1.2 Plate Tectonics Flashcards
What are the different processes which move tectonic plates?
Mantle convection
Slab pull theory
Subduction
Seafloor spreading
What is mantle convection?
Has long been thought responsible for movement but now less accepted. Heat produced by the decay of radioactive elements in the core heats the lower mantle, creating convection currents. The hot liquid magma currents move around the asthenosphere in circles, causing the plates to move. This is because when convection cells make contact with the base of the crust, they move the plates by frictional drag.
What is the slab pull theory?
Increasingly accepted theory.
Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools. This causes it to sink in the mantle under its own weight- pulling the rest of the plate further down
What is subduction?
As new crust is being formed in one place, it’s being destroyed in another- subduction. As two oceanic plates or oceanic and continental plates move towards each other, one slides under the other where it melts in the subduction zone
What is seafloor spreading?
Hot magma is forced up from the asthenosphere and hardens forming new oceanic crust. This new crust pushes tectonic plates apart
Where does seafloor spreading occur?
At divergent (constructive margins)
What does seafloor spreading cause?
In oceans they cause mid ocean ridges and on land, it causes rift valleys
What are the three different types of plate boundaries?
Convergent- where two plates collide (destructive)
Divergent- where two plates move apart (constructive)
Transform- where two plates slide past each other (conservative)
What is the lithosphere?
What is the Asthenosphere
What is the mesosphere
Lithosphere: Crust
Asthenosphere: Upper mantle
Mesosphere: Lower mantle
What is the evidence that plates move?
Continental fit (e.g. Africa and South America)
Glacial Evidence
fossil correlation
Earthquake depths
Location of coal seams
Paleomagnetism
Geological fit
How does glacial evidence show plates move?
evidence of glaciation in tropical Africa
How does fossil correlation show plates move?
Matching fossils of animals dates back to the same time but in continents now thousands of miles apart
How does earthquake depths show plates move?
depths can reach 700km deep in the benioff zone, indicating subduction
How does location of coal seams show plates move?
Coal seams are found in temperate (cooler) climates yet they need tropical climates to form
How does geological fit show plates move?
There are matching rock types far apart e.g. South America and Africa
What is paleomagnetism and how does it show that plates move?
This confirms the seafloor is spreading.
1. Every 40,000 years, the earth’s magnetic fields change direction, causing the magnetic north and south poles to swap
2. When lava cools and becomes rock, minerals inside the rock line up with the earth’s magnetic direction
3. Scientists have found the same pattern of magnetic direction either side of ocean ridges: this is only something that could happen if new rock was being formed at the same time on both sides. It shows a conveyor belt movement of rock away from the ocean ridge
What happens when an oceanic plate meets a continental plate?
Because the oceanic crust is denser, it’s subducted by the continental plate into the mantle and melts
What marks the place where oceanic plates start to sink below the continental plate (or other oceanic plate)?
Deep ocean trenches
What land formation aside from deep ocean trenches does a oceanic plate meeting a continental plate cause?
A chain of fold mountains- as the two plates collide, the continental plate is folded up
What hazards are caused at a plate margin where an oceanic crust meets a continental one?
The friction between plates causes intermediate and deep earthquakes in the benioff zone.
Volcanic eruptions also occur as magma created by the melted plate pushes up through the continental crust, where it causes explosive reactions
What happens when an oceanic plate meets an oceanic plate (destructive)
The faster or denser plate is subducted by the other
What hazards occurs at a margin where an oceanic plate meets an oceanic plate?
Underwater volcanoes because the subducted plates melts, creating magma which rises from the benioff zone to form underwater volcanoes
The subduction also causes shallow-to-deep focus earthquakes
What land formation occurs where an oceanic plate meets an oceanic plate?
An Island Arc
The underwater volcanoes will, over millions of years, rise above sea level as they grow to form separate island volcanoes
What is an example of a hazard which was caused by an oceanic plate colliding with an oceanic plate
The 2004 Indian Ocean tsunami was caused by an earthquake when two oceanic plates collided- The Indian plate was sinking beneath the Burma plate
What happens when a continental plate meets a continental plate?
A collision margin- as both plates have the same density and are less dense than the asthenosphere beneath them, neither plate is subducted. Instead, they collide and sediments between them are crumpled and forced to form fold mountains like the Himalayas
Why is there still some plate subduction where a continental plate meets a continental plate?
When the compressed and therefore denser sediments result in plate subduction beneath them
What volcanic activity and hazards are likely to occur at a margin where a continental plate meets a continental plate?
There is no volcanic activity, but any earthquakes are likely to have a shallow focus increasing severity
What happens at a conservative margin? Why is this so tectonically hazardous?
Two plates slide past each other.
Why are conservative margins so tectonically hazardous?
It’s very tectonically active, with the two plates sometimes sticking as they move past each other, causing stress and pressure to build up, which is suddenly released. This causes strong, shallow focused earthquakes
What is one of the most famous examples of a conservative boundary?
The San Andreas fault
What happens at constructive margins?
Two plate move apart, leading to the formation of new crust
What is formed at constructive boundaries?
Mid ocean ridges/rift valleys
What is a rift valley?
when plates move apart on continents, the crust stretches and breaks into sets of parallel faults. The land between these faults then collapses causing steep-sided valleys
What can happen in a mid-ocean ridge?
Regular volcanic eruptions create submarine volcanoes along these mid ocean ridges, some of which grow to create new islands, such as Iceland on the Mid Atlantic ridge
What are the characteristics of oceanic plates?
Basaltic
high desnity
7-10km thick
What are the characteristics of a continental plate?
granite
low density
older
35 to 100km thick
What’s the inner core like?
5150km deep to centre
7000 degrees (radioactive decay)
very dense 13.5g/cm3
20% nickel
80% iron
solid
What’s the outer core like?
2890km to 5150km
4400 to 6100 degrees
dense - 9.9-12.3g/cm3
12% sulphur
88% iron
liquid
generates a magnetic field
What’s the mantle like?
700km to 2890km deep
870 degrees
medium density 3.3-5.4g/cm3
upper is made of olivine
lower is made of magnesium silicate
phases of liquid and solid in layers
What’s the crust like?
up to 70km deep on average
surface temp is 400 degrees
less dense continental- 2.7g/cm3
more dense oceanic: 3.3g/cm3
Made of granite or basalt
solid
What % of earthquakes happen at plate margins?
95%
What % of earthquakes happen at the Pacific ring of fire?
70%
What plate margins have the most powerful earthquakes?
Destructive, collision and conservative due to sudden release of tension
Which margins have the deepest focus earthquakes?
Destructive
What is the OFZ?
Oceanic fracture zone: belt of activity through constructive boundaries along mid ocean ridges, coming onshore in Africa at the red sea rift
What is the CFZ?
Continental fracture zone- belt of activity following mountain ranges from Spain, Alps, Middle East, Himalayas and around the Pacific
What are intraplate earthquakes?
Very rare
occur along old fault lines. This could be because of solid crust cracking or a plate being stretched
What are hotspots
An area in the mantle from where heat rises as a hot thermal plume from deep in the earth. High heat and low pressure at the base of the lithosphere allow rock to melt causing eruptions
An example of where an intra-plate earthquake happened?
Church Stretton in Shropshire
How do hotspots create an island chain?
Volcanoes the hotspot has caused move with the plate as it moves and a new volcano forms over the hotspot, creating a chain of volcanoes, atolls and seamounts
