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GL1 I Matter
KI1 b The crust
GL1 I KI 1b crust
• Specifications:
• The crust is a thin layer of distinctive
composition overlying the mantle;
• Continental and oceanic crust can be
recognised
• And distinguished by differing
thicknesses, composition and structure
The crust
OCEANIC
and
CONTINENTAL
The crust
• Solid rock
• Very thin compared to size of Earth
• If the Earth was the size of a football,
the crust would be the equivalent
thickness of a postage stamp stuck on
it!
• Density between 2.7g/cm3 to 3.2g/cm3
WARNING!
• In OLD textbooks (and many recent
geography textbooks!)
• They refer to SIAL and SIMA
• Please IGNORE these…..
• They are based on outdated information
and will be marked wrong in geology
exams
• They often also show continental crust
floating on oceanic crust, this is also
incorrect.
CONTINENTAL CRUST
• Forms the continents, and extends a little
beyond them (the continental shelf and
continental slope)
• Mainly GRANITIC rock type
• Density 2.7g/cm3
• Formed over a long period of time
• Thickness varies, up to 70km thick in
mountain ranges
• Mountain ranges have “roots”
Thick crust in mountain ranges
Crust which is not as high
has less underneath it
ISOSTASY
• Is the balance between the crust and
mantle
• As material is eroded off the mountain top,
the root moves up as mantle flows
underneath it.
OCEANIC CRUST
• Much thinner than continental crust
• Average 6km
• Younger than continental crust, max age
220 million years
• Mainly composed of BASALT
• Which is a slightly denser rock 3.0g/cm3
Age of ocean crust
At the bottom of the crust
• Boundary between the Crust and the
Mantle is the
• MOHO DISCONTINUITY
• Named after Mohorovičić
• Depth varies, deeper under mountains
http://geology.com/articles/mohorovicicdiscontinuity.shtml
• Moho shows a “jump” in the speed of P
waves
• It’s like them taking the motorway
Discontinuities
• How do you know that
you have passed from
the crust into the
mantle?
• Density change 2.7 or
3.0 to 3.3.
• This change shows up as
a change in seismic wave
velocities and is called a
discontinuity.
Discontinuities
• The waves are also
refracted.
• In particular this is
called the Mohorovicic
discontinuity or Moho.
• Several other
discontinuities exist
within the Earth (but we
are only expected to know
the Moho)
The crust is broken up into PLATES
Where plates meet there is
seismic activity
Boundaries between plates:
Two plates move together
DESTRUCTIVE
or
CONVERGENT
BOUNDARY

Two plates move apart
CONSTRUCTIVE
or
DIVERGENT
BOUNDARY

Controlled by CONVECTION CURRENTS in the Mantle
• We’re leading into one of the great
geological concepts
• PLATE TECTONICS
• (plates and the study of their
movements)
• GL1 II Energy KI 3