2.2 Plate Tectonics Flashcards
What is the lithosphere broken up into?
Plates
What are the different types of crust?
Oceanic and continental
Where do volcanoes mostly occur?
Along plate boundaries
What can the earth being a geological machine also be known as?
Plate tectonics
What did the concept of plate tectonics originate with?
The idea of continental drift (the continents moving about the planet)
Who first put forward the idea of continental drift?
Alfred Wegener in 1915
Who was Alfred Wegener?
A meteorologist that came up with continental drift
What evidence shows that continental drift is a correct theory?
- the shapes of continents are fitted
- ancient rocks and fossils
- glaciers
- mountain chains
- magnetic attractions
How are the gaps between today’s continents when fitted together accounted for?
The gaps are due to some pieces of land being submerged due to difference in sea level, as well as erosion and deposition
How can ancient crystalline rocks support continental drift?
Different rock types and ancient mountain belts match up
How can fossils support continental drift?
Distribution of plant and animal fossils found in S. America, Africa, Antarctica, Australia - showing all southern continents were once joined
How can glaciers support continental drift?
Distribution of late palaeozoic glaciations can show movement of glacial ice
How can mountain chains support continental drift?
Mapping sea floors - underwater mountain ranges that line up with continental pattern → spreading ridges formed at constructive plate margins
How can magnetic attractions support continental drift?
In metals, the tiny compasses point towards where the north pole was when the lava cooled → use this to figure out where continents were at the time
Where is today’s present north magnetic pole?
In the south
If the sea floor is spreading, what should be shown on the rock?
The rock should show symmetrical magnetic stripes moving away from the ridge
What is a hot spot?
An area of very high heat flow in the mantle below the plates - with the heat burning through the plate to create volcanic activity at the surface
What can hot spots also be known as?
Mantle plume
Why can hot spots indicate direction of plate movement?
Although plates move - hot spot stays in same place e.g. direction that Hawaiian islands get older indicate direction of plate movement
What led geologists to believe that the lithosphere and crust were moving?
Patterns of earthquakes and volcanoes
How can plates move?
Convection currents in the lower mantle
What is the process of convection currents in the lower mantle?
- hot material rises upwards through the mantle
- as this nears the lithosphere, it moves sideways and carries a plate with it (this happens as mid-ocean ridges)
- this material cools and descends at certain points - dragging lithosphere with it at subduction zones
- lower part of mantle heated and material rises again to complete convection cell
How do mid-ocean ridges form, with relation to convection currents?
When hot material in the mantle rises and then move sideways and carries a plate with it
How do subduction zones form, with relation to convection currents?
When material at top of mantle cools and descends, dragging lithosphere down with it
Is the mantle a liquid?
No
How do convection currents occur in the mantle, if the mantle is a solid?
Slow deformation of the solid mantle
What is slab pull?
The weight of the crust pull the plate downwards
How is it thought that constructive plate margins begin as?
As grabens or rift valleys on land
What are rift valleys?
Where the crust is being split under great tension, with sets of normal faults forming opposite one another, and blocks of crust sinking in between
How does an ocean develop as a result of plate tectonics?
- it must start as a rift valley
- this widens - water floods in and forms a long narrow sea
- sea widens by sea-floor spreading and opens up into an ocean
Which way do the plates move in constructive boundaries?
Move apart - with magma brought up from the upper mantle
What happens to magma that is brought up from the mantle at constructive margins?
It crystallises on the surface as basalt pillow lavas, and beneath as dolerite dykes or plutons of gabbro
What is formed when crust is pushed away in opposite directions at constructive margins?
Mid-oceanic ridges - marked by lines of volcanoes
What are the fractures in the line of the mid-ocean ridge?
Transform faults
How are transform faults formed?
Formed at the same time as the ridge, and are curved to allow the plates to move at different speeds across the earths spherical surface
What happens when one plate moves in the opposite direction to another on either side of a transform fault?
Great pressure build up and shallow earthquakes result
What do conservative margins result in?
Build up of pressure as friction prevents rocks moving along these large transform fault zones
How is pressure released at a conservative margin?
