Part 5 - Plate Tectonics Flashcards
What are plate tectonics?
The theory that the earths lithosphere is divide into a number of plates of irregular shape and size that move over the weaker interior (the asthenosphere)
What is the importance of the asthenosphere in allowing lithospheric plates to move?
It is weak and deforms plastically allowing the lithosphere to slide across it, kind of like tar
What is an earthquake?
Vibrations that occur when a sudden fracture allows adjacent rock masses to slip past each other. The ground can shake very violently and cause poorly constructed buildings to collapse but distant earthquakes can be measured
What is used to measure an earthquake to analyse the seismic waves that have traveled through the earth?
Seismometer
Would you expect most earthquakes to happen in the lithosphere in the asthenosphere?
Sudden fracture is more a characteristic of elastic than of plastic , so most earthquakes happen in the lithosphere
What is an earthquakes focus?
An earthquakes focus is the point inside the earth where the initial rupture begins
The greatest energy is released at this point, and this is where the strongest seismic waves originate
Where do the strongest seismic waves originate in a earthquake?
In the earthquakes focus
What is an earthquakes epicentre?
Often misused in the news
It refers to the point on the earths surface directly above the focus
What is an earthquakes magnitude a measure of?
An earthquakes magnitude is the measure of the size of the seismic vibrations at the epicentre
What does each one magnitude represent?
Each one magnitude represent ten times greater shaking at the focus
What is the magnitude of an earthquake often referred to as?
The Richter scale
What is a divergent plate boundary?
It is the line between 2 moving plates, where the asthenosphere wells upwards
What is the divergent plate boundary also known as and why?
The constructive plate boundary
Because material is being added to plates
When part of the asthenosphere mantle wells up through the lithosphere in a divergent plate boundary, why does it melt?
What is this melting known as?
It is a consequence of the drop in pressure
Decompression melting
What happens to peridotite composition (silica, oxygen) begins to melt as it rises up through the crust?
It produces a slightly higher silica content 45% - 49%
This results in a the rock basalt
What happens to the residual mantle left behind?
It has reduced silica content as it gave some to the oceanic crust
What is partial melting in terms of plate spreading?
It is the ability for a rock to melt in a way that it produces mineral enriched rock
Such as the melting of the mantle which produces enriched silica and makes up the oceanic crust
What are the two most abundant gases expelled in volcanic eruptions?
Hydrogen and oxygen
What is a subduction zone?
It is when one plate is descending at an angle below another plate
How far can a subduction zone extend?
700 km deep
Why does a plate only descend 700 km and no more?
Because the heat from the surrounding asthenosphere warms the descending plate until it ceases to be unrecognisable
What is a destructive plate boundary?
It is the point where two plates meet and the plate becomes destroyed as it is deep in the earth and heated up by the mantle
What plate zones are volcanoes usually found?
Subduction zones
