Lecture Fourteen - Igneous and metamorphic geology III Flashcards
What three factors contribute to the type of igneous rock formed?
Magma composition.
Environment - intrusive vs extrusive.
Cooling rate (and volatile content).
What is the spectrum of igneous rocks and magmas?
Intrusive (plutonic).
To
Extrusive (volcanic):
Non explosive (coherent, glassy or crystalline).
or
Explosive (fragmental and gyroclastic).
What characteristics do intrusive magmas have?
Magma that doesnt make it to the surface and cools (freezes) in intrusions under the ground.
Different types of intrusions are distinguished based on their shape and size.
1) Discordant = Intrusions penetrate and cute across pre-existing rock layers.
- Dykes - Near vertile, tabular.
- Plutons - Irregular blob shaped. Numberous plutons may combine into:
>Stocks (
>Batholiths (>100km^2).
Define country rock, baked zone, stoping and xenolith.
Country rock or wall rock = Pre-existing rock into which the magma intrudes.
At the contact between intrusion and wall rock, there can sometimes be a ‘baked’ zone and contact metamorphism can occur (hornfels = contact metamorphosed mudstone).
Stoping = Blocks of wall rock break off and sink into the magma.
Xenolith = Country rock fragmnet that is surrounded by new igneous rock.
How can rocks formed in intrusive settings be distinguished by their texture?
Texture (size of crystals):
Intrusive megmas have cooled very slowly (weeks-months for shallow dykes/sills to a million years forlarger batholoths).
Able to grow large crystals:
Phaneritic = Interlocking crystals that are all visible with the naked eye (>0.5mm).
Aphanitic = Crystals too small for the naken eye (<0.5mm).
Porphoritic = Larger crystals (phenocrysts) surrounded by a finer ground mass of crystals/glass.
How can rocks formed in intrusive settings be distinguished by their composition (SiO2 %)?
What are the two kinds of catagories volcanoes can be separated into?
Coherent = Non-explosive, non-fragmental.
Can be glassy - cooled very quickly.
Can or crystaline - cooled more slowly (but still fine grained).
Fragmental = Explosive or pyroclastic.
What are the characteristics of glassy volcanic fragments?
Rock made of a solid mass of volcanic glass or tiny crystals by glass.
Volcanic glass - supercooled magma before crystals can form (near liquid temperatures).
1) Obsidian - Silicic (felsic) glass erupted as decite-rhyolite lava domes/flows. Black or brown. Fractures chonchoidally, sharp fragments -> used as tools and weapons by pre-inductrial people.
2) Tachylite - Mafic glass. Similar to obsidian but from mafic magmas (mach rarer).
3) Pumice - Highly vesicular, formed by eruption of bubbly/frothy magma or cooling frothy lava. Light grey or tan (low density). Dacite-rhyolite magmas.
4) Scoria - Similar to pumice, but darker in colour, often larger vesicles, from mafic (basaltic) magmas/lavas.
Why does magma explode (depressurisation)?
Pyroclastic - pyro = fire, clastic = fragmented rock with solid grains stuck together.
Internal gas pressure -> depressurisation.
Magma at depth is under great pressure.
When pressure is released as it rises to the surface, it explodes.
Fragments the magma or lava -> Explosion with a mixture of juvinile magma clasts (pumice or scoria), ash (<2mm in diameter), free crystals +/- country rock (crustal) lithic clasts.
The resulting deposits consists of carying amounts of these compnents deposited by air fall or avalanch flow -> Tuffs and Ignimbrites.
Why does magma explode (depressurisation)?
gravitational instability.
Sometimes (semi)-coherent ectruded lavas can become unstable and fall and fragment.
Consists of larger framents of colcanic debris -> volcanic breccias.
Common with collapse of extruded lava domes or viscour lavas.
May be at least helped by internal gas pressures.
‘Autobrecciation.’
What kinds of magmas are more likely to produce explosive/pyroclastic eruptions and rocks?
If a magma can build up presure by not allowing the gas bubbles to escape, there is a more violent erruption.
Felsic magmas - are more viscours, so gas can’t escape easilt, high degree of fragmentation and explosions are likely.
Mafic magmas - more commonly form non-explosive (effusive) lava flows whereas felsic magmas tend to erupt explosively.
Other factors are important - magma (of any composition) and water or ice = highly explosive.
Basaltic magmas - Relatively low water content. Low viscositiy, so gas can escape more easily, mainly effusive eruptions. Low
Rhyolite magmas - Relatively high water content. High viscosity, so gas bubbles cannot escape as easily. Eften explosive.
What percentages of crystals will magma erupts/not errupt at?
If a magma has <45% crystals, most magma can erupt.
if a magma has 45-60% crystals, some liwuif can erupt, but the rest becomes an intrusive igneous rock.
If a magma has >60% crystals it becomes too rigid to erupt and will likely form an intrusion.
What are xenoliths?
Can be pieces of crustal or mantle rocks.
Can be found in both plutonic and volcanic rocks.
Rising magmas break off and incorporate pieces of rocks they are passing though.
Provide information about mantle or deep crust.
Often have sharp/anglular edges or margins.
What is a magmatic envlave?
A patch of minerals in an igneous rock where the melt has not mixed fully (enclaves form due to magma mixing).
Related to, but often not identical in composition to the host rock (enclaves have a distinct genisis from hots rock).
Often have rounded edges.
