EPSC201 - Lecture 14 Notes
If an earthquake is along a transform fault, they
don’t generate as much surface waves. Surface
waves are responsible for damage. If its along
a transform fault, the direction it should slide is
already defined and set up to move.
Here are some types of intrusive rock
Laccolith – a huge underground body of mag-
ma, that can cool, and pushes up the sedimen-
tary layers above it
Batholith – main magma chamber that feeds all
these other terms. If the sedimentary layers
wear down enough, it will become exposed.
They look like rounded out-
crops formed from igneous rocks. These can be very large, up to 50 km wide. So
these are large bodies of magma.
Dike – vertical wall of hard rock, it would have fed a fissure eruption of magma go-
ing upwards, can be thin but continue onwards for kilometers. They feed volca-
noes by flowing upwards.
Sill – horizontal intrusions of magma that
go in-between layers of sedimentary rock, fed by dikes, can be
very thick, or very thin. Hundreds of meters to cm thick.
Diabase – intrusive basaltic rock, does not make it to the sur-
face – form of gabbro
Gabbro – intrusive igneous rock chemically equivalent to basalt
Basalt – extrusive, the magma makes it out to the surface
Why do we get magma formation and volcanoes? Why do we
get hot spots, such as Hawaii and Iceland?
With spreading activity, you get basalt. With subduction, you
get andosite and rhyolite.
At spreading centers, we have decompression melting. As the
plates spread, mantle moves upwards, and releases pressure.
This causes the mantle to melt. Rocks are great insulators. As the mantle moves upwards,
the surrounding rocks stay hot (the temperature they were
when deep). So the mantle rises up, and is under less tem-
perature, but is at the same hot temperature. This causes the
mantle to melt into magma, and is called decompression
Shown here is a geotherm. From this, we see that as pres-
sure increases going deeper into the earth, higher tempera-
tures are required to make the rock liquid.
At the liquids line, everything right of the line is liquid, which
shows that at higher temperatures, the rock turns to liquid.
Anything on the solidus line that is moved upwards due
to mantle upwelling (decompression melting) will then
turn to liquid. This graph explains why decompression
melting works. However, the mantle must move enough
to reach the liquidus line. If it only moves a little bit, and
doesn’t reach the next line, it won’t melt completely. Of-
ten, the mantle will be in a state of partial liquids and
Subducting slabs come from the surface, which is at a
much colder temperature then the mantle. Nonetheless,
we still observe magma formation due to melting of the
subducting slab. The friction of the subducting slab also
causes a small amount of heating, but is not the source
The source of melting for the subducting plate is water.
If the subducting plate has water content, the melting
point of the solidus is much lower. The above geotherm
assumes the conditions are anhydrous. However, if
there is water present then things melt much easier.
Subducting plates often come from ocean, which causes
them to have high water content.
Here the subducting plate is under the ocean. This
drags down seawater, which hydrates them. Note the
different geotherms for hydrated and dehydrated plate
melting. The hydrated form melts much easier. Amphibole can be considered a hydrated form of pyroxcine. When water is added, this transformation
can occur. As amphibole is heated, it realeases its water, and dehydrates back into pyroxcine. This
water is released into the surrounding mantle.
Key point – amphibole releases its water into the mantle when heated, causing melting due to in-
crease in water content
Subduction melting does not cause that much rock to be melted. Decompression melting does cause
a lot of rock to be melted. In the case of the spreading centre, it is easy to get the melting going. A lot
of mantle rises up and melts. At the spreading center, the rock is mostly basalt. At the Subduction
zone, there is rhyolites and andeosites. At the spreading centre, the rock is similar to most of the man-
tle on the Earth. Basalt has high amounts of silica, and less olivine. So the magma at spreading cen-
ters, has high amounts of silica. By contrast, Subduction melting melts only a small amount of rock,
but generates magma high in silica. Decompression melting produces magmas are poor in silica.