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ASTRONOMY 161
Introduction to Solar System Astronomy
Class 15
The Earth
Monday, February 12
The Earth: Physical characteristics
Mass = 5.97×1024 kg
Mean radius = 6,373 km
Polar radius = 6,357 km
Density = 5,515 kg/m³
Sidereal rotation period = 0.9973 d (23.934 h)
Axial tilt = 23.4°
The Earth: Key Concepts
(1) The study of seismic waves tells us about the
Earth’s interior.
(2) The Earth is layered into crust, mantle, inner core,
and outer core.
(3) The Earth is layered because it underwent
differentiation when molten.
(4) The lithosphere is broken into plates that move
relative to each other.
(5) The motion of liquid metal in the outer core
produces a magnetic field.
(1) The study of seismic waves
tells us about the Earth’s interior.
How can we study the deep
interior of the Earth?
Average density of the Earth
= 5500 kg/m3
Earth is too dense to be
solid rock:
Basalt = 3300 kg/m3
Granite = 2700 kg/m3
Earthquakes produce two types of seismic waves:
P waves [Primary, Pressure]: Sound waves travel
through solids and liquids.
S waves [Secondary, Shear]: Transverse (side-to-side)
waves that do NOT travel through liquids.
Seismic waves
radiating through
the Earth after an
earthquake:
Note: S waves do
not travel through
the outer core!
(2) The earth is layered into a crust, mantle,
inner core, and outer core.
From the outside in:
1) Crust:
solid rock
5 km thick basalt (ocean floor)
35 km thick granite (continents)
2) Mantle:
partly solid rock, partly semisolid (plastic) rock
2900 km thick
3) Outer (liquid) core:
molten iron and nickel
2200 km thick
4) Inner (solid) core:
solid iron and nickel
1300 km in radius
The center of the
Earth is as hot as
the surface of the
Sun.
Why doesn’t the
inner core melt??
The high pressure
in the inner core
keeps it solid.
(3) The Earth is layered because it
underwent differentiation when molten.
When young, the
Earth was heated
by the impact of
planetesimals. For
a while, the Earth
was molten.
In a liquid, dense
stuff sinks, lowdensity stuff floats.
When differentiation was complete, Earth cooled and
(partly) solidified.
Crust of the ocean floor floats on the mantle “like skin
on cocoa”; lower-density continents are the
“marshmallows”.
(4) The lithosphere is broken into
plates that move relative to each other.
The crust plus the upper
mantle form the lithosphere
(solid but brittle).
Beneath the lithosphere is the
asthenosphere (plastic).
The asthenosphere,
heated from below,
undergoes convection.
Convection
Convection currents in the asthenosphere have broken
the lithosphere into sections called plates.
There are 15 large plates.
Continuing convection in the asthenosphere causes
plates to move relative to each other.
The study of plate motion is called plate tectonics.
The motion of continents
was first suspected by
Sir Francis Bacon (17th cent).
Best known for leading the
scientific revolution with his
new 'observation and
experimentation' theory.
America and Europe
are moving apart by
3 centimeters per year
(= 30 km per million
years).
This “continental drift”
is measured using
GPS (global
positioning
systems).
200 million years
ago, the Americas,
Europe, and
Africa formed a
single
supercontinent,
PANGAEA.
The boundaries
between plates
are geologically
active, with many
volcanoes and
earthquakes.
Example: the Ring
of Fire around the
Pacific Ocean.
Types of plate boundaries
1) Oceanic rift: plates moving apart, new rock
forming.
2) Subduction zone: plates moving together,
ocean crust shoved under continent.
3) Mountain-building zone: plates moving
together, continents collide & buckle upward.
4) Transverse fault: plates slipping past each
other, with lots of friction.
The summit of Mount Everest is made of
marine limestone.
It has been lifted
9 kilometers
high!
The San Andreas
transverse fault:
Motion jumps
during a quake:
(5) The motion of liquid metal in the
outer core produces a magnetic field.
A compass points to
the Earth’s North
Magnetic Pole
(not the true
North Pole!)
Why is the Earth a
giant magnet?
Remember: Moving charged particles create a
magnetic field.
Inside the Earth, convection currents exist within the
liquid outer core. These currents carry electrons
around, creating a magnetic field.
The Earth’s
magnetic field
stretches far
beyond the
surface.
Our magnetic field
deflects the solar
wind (electrons
and protons
streaming away
from the Sun).