In the form of an earthquake
Which direction are plates at destructive margins moving?
Towards each other
What does the plate destroyed at destructive plate boundaries compensate for?
The new plates forming along spreading ridges
What are subduction zones?
When one plate is going underneath another
What are Benioff zones?
Inclined zones of seismicity - when earthquake foci occur in lines at distinct angles leading down from plate margins
When do trench systems occur?
When an oceanic plate is subducted underneath another oceanic plate
What is marked when an oceanic plate subducts another oceanic plate?
A deep ocean trench
How can earthquakes and volcanoes form from the subduction of oceanic-oceanic plates?
- earthquakes - friction with the other plates (and so a Benioff zone formed)
- volcanoes - when depth reached where temperature high enough plate will melt, forming magma, which will rise through cracks
What type of volcanoes are formed from the subduction of oceanic-oceanic plates?
Andesitic
What are volcanic arcs?
Chain of volcanic islands created in an arc shape, due to the curvature of the earth
Where do oceanic-continental mountain belts occur?
When an oceanic plate is subducted under a continental plate
Why is the oceanic plate subducted under the continental plate?
It is denser and thicker than the continental plate
What happens to most of the magma when the subducted plate melts in a oceanic-continental subduction zone?
Although some reaches the surface to form andesitic volcanoes, most builds up in the continental crust as large granite plutons
What will the granite plutons, formed when an oceanic plate is subducted underneath a continental plate, do to the surrounding rock?
Produce contact metamorphism of the rocks around them
How are fold mountains formed?
When sediments that build up along oceanic-continental subduction margins are buckled into great folds and faults
How is regional metamorphism related to destructive plate boundaries?
The immense pressures and temperatures created by movement along destructive plate margins also produce regional metamorphism on a large scale
When do continental mountain belt systems occur?
When the subduction of an oceanic plate results in one continent moving towards another - until it eventually collides with it
Why are two continental plates squeezed against one another at destructive plate margins?
Neither can be subducted
What forms when two continental plates are squeezed against one another?
The sediments buckle and form fold mountains - also earthquake activity and intrusion of granitic plutons
Why is there little volcanic activity at continental-continental destructive plate boundaries?
There is so must crust to be penetrated
Why is thrust faulting common at continental-continental destructive plate boundaries?
Due to the great pressure involved
What is the Joides Resolution 360 research cruise?
Boat that does ocean drilling of igneous rocks to understand the processes that create mid-ocean ridge basalt
What does the mechanical behaviour of the outer earth involve?
- the lithosphere - cold, rigid outer shell
* underlain by the asthenosphere - weaker layer
What is the lithosphere composed of?
Crust and upper mantle
What is the asthenosphere composed of?
Upper mantle
Are the mechanisms of how tectonic plates move completely understood?
No
Who discovered evidence for sea floor spreading, and what year?
- Hess - 1960
- Vine and Matthews - 1963
- J. Tuzo Wilson - 1965
Features at divergent plate boundaries?
- basalt extrusion
- sea floor spreading
- ocean ridges
- melting of upper mantle to form basaltic magma
- high heat flow
- rift valleys
- abyssal plane
Example of a divergent plate boundary?
Mid-Atlantic Ridge
Features at conservative plate boundaries?
- earthquake activity
* transform faults
Example of a conservative plate boundary?
San Andreas fault zone
Features at oceanic-oceanic convergent plate boundaries?
Island arc/trench systems
Example of a oceanic-oceanic convergent plate boundary?
Java-Sumatra/Caribbean
Features at oceanic-continental convergent plate boundaries?
- subduction zones
- Benioff zone
- partial melting producing andesitic and granitic magmas
Example of a oceanic-continental convergent plate boundary?
The Andes
Features at continental-continental convergent plate boundaries?
- mountain building
- folding
- thrust faulting
- partial melting of crust producing granites
- associated regional metamorphism
Example of a continental-continental convergent plate boundary?
The Himalayas
What is the only type of material that can be subducted? Why is this?
Oceanic plate material - because the granitic crust of the continents is not dense enough to descend into the asthenosphere
