‘Average’ continental crust = intermediate
Feldspar, pyroxenes or micas, lesser Quartz and amphiboles
How does degree of partial melting determine magma composition?
Total melting of mantle peridotite (ultrabasic)→ ultrabasic magma
Total melting of ‘average’ crustal rock (intermediate) → intermediate magma
*In reality, total melting rarely happens as the parent rock is made up of a variety of minerals with different melting points, and temperatures aren’t high enough.
What happens when we have partial melting?
Bowens reactions series:
-melting temperatures
Lower temperature minerals will melt and rise, leaving behind a residue of higher temperature minerals
(Magmatic differentiation)
-Mantle peridotite= basaltic melt (Pg Feldspars and pyroxenes)
-Intermediate lower continental crust= felsic melt (K and Pg Feldspars, Quartz, micas)
Why do magmas rise?
- They are less dense
Melting causes the break-up of the organised mineral crystal structure,
Atoms are more randomly and less efficiently organised – a more open and less tightly (densely) packed structure. - They are viscuous (can flow)
Viscosity varies with composition which has implications for:
-eruption style (explosive or non explosive)
-architecture of volcanoes
Where do magmas go? What is it controlled by?
1 move and crystallise beneath the Earth’s surface= intrusive igneous rocks
Deep (>2 km) – “plutonic” – tend to be coare grained
Shallow (<2 km) – “hypabyssal” – between plutonic and volcanic
OR
- erupt on the Earth’s surface – “volcanic” – tend to be fine grained
Controlled by:
-Rate of magma production
-Temperature of the host “country” rock
-Size and shape of the conduits (surface area: volume ratio)
How do rates of cooling and crystallisation of magmas vary?
Intrusive melts cool slowly
This is not the case for extrusive (volcanic) rocks.
This is because rock is a very poor conductor – the magma loses its heat to the country rock slowly.
slow cooling rates result in “Fractional Crystallisation”.
As the temperature descends, high temperature minerals form at their melting T, leaving behind a melt that is enriched in the components of lower T minerals.
Why are magmas complex mixtures?
have already undergone differentiation from their parent material during the partial melting
What textures do intrusive igneous rocks have? Why?
“phaneritic” texture
-this is because the slow cooling results in the ready formation of crystals out of the melt (visible to naked eye)
porphorytic texture
-phenocrysts sitting in a finer ground mass results ecasue the feldspars higher melting temp than the quarz and the mica so therefore one of the earlier crystals to form so abundant space for formation of these large crystals
cumulate texture
-identified in the field not hand specimens
-layers of systematically lower and lower temperature minerals
What is magma mixing?
Magma mixing is another example of magmatic differentiation.
It is the physical mixing of two magma types to give a new homogeneous one.
e.g. acidic (rhyolitic) magma formed by partial melting continental crust mixes with basic (basaltic) magma formed by partial melting in the mantle to form a melt with intermediate (andesitic) composition.
What is stoping?
-differentiation process
Partial melting of wall rock and “stoping”.
Heat provided by the magma can melt the wall rock
blocks of wall rock can be incorporated into the melt (stoping)
The material dissolves and mixes into the melt to form a new composition.
More common in basic intrusives.
