Transcript Explain the relationship between igneous activity, magmatic

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 = tuff.
Pyroclastic flow deposit =
ignimbrite.
Large blocks close to the vent =
(vent) agglomerate.
Oceanic v Oceanic 3
Will the basic/intermediate magma undergo
fractional crystallisation/magmatic differentiation?
Yes.
So what will happen to the magma over time
feeding a particular volcano?
It will become more SiO2 rich.
What will that do to the style of eruptions?
Become more explosive.
Oceanic v Oceanic 3
What intrusive rocks
may form?
Plutonic intrusions that
form deep down in
batholiths.
What will be the crystal
size?
Coarse.
They will tend to be
intermediate so what
will the rock be?
Diorite.
Oceanic v Oceanic 4
Hypabyssal intrusions
(just below surface and
small – dykes and
sills).
What will their crystal
size be?
Medium.
So what will the rock
type be?
Microdiorite.
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 = tuff.
Pyroclastic flow deposit
= ignimbrite.
Large blocks close to
the vent = (vent)
agglomerate.
Ocean v Continental Collision 4
What intrusive rocks may
form?
Plutonic intrusions in
batholiths will be coarse
and will undergo fractional
crystallisation and so a
wide range of rock types
may occur.
Remember they can form
zoned intrusions with basic
at the margin acid in the
core and intermediate
between he two.
Ocean v Continental Collision 5
What will the rock types
be:
Acid = granite
Intermediate = diorite
Basic = Gabbro
There will also be
hypabyssal intrusions
forming medium grained
equivalents:
Acid = Microgranite
Intermediate = microdiorite
Basic = microgabbro or
dolerite.
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?
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
the earth/plutonic.
Continent v Continent Collision 3
This acid magma cools at
depth in batholiths.
What rock type will it
form?
Granite.
The magma may undergo
some fractional
crystallisation and so
there can be some zoning
with possibly intermediate
rocks at the edge and very
acid rocks in the centre.
Smaller intrusions may
produce microgranites.