Plate Margins

We must now look at the 3 main types of plate margin:
Constructive.
Destructive.
Conservative.
Credit: Hawaii Natural History Association

Each type of plate margin has different rocks and types
of hazard
Plate Margins
Credit: U.S. Geological Survey
Destructive 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:

Credit: Hawaii Natural History Association
Oceanic v oceanic
Oceanic v continental
Continental v continental

Each will produce different hazards and rocks.
Golden 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 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 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 collision
Credit: U.S. Geological Survey
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 2
The 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 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.
Credit: U.S. Geological Survey
Oceanic 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.
Credit: U.S. Geological Survey
Oceanic 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).
Credit: U.S. Geological Survey
Oceanic v Oceanic examples
Credit: U.S. Geological Survey

Montserrat/Caribbean arc: Soufriere Hills.
 Aleutian islands.
 Indonesia: Krakatoa
 Philippines: Pinatubo
 Kamchatka.
Oceanic v Continental Crust collision

The features/landforms:
 Many are the same:
Trench.
Subduction zone.
Benioff zone
Continent based
volcanoes.
Fold mountains.
Credit: Hawaii Natural History Association
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)
Credit: Hawaii Natural History Association
Oceanic v Continental Crust 3





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
Oceanic v Continental Crust 4

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.
Credit: U.S. Geological Survey
Oceanic v Continental Examples

Volcanoes along the
Andes:
Eruptions of Mt.
St. Helens,
Popacatapetl (Mexico)
Nevada del Ruiz (Columbia)

The Rockies/Cordillera:
Credit:/ U.S. Geological Survey
Mt. St. Helens
Credit:/ U.S. Geological Survey
Continental v Continental Crust
Credit: U.S. Geological Survey
Credit: U.S. Geological Survey
Continental v Continental Crust

The features/landforms.
Mountain ranges

Everest
The processes:
Mountain building
(orogenesis).
No subduction
because continental
crust will not subduct.
Credit: U.S. Geological Survey
Continental v Continental Crust

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.
Credit: U.S. Geological Survey
Continental v Continental Crust

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 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 Margin

What kind of volcanic
activity will there be?
 None.
 Why not?
 Nothing is melting.
 Will there be any
earthquakes?
 Yes and very large.
Credit: U.S. Geological Survey
Constructive Plate Margin
Credit: U.S. Geological Survey
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 Margin
Credit: U.S. Geological Survey
Along conservative margins there is neither a loss nor a gain and the plates slip past each other, these margins are marked by transfo
Constructive Plate Margin
Credit: U.S. Geological Survey
Mid ocean ridges are characterised by:
•Active volcanoes (submarine volcanoes and in places
emergent volcanic islands)
•Small earthquakes
•Lack of sediment
•High heat flow
Constructive Plate Margin
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.
Credit: U.S. Geological Survey
Constructive Plate Margin
 Iceland
forms
example of Mid
Ocean Ridge that
has grown into a
volcanic complex
above sea level
Credit: U.S. Geological Survey
Passive Plate Margins
Credit: Hawaii Natural History Association





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).