Geology Reference
In-Depth Information
Figure 7.2 Lithostatic Pressure
Vertical pressure (kbar)
0
0.5
1.0
1.5
2.0
2.5
Surface
0
5
10
1 kilobar (kbar) = 1000 bars
Atmospheric pressure at sea level = 1 bar
a Lithostatic pressure is applied equally in all directions in Earth's
crust due to the weight of overlying rocks. Thus, pressure increases
with depth, as indicated by the sloping black line.
Figure 7.3 Differential Pressure Differential pressure results
from stress that is unequally applied to an object. Rotated garnets
are a good example of the effects of differential pressure applied
to a rock during metamorphism. In this example from a schist in
northeast Sardinia, stress was applied in opposite directions on the
left and right side of the garnet (center), causing it to rotate.
associated with intense deformation; and regional
metamorphism , which occurs within a large area and is
associated with major mountain-building episodes. Even
though we will discuss each type of metamorphism sepa-
rately, the boundary between them is not always distinct and
depends largely on which of the three metamorphic agents
was dominant (
b A similar situation occurs when 200-ml cups composed of
Styrofoam™ are lowered to ocean depths of approximately 750 m and
1500 m. Increased water pressure is exerted equally in all directions
on the cups, and they consequently decrease in volume while
maintaining their general shape.
Figure 7.4).
Contact Metamorphism
Contact (thermal) metamorphism takes place when a body
of magma alters the surrounding country rock. At shallow
depths, intruding magma raises the temperature of the
surrounding rock, causing thermal alteration. Furthermore,
the release of hot fl uids into the country rock by the cooling
intrusion can aid in the formation of new minerals.
Important factors in contact metamorphism are the
initial temperature, the size of the intrusion, and the fl uid
content of the magma, the country rock, or both. The
initial temperature of an intrusion depends, in part, on its
composition; mafi c magmas are hotter than felsic magmas
(see Chapter 4) and hence have a greater thermal effect on
the rocks surrounding them. The size of the intrusion is
also important. In the case of small intrusions, such as dikes
and sills, usually only those rocks in immediate contact
with the intrusion are affected. Because large intrusions,
such as batholiths, take a long time to cool, the increased
2Mg 2 SiO 4 + 2H 2 O
Mg 3 Si 2 O 5 (OH) 4 + MgO
olivine
water
serpentine
carried away
in solution
The chemically active fl uids important in the metamor-
phic process come primarily from three sources. The first
is water trapped in the pore spaces of sedimentary rocks as
they form. The second is the volatile fluid within magma.
The third source is the dehydration of water-bearing miner-
als such as gypsum (CaSO 4 2H 2 O) and some clays.
THE THREE TYPES OF METAMORPHISM
Geologists recognize three major types of metamorphism:
contact ( thermal ) metamorphism , in which magmatic heat
and fl uids act to produce change; dynamic metamorphism ,
which is principally the result of high differential pressures
 
 
Search WWH ::




Custom Search