lecture-8 1 lecture #08- subduction zones. lecture-8 2 subduction zones f when two tectonic plates...
TRANSCRIPT
![Page 1: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/1.jpg)
Lecture-8 1
Lecture #08- Subduction Zones
![Page 2: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/2.jpg)
Lecture-8 2
Subduction Zones
When two tectonic plates converge often one will get buried or subducted beneath the other
The plate boundary regions where this occurs are called subduction zones
![Page 3: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/3.jpg)
Lecture-8 3
Subduction Zones
There are two types of lithosphere, oceanic and continental, so there are three possibilities at a convergent boundary:– oceanic and oceanic
– oceanic and continental
– continental and continental
In which of these cases can subduction occur ?
![Page 4: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/4.jpg)
Lecture-8 4
Subduction Zones
Subduction zones only occur at convergent boundaries between oceans and continents, and oceans and oceans
When oceanic lithosphere converges with continental lithosphere it is the oceanic material that is always subducted beneath the continental material.
When the convergent boundary is between two oceans it the older (heavier) plate which usually subducts.
![Page 5: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/5.jpg)
Lecture-8 5
Subduction Zones
Examples of an oceanic lithosphere subducting beneath a continental lithosphere:– South America subduction zone: Nazca plate
(oceanic) subducting beneath South American plate (continental)
– Aleutian subduction zone: Pacific plate (oceanic) subducting beneath North American plate (continental) – in Alaska
![Page 6: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/6.jpg)
Lecture-8 6
Subduction Zones
Examples of oceanic lithosphere subducting beneath oceanic lithosphere of another plate:– Marianas subduction zone: Pacific plate
subducting beneath Phillipine Sea plate in western Pacific
– Tonga subduction zone: Pacific plate subducting beneath Australian plate in western Pacific
![Page 7: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/7.jpg)
Lecture-8 7
General Picture of Subduction
![Page 8: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/8.jpg)
Lecture-8 8
General Picture of Ocean-Ocean Convergence
![Page 9: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/9.jpg)
Lecture-8 9
General Picture of Ocean-Continent Subduction
![Page 10: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/10.jpg)
Lecture-8 10
Second General Example of Ocean-Continent Subduction
![Page 11: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/11.jpg)
Lecture-8 11
The Termination of a Subduction Zone: Indian-Eurasian Boundary
![Page 12: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/12.jpg)
Lecture-8 12
Subduction Zones
Two dominant features associated with subduction zones are:
– deep earthquakes
– volcanoes
![Page 13: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/13.jpg)
Lecture-8 13
Subduction Zones and Deep Earthquakes
Earthquakes can only occur in brittle material (high viscosity)
It follows that earthquakes happen only in the lithosphere, which is usually 100-200 km thick
However, we observe earthquakes down to a depth of 700 km ???
![Page 14: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/14.jpg)
Lecture-8 14
Subduction Zones andDeep Earthquakes
It turns out the the deep earthquakes we observe (depth > 200 km) are occurring in lithosphere that has been subducted.
Deep earthquakes do not occur in any place except for subduction zones since this is the only place where brittle material (lithosphere) exists below its normal depth.
![Page 15: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/15.jpg)
Lecture-8 15
Subduction Zones andDeep Earthquakes
Deep earthquakes occur in planar (2D) arrangements called Wadati-Benioff Zones
Seismologists use the locations of deep earthquakes to map out the geometry of subducting lithosphere.
![Page 16: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/16.jpg)
Lecture-8 16
Sometimes Slab Geometry is Simple
![Page 17: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/17.jpg)
Lecture-8 17
Often it is Complicated (South America)
![Page 18: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/18.jpg)
Lecture-8 18
Often it is Complicated (Tonga)
![Page 19: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/19.jpg)
Lecture-8 19
Subduction Zones and Volcanoes
Volcanic activity is associated with all active subduction zones
We see dormant and “fossil” volcanoes at places where subduction used to occur
This type of volcanic activity is fundamentally different than volcanoes at mid-ocean ridges and hot-spots
![Page 20: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/20.jpg)
Lecture-8 20
Subduction Zones and Volcanoes
As oceanic crust ages and moves away from the ridge where it was formed it accumulates sediments which are rich is water
Water also reacts with the newly formed crust and becomes chemically bound to it
![Page 21: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/21.jpg)
Lecture-8 21
Subduction Zones and Volcanoes
Some sediment layers get “scraped off” the oceanic crust when it subducts at a trench; however a large amount of water is retained in the subductiong slab of oceanic material.
Thus, some water gets transported into the mantle while chemically bound to the rocks.
![Page 22: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/22.jpg)
Lecture-8 22
At about a depth of 100 km the temperature becomes hot enough that a chemical reaction takes place and the water is liberated from the material which carried it down into the mantle.
This is called a dehydration reaction.
Subduction Zones and Volcanoes
![Page 23: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/23.jpg)
Lecture-8 23
Subduction Zones and Volcanoes
The free water that has just been liberated immediately starts to percolate upwards and begins to partially melt the asthenosphere above it.
This partially molten material, and water, is much lighter than the surrounding material and begins rising …
![Page 24: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/24.jpg)
Lecture-8 24
Subduction Zones and Volcanoes
When the partially molten material nears the surface it often becomes fully molten because of decreasing pressure – now we call it magma.
The outermost crust at the Earth’s surface is cold, brittle and strong so it is difficult for the magma to break-through
Thus magma will often pond beneath volcanoes in a magma chamber until the pressure becomes high enough for it to break though the outermost crust and erupt
![Page 25: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/25.jpg)
Lecture-8 25
![Page 26: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/26.jpg)
Lecture-8 26
![Page 27: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/27.jpg)
Lecture-8 27
![Page 28: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/28.jpg)
Lecture-8 28
Subduction Zone Summary
Subduction zones occur at convergent plate boundaries; they are “burial grounds”
Oceanic material can subduct beneath oceanic material on another plate or beneath continental material on another plate
Continental lithosphere never subducts
![Page 29: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/29.jpg)
Lecture-8 29
Subduction Zone Summary
Subduction zones are the only place where deep (> 200 km) earthquakes occur
The deep earthquakes “line up” on planar structure that delineate the subducting oceanic plates
These seismicity patterns are called Wadati-Benioff zones
![Page 30: Lecture-8 1 Lecture #08- Subduction Zones. Lecture-8 2 Subduction Zones F When two tectonic plates converge often one will get buried or subducted beneath](https://reader036.vdocuments.us/reader036/viewer/2022070413/5697bfbd1a28abf838ca238a/html5/thumbnails/30.jpg)
Lecture-8 30
Subduction Zone Summary
Volcanoes are also prevalent at subduction zones.
They are formed from water that dehydrates from the subducting slab (at about 100 km) and the percolates upward causing magma formation.
This volcanic material is chemically distinct from MOR volcanoes and hot-spots