Download the Earth - Physical Science 100

The Interior of the Earth
Chapter 30
The
Interior of
the Earth
Quick Quiz
• What are the parts of a continent?
–
shield, stable platform, fold mountains
Yesterday:
The main features of the earth’s surface
are continents and ocean basins.
Today:
What are the main features of the earth’s
interior?
How did we find out what these features are?
Merry Christmas
• How do you figure out what’s in the
box?
–
–
• Ocean?
–
anything with the word abyssal, ocean or
sea in it.
How do we know what’s inside
the Earth?
1.
2.
3.
4.
5.
Drill a hole. . . a very deep hole!
“Weigh” the earth.
“Listen” to earthquakes. (Best)
Examine meteorites.
Study magnetic properties.
–
–
–
Peek
Weigh
Shake
Look for clues around the house (shopping
bags, price tags)
Borrow a metal detector…(they measure
magnetic fields generated in metals)
Drilling a Hole
Ocean drilling program
Deepest hole penetrated 2,111 m (1.31 mi)
Leg 148, E Pacific Ocean
Shallowest water depth: 37.5 m (123 ft)
Leg 143, NW Pacific Ocean
Greatest water depth: 5,980 m (3.72 mi)
Leg 129, W Pacific Ocean
“Weighing the Earth”
(The Cavendish Experiment)
Direct Observations
(Sir Henry Cavendish (1798))
gold
Inclusions in Volcanic Eruptions
Pieces of rock from deep in the Earth are
brought up in magma as it rises to the
surface
348 lb balls of lead
F = GmM , so if you know G, m, d and F you can
d2
figure out the mass of the Earth
Evidences from Earth’s Mass & Density
Density of overall Earth is 5.5 g/cm3
Granite has a density of ~2.7 g/cm3
Continental Crust
Basalt has a density of ~3.0 g/cm3
Oceanic Crust
Peridotite has a density of ~3.3 g/cm3
This represents the rocks from the Upper Mantle
What can we conclude about the
interior of the earth compared to the
crust?
A. The interior is more dense
B. The interior is less dense
C. The interior has the same density
So, what does this tell us about the
Earth’s Interior?
From density alone can we tell which of the
models below is correct?
Gradually
increasing
density
model
Layered
density
model
Inferences from Meteorites
Stony Chondrite
Iron
Stony Achondrite
Evidence from Meteorites
Meteorite - chunk of rock from space that
lands on earth. (“falling star”)
“Stony” meteorites
a. Silicate material
b. “Less” dense
“Metallic” meteorites
a. Iron, nickel
b. “More” dense
Planetesimal
Asteroids, Meteors
Kaboom Lots of little pieces
Silicates (80%)
Iron, nickel (20%)
Earth as a magnet
• Must be magnetic material
–
Iron, nickel or both
• Permanent magnet loses
magnetism at high temperature,
so must be due to current.
• Molten iron/nickel has electric
current.
Conclude, at least some portion of
the earth’s core is molten iron
and nickel.
The two ways to make a magnet
1. Iron and nickel atoms in certain arrangements
N
S
2. electric currents
current flowing
through a wire
generates a
magnetic field
“Listening” to Earthquakes
In the early 1950’s sensitive devices were
developed to monitor nuclear bomb testing
and stations were set up around the world to
detect explosions.
The devices also monitor earthquakes and
have given us a great deal of information
about the earth’s interior.
Earth
Types of Waves
Which damage is from a P wave?
Surface waves
cause damage and destruction
Volume waves
compression waves
primary (P) waves, arrive first
travel through solids and liquids
shear waves
secondary (S) waves, arrive second
will not travel through liquids
Seismograph
What has been learned from seismic waves
(earthquakes) about the earth’s interior?
1. How does the speed of the waves
change with depth?
Does speed change gradually, or are their abrupt
changes in medium?
2. What are the layers like that
influence the waves?
Do the waves stop entirely, speed up or slow
down? What does this tell us.
3. What are the shadow zones and
why do they occur?
Speed of
Seismic Waves
• P faster than S
• Speed generally
increases with depth,
except
–
–
–
Mohorovicic
discontinuity
Asthenosphere boundary
Mantle / core boundary
Earthquake
P waves
shadow
S waves
• S waves cannot travel
through liquid
• Resulting shadow zone
is bowl-shaped
• Implies core is liquid
• S waves stop here!
• Speed constant in inner
core
Shadow
zones, the
movie
• Waves change speed abruptly
when they transition from
solid to liquid
• This causes bending
(refraction)
• Refraction makes it
impossible for waves to arrive
in the shadow zone
• Resulting shadow zone is
doughnut-shaped
shadow
A Real Shadow Zone
Shadow Zones
Areas where
no waves can
be detected
So what do we conclude?
Earth is Layered (Differentiated)
Compositional Layers
Core – made of Iron
Mantle – made of Peridotite
Crust – made of granite & basalt
Mechanical Layers
Inner Core – Solid
Outer Core – Liquid
Mesosphere – Solid, but plastic
Asthenosphere – Mostly solid, plastic, 1-6% liquid
Lithosphere – Solid, brittle
The layers of the earth
P Shadow
zone
between the
dark ovals
Two Depth Characterizations
Composition
crust
Physical
Characteristics
lithosphere
mantle
asthenosphere
Mechanical Layers
Question
A strong earthquake causes “S” (shear waves) and “P”
waves to travel out through the earth. Which
statement(s) is (are) true about the shadow zone(s) on
the opposite side of the earth?
a. neither “S” nor “P” waves are seen beyond about 103o
b. “S” waves are never seen beyond 103o. “P” waves are not
seen just beyond 103o but appear again beyond about
143o. Therefore the shadow zone for “S” waves is circular
but the shadow zone for “P” waves is doughnut shaped.
c. The shadow zone for “S” waves is evidence for a liquid
core in the earth.
d. The shadow zone is the result of a very dense igneous rock
in the upper mantle that prevents both “S” and “P” waves
from getting through.
e. both b and c above.