Transcript Pre-plate - Santee School District

How Do We Know What It
Is Like Inside The Earth?
Pre-GPT scientific thought on the Earth before
1950
Department of Geological Sciences
San Diego State University
What direct evidence tells you about
the inside of the Earth?
How Symmetrical Is the Earth?
How Symmetrical Is the Earth?
Very, a the rotation about its axis is
fairly smooth
Symmetry
would be imposed
by initial spinning
while molten
Any reasonable
Earth
model proposed
must be
symmetrical
The Interior Temperature of the Earth


Is the inside of the Earth hot or cold?
What evidence is there for this?
Evidence that the Earth is hot inside:
Volcanoes
Hot Geysers
Hot sulfurous gasses from vents
Evidence that the Earth is hot inside:
Deep Mines
The temperature in all deeper mines increases with depth:
About 30 oC per 1000 m
This image is in a South
African Gold mine 3km or
2 miles deep in the Earth.
The temperature is 49 C
or 120 F
The limit to the depth of
a deep mine is when
the temperature is to
hot for stand
Deepest mine is
3,585 m or 11,762 ft
Deepest mines only extend a tiny fraction into the average
thickness the crust, never mind the rest of the Earth
Deepest
Mine
3.4 km
.
Earth
Radius~
6,400 km
What About Magnetism?



Is the Earth’s magnetic?
What sort of properties does it have?
Could you detect the Earth’s
magnetic field
Age of Exploration, 1400-1600,
also discovered the compass and
Earth’s magnetic field.
The Earth’s
Magnetic Field: by
compass
Magnetic Dip:
the angle that magnetic
lines
leave or enter
the Earth
Aurora Borealis and Aurora Australis,
northern and southern lights
Most frequent is near 67 N and S latitude
Maximum
N. Pole Coverage
Maximum
S. Pole Coverage
Earth’s magnetic field must
be deep inside,
And uniform and symmetrical
to cause symmetrical compass variations and
the Auras Borealis and Australis
I am
So
proud
What can gravity tell us
about the Earth?
Sir Isaac Newton
Published his
Philosophiae Naturalis Principia
1686
which included the definitive
statement on gravitation
Basic Gravity:
Any two bodies in the universe
attracted each other with a force that is
GREATER
if either is more
GREATER
if they are
MASSIVE
CLOSER
Force,
equal
and
opposite
Hey,
this
Took a
lot of
work
Force,
equal
and
opposite
Gravity: The Formula
Memorize
this for the
quiz
G
m2
D
m1
Cavendish’s 1789 Experiment to Determine
the Mass and Density of the Earth
* Use torsion balance to measure
the force between 4 iron balls
Ball
* Measure the size and mass of the
weight
iron balls
* Calculate “constant” in gravity formula
Ball
mass
Earth
mass
G
Earth
Radius
Earth
Earth Mass
= 6 x 10 24kg
Earth
Average Density
= 5.54 g/cm3
Cavendish’s 1789 Experiment to Determine
the Mass and Density of the Earth:
What does this mean?
Earth
Average Density
= 5.54 g/cm3
Earth
Henry
Cavendish
~ 1789
You
should see
this on my
Facebook
Cavendish’s 1789 Experiment to Determine
the Mass and Density of the Earth:
What does this mean?
Water
= 1 g/cm3
Surface rocks as
Basalt, Granite
~ 3 g/cm3
Earth
Earth
Average Density
= 5.54 g/cm3
Deeper Rocks must be
much denser than 5.54 g/cm3
to compensate for lighter surface rocks
What is the
material at the
center of the Earth?
Iron -Nickel
The Earth is thought to be similar to
Meteorites, about 5% of the meteorites
are Iron-Nickel metal. These are
believed to have been the center
of former bodies in space where the
Iron-Nickel, a dense material, settled to
the center when the body was molten.
A later collision broke apart the body into
many small parts.
Iron-Nickel meteorite found in Oregon
Gravity is significantly affected
by masses close to the Earth’s surface
– even only a kilometer or so under ground
Earth’s
Radius
6,500 km
Ore bodies near the surface
will cause gravity to be more and less –
a way of searching for them
The British
Great Arc
Meridian
Survey of
India
1803-50
The British discovered that their
plumb bobs were not pointing
toward the center of the Earth
What could be an explanation?
Plum bobs, gravity and floating
George Airy
John Pratt
Airy and Pratt proposed different
explanations for the nature of
the crust.
Airy hypothesized that there
was a uniform density crust
to the Earth and it varied in
thickness – he was more right.
Isostasy and Floating Analogy:
The crust floats on the mantel. A plate floats on a portion of mantle.
Figure 2-5
Crust, Mantle, Core, Plates all - float
with lighter on top, heavier below:
Just as objects do in water
Principle of Isostacy: “A body that is less dense than a surrounding fluid
will displace a volume of the fluid equal to that body’s mass”
What Can Sound Tell Us?
Hint: Earthquakes are mostly sound energy
Earthquakes are used to probe the interior of the
Earth, somewhat as a bat uses sound not only
to detect the presence of objects but about the
objects’ size and nature.
• Primary (P) waves:
- compressional
- analogy is sound waves
• Secondary (S) waves:
- transverse (analogy is ocean waves)
- ~1/2 velocity of P waves
- do not pass through liquid
• By the way, surface waves (a third
type slower than above two) cause the
big disasters
(Garrison, 1993)
Prediction for
Homogeneous
Earth Structure
Observation from
Global Seismological
Networks
(Garrison-3.7Top)
(Garrison, 1993)
(Pink Floyd, 1973)
.
(Pink Floyd, 1973)
Connect the dots: Then you get curves of
temperature, density, P and S speed,
properties and molecules
Crust
The End