Download Structure of the Earth

Structure of the Earth
(Why?)
Recall Hypsometric Curve?
Continental
lithosphere is very
different from
oceanic lithosphere.
To understand this, we
need to know more
about the structure &
composition of Earth.
Earthquake (seismic) waves are the key to
understanding earth s structure.
•  Seismic wave paths traveling through
Earth provide important information
about Earths structure
•  They allow us to image and locate the
boundaries between Earths many
layers (e.g., the core-mantle boundary)
•  Changes in wave paths allow indicate
the changes in composition and
rheology of the Earth (e.g., liquid
outer Fe-core)
•  See Chapter 5 and 6 of your textbook.
A detailed reading guide is given for
you on Compass.
From your reading, you will see…
•  That the science leading to our understanding
of Earth’s structure is interesting and
complicated.
•  It involves Snell’s Law and the physics of
wave propagation, wave theory and spherical
geometry combined with experimental highpressure mineral physics.
•  We will not cover the geophysics of seismic
waves and how we know Earth’s internal
structure. I’ve included it in your assigned
readings to satisfy your curiosity.
–  If you have any questions over this material, don’t
hesitate to see me
What you need to know in order to
understand the oceans…
•  Earth can be divided on the basis
of its composition or its
rheology
–  Rheology = behavior in response to
stress (e.g. rigid, plastic, fluid)
•  Earth’s composition layers –  Crust, Mantle and Core
•  Earth’s rheologic layers
–  Lithosphere, Asthenosphere,
Mesosphere, Liquid Outer Core,
Solid Inner Core
•  The details of these two ways of
seeing Earth’s structure
follow…
Earth’s composition layers
•  Composition layers on left side of
figure:
•  Oceanic Crust: 0-10 km thick
–  Si - Mg - Fe-rich (dense)
•  Continental Crust: 35-75 km thick
–  Si - Al - Ca - Mg - Fe (not dense)
•  Mantle: ~2900 km thick
–  Si - Mg - Fe –  Very rich in Mg-Fe (very dense)
–  Extends from base of crust
(Moho) to the top of the core
(CMB)
•  Core: ~3500 km thick
–  Fe (Ni-S) - rich (supper dense)
Earth’s Rheologic layers (on the right side of the figure)
• 
Lithosphere: 100-150 km thick
–  Includes crust and rigid upper mantle
–  This is the rigid plate of plate tectonics
–  Base is defined by 1280ºC boundary
• 
Asthenosphere: 100/150 to 700 km depth
–  This is a plastic region that the lithospheric
plates ride/float on
–  Begins at ~100 km depth in oceans, 150 km
depth under the continents
• 
Mesosphere extends from 700 to ~2900 km
depth
–  Often called the Lower Mantle
–  At 700 km depth the pressure is so great that
the minerals are forced to compress their
structure (thus change to denser minerals)
–  This is a plastic layer denser than the
asthenosphere
• 
Liquid Outer Core: 2900 to 5155 km
–  Made of dense, convecting liquid iron
–  Generates Earth’s magnetic field
• 
Solid Inner Core – 5155 to 6370 km
–  Solid, crystalline Fe
The lithosphere includes crust
and part of the mantle.
A.  True
B.  False
The Asthenosphere is
composed entirely of
mantle.
A.  True
B.  False
Now we can begin to understand the hypsometric curve
• 
Why is the ocean floor deep compared
to the continental surfaces?
–  In other words, why do continents
(e.g., Antarctica), rise above the
ocean floor?
• 
To answer this question, we need to
understand a few things:
–  If the asthenosphere is a plastic, then
the rigid lithosphere floats on it.
–  A perfect analogy to this concept is a
block floating on putty, or
–  Another analogy: blocks floating in
water
–  Archimedes - a Greek philosopher
played around with this…
Buoyancy Model
What will happen if the thickness of
the block is increased?
A.  The base of the block will sink
deeper.
B.  The top of the block will rise
higher
C.  Both A and B
D.  Nothing will happen, the position
of the block will not change.
model
ZU
ZA
ZB
Isostacy:
• 
concept of isostacy…
–  Continental lithosphere
sinks deeper and rides
higher than oceanic
lithosphere due to it s
thickness and lower
density.
–  In this picture the thick
board represents the
continental lithosphere.
–  The thinner boards
represent the oceanic
lithosphere.
–  Note, if you remove the
top of a continent, it
bobs up in such a way
that the root is shallower,
and the surface is still
above the oceanic
lithosphere.
Examples of Isostacy:
Thick ice sinks deeper
and rides higher than
thin ice
More Isostacy:
animation
Now we can understand the hypsometric curve
• 
Ocean floor is deeper than
continents because –  Ocean lithosphere is thinner than
continental lithosphere
–  Oceanic lithosphere is denser than
continental lithosphere
–  Thus, due to relative buoyancy the
oceanic lithosphere does not ride as
high on the asthenosphere compared
to the continental lithosphere. • 
This is the concept of isostacy…
–  Continental lithosphere sinks deeper
into the asthenosphere and rides
higher than oceanic lithosphere due to
it s thickness and lower density
Now we understand this graph and image!
Review Questions
• 
• 
• 
• 
• 
What does the hypsometric curve show you?
What are the composition layers of the Earth?
What is the radius of the Earth?
How is the asthenosphere different from the lithosphere?
How is the mesosphere different from (a) the
asthenosphere and (b) the lithosphere?
•  Why are oceans deep compared to continents?