Volcanoes & Mantle Melting Flashcards
Types of eruption:
Effusive - slow runny lava e.g. Pahoehoe or a’a - basaltic
Explosive - lots of volatiles -rhyolitic
How do different compositions of lava get formed?
All lavas apart from basalt have to have been altered. This can occur through magmatic fractionation in large chambers or by percolating through continental crust.
What are Large Low Shearwave Velocity Zone?
Areas at the bottom of the mantle where shear waves slow down, implying there is a lower density material, or P/T changes. These could be evidence for upwellings related to the downwellings of slabs.
three ways mantle can be melted:
1 - anomalous higher temperature areas like hotspots
2 - adiabatic (no heat lost) decompression melting decreasing pressure - MORs
3 - moving solidus by adding volatiles - subductions
What is the mantle melting regime?
A triangle of melt is formed as rocks closer to the centre of the upwelling will melt more. The higher the rock gets in the regime, the more it partially melts and the maximum melt is the shallowest depth reached by a rock.
In order to calculate total melt thickness and so the crustal thickness, it is assumed that all melt formed in all the regime finds its way to the ridge through fractures. In this way, the thickness is a function of the depth of the solidus.
What are Ophiolites
obducted oceanic crust.
What are Rift Systems?
Breaking apart of plates, with swarms of seismicity and volcanism in the form of dykes.
What is the structure of a magma chamber?
There are networks of several melts in a large mush slowly transferring upwards. One large chamber could exist but it would take a long time to build up.
In contninental settings, magmatic differentiation must occur so felsic magmas are low enough density to rise.
The lower the density they quicker it moves.
Differences between mafic and felsic melts?
- Mafic magmas are denser (2700 vs 2450)
- Felsic melts hold more water due to higher viscosity. This decreases polymerisation and viscosity in itself.
How do magmas deform?
In Bingham flow - linearly, but only after resisting for a certain yield stress. Mafic rocks flow
