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Journey to the Surface
of the Earth, part 2
Or, Earth from the
Inside-Out
The Earth’s Interior
 Yesterday
we discussed these points:
– How we know about the interior of the Earth…
– How Earth’s interior took on the characteristics it
has…
– The layers of the Earth as defined by composition
 Today
we will discuss:
– The layers of the Earth as defined by
behavior…
Layers Due to Behavior
 The
Inner Core
– Solid sphere at Earth’s center
 High pressure maintains solidity even though
temperatures are high enough to melt metal
– Only 1200 km thick
– Boundary between the inner and outer core
is called the Lehmann Discontinuity
Layers Due to Behavior
 The
Outer Core
– Liquid, metallic core
– High temperature creates convection
currents
 Vertical motion in a fluid like boiling water
 Hot liquids are less dense (and so rise); cool
liquids are more dense (and so sink)
Layers Due to Behavior
– Motion of the liquid metal produces a
magnetic field – Earth’s
“magnetosphere”
– Without the magnetic field, Earth would
be subjected to deadly radiation from
the sun
– Therefore, without the outer core, there
would be NO LIFE on Earth
Layers Due to Behavior
– The magnetic field is due to the combined
properties of the outer core
 It is metallic AND it is liquid AND in motion
 ALL THREE are required to produced the
magnetic field
– For example – Mercury has an iron core, but no
magnetic field because it is solid!
– Venus has a liquid iron core, but it has no magnetic
field because there is very little motion within the
liquid
The Lower Mantle
 The
lower mantle is a thick (over
2200 km) layer of mostly solid rock
– However, rock is not rigid – it can flow
like a thick liquid (like cold honey)
– Is responsible for heat transfer from the
core outward
The Asthenosphere
 The
asthenosphere is part of the upper
mantle
– It is consider a “plastic” zone – not complete
molten, but not really rigid
– Rocks in the asthenosphere are very close to
their melting points and so deform easily
– Convection in this region is thought to be
the driving force of plate tectonics
The Asthenosphere
The Lithosphere

The lithosphere consists of the uppermost
mantle and the entire crust
 It is a solid, rigid layer that is broken into
several “plates”
– Lithospheric plates are large segments of the
lithosphere that consist of oceanic and/or
continental crust
– Seven major and many minor plates (has changed
throughout Earth’s history)
– Plates are in constant motion
The Lithosphere
Motion in the Lithosphere
 In
the lower layers of the Earth (core,
mantle), motion is dominated by
convection
 At the surface, this motion translates into
lateral motion of the plates
Motion in the Lithosphere

Two basic forces are thought
control plate motion at the
surface of the Earth:
– Ridge-push
 As new crust is generated at a midocean ridge, the new crust (assisted
by gravity) pushes the older crust
aside
– Slab-pull
 When oceanic crust is forced under
continental crust, the suction of the
downward slab pulls the rest of the
plate toward the trench
Motion in the Lithosphere
 Both
forces act in tandem (together)
along with convection to move the plates
Summary

What are the five layers of the Earth as defined
by behavior?
 Why is the inner core solid?
 What is the importance of the outer core?
 What is the major role of the lower mantle?
 How is the asthenosphere different from the
lithosphere?
 What is the importance of the asthenosphere?
Summary
 What
are lithospheric plates?
 What are the two forces that control
plate motion?
 What is the basic reason for plate
motion?