GEOL1202 Planet Earth Assignment Plate Tectonics and Metamorphism This is an geology assignment associated with the plate tectonic movement. You should fol

GEOL1202 Planet Earth Assignment Plate Tectonics and Metamorphism This is an geology assignment associated with the plate tectonic movement. You should follow the instructions carefully. Geology 1202 Planet Earth
Assignment – plate tectonics and metamorphism
We have looked at the three main factors controlling metamorphism; that is temperature,
pressure and stress, and some of the metamorphic rocks. In this assignment we will take a closer
look at one of the tectonic environments that can provide these necessary conditions.
One situation in which a variety of
metamorphic conditions can exist is
at a subduction zone where one
plate is descending beneath
another. Based on what we know
about the properties of the rocks
involved we can make predictions
about what goes on in this area.
The plate that is descending has
been at the Earth’s surface and is
relatively cold. As it moves down
the subduction zone, it is heated by
the hot asthenosphere above and
below and may begin to undergo
partial melting. Hot magma
penetrates up through the upper
plate from the asthensophere below, bringing heat near the surface.
The red contours (called isotherms) on the cross-section at right show these patterns of
temperature change in the subduction zone. (these lines are similar to the contours you looked at
on topographic maps – instead of marking the change in elevation they mark the change in
temperature) Two isotherms are marked on the figure – 300 and 600 degrees celsius.
Geothermal gradients
O
If you could drill downward at the points labelled A, B
and C on the diagram above you would encounter very
different conditions at each one, even though they
would all start an the same temperature and pressure
at the surface of the Earth.
The diagram at right shows the distribution of
metamorphic facies with temperature and pressure.
Lines A, B, and C show the geothermal gradients that
would be encountered at the three marked positions
on the diagram of a subduction zone above.
0
800
0
0
C
hornfels facies
6
8
5
10
amphibolite
facies
4
te e
ni lyit
e
l
eh e s
pr mp cie
u
a
f
p
green
sch
facie ist
s
PRESSURE IN KILOBARS
A
zeolite
facies
volcanic 15
plutonic
complex
20
25
plate
interior
granulite
facies
30
upper part of
subduction zone
10
35
eclogite
facies
DEPTH IN KILOMETRES
B
2
st
hi
sc s
ue ie
bl fac
At point A, the rocks would be relatively cold at great
depth. At C, an area of active volcanism, the
temperature would go up much more rapidly with
depth. We would describe the geothermal gradient as
high. At point B, conditions would be somewhere
in-between.
TEMPERATURE IN C
200
400
600
What do I have to do?
The object of this assignment is to predict the facies of metamorphism that occurs at different
points near a converging plate boundary, or subduction zone. To predict the metamorphic facies
that will develop, it is necessary to know the temperature and the pressure.
A
D
B
E
C
This is the same diagram of a subduction zone you have seen earlier, with isotherms to indicate
the variation in temperature. Use the diagram to estimate the temperature conditions at points A
to E (indicated by the black dots). If your point is exactly on an isotherm, this is easy. However, if
it is between isotherms, you will have to estimate, based on the distance to nearby isotherms.
For example, if your point is halfway between the 300 and 600° isotherms you would estimate the
temperature at 450°. For simplicity assume that rocks at the surface are 0°. Enter these values
in Table 1 (next page).
The next thing you need to know is the pressure. Pressure is due to the weight of overlying rocks,
so it increases with depth. Your chart of metamorphic facies includes both pressure (on the left
hand side) and depth (on the right). We will use depth measurements to estimate the influence of
pressure on the type of metamorphism. For each of the points A to E on the cross section, use
the vertical scale to measure the depth below the surface – we will discount the overlying water
so measure from the surface of the crust. Enter these values in Table 1.
With this information you can use the Pressure-Temperature diagram to determine what
metamorphic facies would be produced at each point. For each point draw a vertical line down
from your estimated temperature (along the top of the graph). Next draw a horizontal line across
from your estimated depth (indicated on the right side of the graph) to the left side Record the
pressure (as indicated by the scale on the left side of graph) in Table 1.
Where the two lines intersect is the metamorphic facies – label this point with the appropriate
letter (A to E). Enter the facies in Table 1.
EXAMPLE
Below is an example of how it should be done using
point X
estimate
the
temperature
based on
the Isotherms
measure
the
depth
X
use the
scale to
convert the
depth to
kilometres
O
800
0
hornfels facies
8
amphibolite
facies
6
green
sch
facie ist
s
4
5
10
te te
i
i
en lly
h
e
s
e
pr mp cie
u
a
p f
st
hi
sc s
ue ie
bl fac
PRESSURE IN KILOBARS
2
zeolite
facies
0
15
20
25
X
granulite
facies
30
10
35
eclogite
facies
12
draw a vertical
line at the
correct temperature
40
DEPTH IN KILOMETRES
0
TEMPERATURE IN C
200
400 450 600
draw a horizontal
line at the
correct depth
STUDENT NUMBER:
NAME:
O
800
0
0
hornfels facies
10
e-
amphibolite
facies
green
sch
facie ist
s
4
it ite
n
e lly
h
e e s
pr mp cie
pu fa
6
st
hi
sc s
ue ie
bl fac
PRESSURE IN KILOBARS
2
5
zeolite
facies
8
15
20
25
granulite
facies
DEPTH IN KILOMETRES
0
TEMPERATURE C
200
400
600
30
10
35
eclogite
facies
Table 1
Point A
Point B
Point C
Point D
Point E
Temperature (celsius)
Depth (km)
Pressure (kb)
Metamorphic Facies
What do I submit?
Submit a photograph of your annotated PT diagram and the competed Table. Do not forget to fill in
your name and student number on the sheet.

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