Plate Margins We must now look at the 3 main types of plate margin: Constructive. Destructive....

Plate MarginsPlate Margins We must now look at the 3 main types of plate margin:

Constructive.

Destructive.

Conservative.

Each type of plate margin has different rocks and types of hazard

Credit: Hawaii Natural History Association

Plate MarginsPlate Margins

Credit: U.S. Geological Survey

Destructive Plate MarginsDestructive Plate Margins May also be called

convergent plate margins.

Why?

What makes them

converge? There are 3 types of destructive plate margins depending on what types of plate are

involved:Oceanic v oceanic

Oceanic v continental

Continental v continental

Each will produce different hazards and rocks.


Golden Rule 1Golden Rule 1

Why do plates/rocks melt:

1. Because the temperature has increased.

2. Because the pressure is decreased.

3. Because it is saturated in water.

If one or more of these situations exists then the plate/rock will melt.

Golden Rule 2Golden Rule 2 What kind of magma is produced when a plate/rock melts? Remember how we classify igneous rocks.

Acid: > 66% quartz rich

Intermediate: 52 – 66 %

Basic: 44 – 52% quartz poor

Ultrabasic: < 44% Quartz is one of the first minerals to melt, so When the initial magma melts the one it produces will be

richer in SiO2 and so will move towards the acid end of the scale.

If you melt an ultrabasic rock it will produce a basic magma. If you melt a basic rock the magma will be more intermediate.

Golden Rule 3Golden Rule 3

The more SiO2 in a magma the more explosive will be the volcanic eruptions.

Why? Because the magmas are more viscous/sticky. Gasses come out of solution when a magma reaches

the surface (P drops) but they cannot escape easily. They therefore expand putting pressure on the magma

chamber. Eventually the P is so great that the top/plug is blown

off or the side slips and a violent eruption occurs.

Oceanic v Oceanic collisionOceanic v Oceanic collision


Oceanic v Oceanic collision

Oceanic v Oceanic collision

The features/landforms:Subduction zone: describes the whole area where

subduction is taking place.Ocean Trench: is the deep valley formed in the ocean floor as the subducted plate bends.Benioff zone: zone of earthquakes set off by the solid lithospheric plate forcing it’s way through the mantle.Island Arc: An arc of islands running parallel to the trench/subduction zone created by volcanic activity fed by magma from the melting subducted plate.

Oceanic v Oceanic 2Oceanic v Oceanic 2The processes:

Subduction: Occurs where a piece of oceanic lithosphere bends and sinks beneath another plate.

– Why do oceanic plates subduct?

– Because oceanic crust is denser (3.0).

Credit: U.S Geological Survey

Oceanic v Oceanic 3Oceanic v Oceanic 3 Seismic activity:

As the cold/solid oceanic plate sinks through the mantle there is frictional resistance.

When the resistance is overcome and the plate moves the energy is released as seismic waves = earthquake.

This is the Benioff zone.


Oceanic v Oceanic 4Oceanic v Oceanic 4 Melting:

As mentioned earlier there are 3 reasons why crust may melt.

Which is occurring in this situation?– Increased T as the plate sinks deeper.

– Also the plate is saturated in water.

The melting oceanic crust is basic so what kind of magma will form when it melts?

More SiO2 rich and so will move towards being intermediate.

This magma will move upwards through the overlying mantle and thin crust to the surface.


Oceanic v Oceanic 5Oceanic v Oceanic 5

Explosive volcanic activity. Basic/intermediate magma

will be fairly violent. Enough volcanic activity

will occur to build volcanic islands above sea level in an arc parallel to the plate margin (island arc).


Oceanic v Oceanic examples

Oceanic v Oceanic examples

Montserrat/Caribbean arc: Soufriere Hills. Aleutian islands. Indonesia: Krakatoa Philippines: Pinatubo Kamchatka.


Oceanic v Continental Crust collision

Oceanic v Continental Crust collision

The features/landforms: Many are the same:

Trench.

Subduction zone.

Benioff zone

Continent based volcanoes.

Fold mountains.


Oceanic v Continental Crust 2


The processes. Again very similar to

ocean v ocean with a subtle difference.

Subduction (but only the oceanic crust will subduct).

Seismic activity.

Melting:

Mountain building (orogenesis)




What happens to the melt as it enters the continental crust?

Remember that the magma is now intermediate(ish).

It travels up through the continental crust that has what composition?

Acid. The hot magma melts the

continental crust and adds SiO2 rich crust to the intermediate magma to make magma that is very intermediate or acid. Credit: U.S. Geological Survey



Acid/intermediate magma will produce what kind of volcanic activity?

Very explosive and dangerous.

Mountain building (orogenesis).

This involves crust being compressed and either folding or faulting.Fault movement sets off earthquakes.

There will therefore be earthquakes linked to both the Benioff zone and orogenesis.


Oceanic v Continental Examples

Oceanic v Continental Examples

Volcanoes along the Andes:

Popacatapetl (Mexico)

Nevada del Ruiz (Columbia)

The Rockies/Cordillera:Mt. St. Helens

Credit:/ U.S. Geological Survey

Credit:/ U.S. Geological Survey

Eruptions of Mt. St. Helens,

Continental v Continental CrustContinental v Continental Crust



Continental v Continental Crust


The features/landforms.Mountain ranges

The processes:Mountain building (orogenesis).

No subduction

because continental

crust will not subduct.


Everest



Will there be volcanoes? The crust is up to 90 km

thick and so is hot enough at it’s base to melt.

However, the melt produced is so viscous and the distance to the surface so far that magma will not make it to the surface.

Therefore NO VOLCANOES.




Will there be earthquakes?

Orogenesis is still taking place.

All the energy is being put into deforming the rocks.

So YES there will be many large earthquakes.

Conservative Plate MarginsConservative Plate Margins This is where 2 plates are sliding past each

other horizontally but are not being created or destroyed.

There is only one really good example in the world: California, San Andreas Fault.

Credit: U.S. Geological Survey(Photographer – Robert E Wallace Credit: U.S. Geological Survey

Conservative Plate MarginConservative Plate Margin

What kind of volcanic activity will there be?

None. Why not? Nothing is melting. Will there be any

earthquakes? Yes and very large.


Constructive Plate MarginConstructive Plate Margin

May also be called: divergent plate margins.Why?New crust is generated along ocean ridges. Examples are:The Mid Atlantic Ridge and the East Pacific Rise.


Constructive Plate MarginConstructive Plate MarginAlong conservative margins there is neither a loss nor a gain and the plates slip past each other, these margins are marked by transform faults.Along conservative margins there is neither a loss nor a gain and the plates slip past each other, these margins are marked by transform faults.

Mid ocean ridges are characterised by:

•Active volcanoes (submarine volcanoes and in places emergent volcanic islands)

•Small earthquakes

•Lack of sediment

•High heat flow



The new crust formed at this plate margin, along with a layer of the upper most part of the mantle, moves away from the ridge and new material is added to the trailing edge.



Iceland forms example of Mid Ocean Ridge that has grown into a volcanic complex above sea level


Passive Plate MarginsPassive Plate Margins

Nothing is happening. 2 pieces of crust are fused together. Like the N. American continental plate and the Atlantic

oceanic plate. However, in time the margin may become active. It is already starting to subduct in the Caribbean arc

region (Montserrat).


Plate Margins We must now look at the 3 main types of plate margin: Constructive. Destructive....

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