EESA06H3 Chapter Notes - Chapter 4: Epicenter, Outer Core, Ultramafic Rock

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12 Aug 2016
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The Crust
oThe crust is thinner beneath the oceans (5-8 km thick) than beneath the continents( <70 km
thick)
oSeismic waves travel faster in oceanic crust (7km per second [seismic P waves]) than in
continental crust (6km/second [seismic P waves])
ob/c of this velocity difference, it is assumed that the 2 types of crust are made up of different
rocks.
oThe upper part of the oceanic crust is BASALT and suggest that the lower part is GABBRO
(course-grained equivalent of basalt)
oSeismic P waves travelling in continental crust (more slowly) travel at the same speed at which
they travel through granite & gneiss.
oContinental crust is often called “GRANITIC” but the term should be put in quotation marks b/c
most of the rocks exposed on land are not granite.
oContinental crust
- Highly variable/complex , consisting of a crystalline basement composed of granite, other
plutonic rocks, gneisses, and schists, all capped by a layer of sedimentary rocks like icing on
a cake.
oSome geoscientists use the term
- FELSIC rocks high in feldspar and silicon for CONTINENTAL CRUST
- MAFIC  rocks high in magnesium and iron (ferric) for OCEANIC CRUST
oContinental crust thickest under geologically young mountain ranges such as the Andes &
Himalayas bulging downward as a mountain root into the mantle.
oContinental crust also less dense than oceanic crust (important in plate tectonics)
oThe boundary that separates the crust from the mantle beneath it is called the MOHOROVICIC
DISCONTINUITY (“MOHO”)
oMantle lies closer to earth’s surface beneath the ocean than it does beneath the continents.
oProject Mohole
- To use specially equipped ships to drill through oceanic crust and obtain samples from the
mantle
- Abandoned due to high costs; but ocean floor drilling has become a routine since
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