Download Magma Type and Plate Margins

Explain the relationship between
igneous activity, magmatic
composition, rock types and plate
boundaries.
Constructive Plate Margins
What is it that melts?
Upper mantle/peridotite.
What is it’s composition?
Ultrabasic.
Why does it melt?
Reduced P as crust above is
stretched and thinned.
Increase in T due to hot rising
convection currents.
Constructive Plate Margins 2
What kind of magma is produced
by melting the ultrabasic
mantle?
Basic.
What does the magma do?
Rises up through the thin crust
to the surface.
It may erupt underwater to
produce what?
Basalt pillow lavas.
Constructive Plate Margins 3
Some of this basic magma will cool
the the fractures feeding up to the
surface.
What rock and structure will form?
Sheeted dolerite dykes.
Some magma will cool in the magma
chamber to form what rock?
Gabbro.
Ocean v Ocean Collision
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.
Then what will happen?
Oceanic v Oceanic 2
Explosive volcanic
activity.
Basic/intermediate
magma will be fairly
violent.
What will the products be:
Lavas will be as important
as pyroclastics.
Lavas: Mainly andesites.
Pyroclastic deposits:
Ash
Pyroclastic flow deposit
Large blocks close to the
vent
Ocean v Continental Collision
The subducting
oceanic plate will melt
just as in ocean v
ocean.
The magma produced
will be the same. Which
is?
Basic to intermediate.
It will rise up into the
continental crust and
this is where the
differences occur.
Ocean v Continental Collision 2
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 (which
has low T minerals) and
adds SiO2 rich crust to the
intermediate magma to
make magma that is very
intermediate or acid.
Ocean v Continental Collision 3
There will be a greater
proportion of
pyroclastics compared
top lavas.
What are the products?
Lavas: Mainly
andesites, some
rhyolites.
Pyroclastic deposits:
Ash
Pyroclastic flow deposit
Large blocks close to
the vent
Continent v Continent Collision
What is melting?
The base of the
continental crust.
Why is it melting?
Because the crust is
so thick that it is hot
enough to melt.
What is the
composition of the
rock that melts?
Acid/intermediate.
So what will the
resultant magma be?
Acid.
Continent v Continent Collision 2
1.
2.
3.
What happens to this
magma?
It will intrude higher.
Will it reach the surface
and form volcanoes?
No.
Why not?
For 3 reasons:
The magma is very
viscous.
The magma is relatively
cool (it is just hot enough
to melt).
The crust is too thick.
So it crystallises deep in