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EARTH’S INTERNAL STRUCTURE AND MAGMA FORMATION
PROCESSES
Continental crust
Oceanic crust
Crust-mantle boundary (MOHO)
Upper mantle (down to 660-670 km)
Mantle is subdivided mainly based on seismic wave velocities
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Nature
451
, 26
6-26
8 (1
7 Ja
nuar
y 20
08)
Information from seismic waves
“CAT-scan” of the mantle, using SEISMIC TOMOGRAPHY
Faster seismic velocity
Colder and stronger rocks
Slower seismic velocity
Warmer and weaker rocks
http
://w
ww
.nat
ure.
com
/nat
ure
/jour
nal/v
451/
n71
76/f
ull/
nat
ure
0658
3.h
tml
Nature
451
, 26
6-26
8 (1
7 Ja
nuar
y 20
08)
Red = Slow seismic velocityBlue = Fast seismic velocity
Seismic tomography shows that some parts of the mantle are hotter than others
Would result in heat transfer
Conduction in lithosphere
Convection in asthenosphere
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Fig. 2.13, page 31
CONVECTION
• Heat transfer that results when warmer, less dense material rises, and cooler, denser material sinks
http://www.columbia.edu/itc/ldeo/mutter/jcm/Topic3/Topic3.html
CONVECTION
• Convection within the mantle used to be considered by some to be a major driving mechanism of plate tectonics (“ridge push” mechanism, page 30)
http://www.columbia.edu/itc/ldeo/mutter/jcm/Topic3/Topic3.html
Melting by lowering pressure Melting temperature of a solid goes
UP under high pressure (becomes difficult to melt)
Melting temperature of a solid goes DOWN under low pressure (becomes easier to melt)
Is the major process of magma generation at divergent boundaries and hot spots (pages 30-31)
At divergent boundaries and hot spots, lithosphere becomes thinner
due to pulling from subducting slabs
Pressure is reduced and hot rocks from asthenosphere move upward (convection)
Rocks start to melt
Melting caused by decreasing pressure is called DECOMPRESSION MELTING
Example: Hawaii (hot spot)Mid-Atlantic Ridge (divergent boundary)
A little note on hot spots …
• Hot spot volcanoes are NOT associated with any plate boundary (example: Hawaii, Yellowstone, Galapagos island etc.)
• They develop over columns of very hot, plastic rock called MANTLE PLUMES (pages 39-44)
LINK TO: http://karel.troja.mff.cuni.cz/staff/HANKA_CIZKOVA/Anim/animace.htm
Dr. Hana Cizkova (Kyvalova)
Adding water under pressure
Melting temperature of a solid goes DOWN when water is added (wet rocks melt more easily than dry rocks)
Is the major process of magma generation at subduction zones (page 37, figure 2.18)
Increasing temperature:
When magma passes through and melts other solid rocks
Not a major process of magma formation
