Transcript astron_ch_7c (1)

Sponge:
List the layers
of the Earth’s
atmosphere.
Earth’s Interior Crust - ~15 km thick,
8 km under oceans,
20-50 km under
continents,
3
density of 3 g/cm
Mantle - dense, but
rocky material,
80% of Earth’s
volume,
3
density 5 g/cm
Outer core - liquid
3
density ~ 8 g/cm
Inner core - solid
density is compressed
3
to 12 g/cm
The inner and outer
core are both the
same temperature
and are made of
the same material:
nickel and iron.
This separation of
the parts of a planet
(or moon) by
density is called
differentiation.
The Earth was molten in
the past which allowed
differentiation to occur.
The Earth was molten due
to the bombardment by
other materials and
radioactive decay.
The upper mantle is
made of basalt
3
(3.5 g/cm ); the
continents are made of
3
granite (3 g/cm ) which
can float on the mantle.
The Earth’s central
core temperature is
nearly equal to the
Sun’s surface
temperature.
5000K 8500°F
Surface Changes
Plate Tectonics - the Earth
is covered by surface
plates which move
individually. This
movement was detected
by comparison to quasars.
The plates are not
necessarily the continents
themselves.
Some are continents plus
ocean floors (Indian plate).
Some are all ocean floor
(Nasca plate).
All of the plates together form
the Lithosphere. The
lithosphere is the portion of
the Earth that undergoes
tectonic activity. It is made
up of the crust and a small
part of the upper mantle.
The semisolid part
of the mantle over
which the plates
slide is called the
Aesthenosphere.
One of the largest divisions
between plates is the MidAtlantic Ridge which
separates the North
American Plate from the
Eurasian Plate and the
South American Plate
from the African Plate.
Collisions between plates cause
(1) mountain ranges
(like the Himalayas) or
(2) subduction zones where
one plate slides under the other,
ultimately to be destroyed.
This is responsible for most
deep ocean trenches.
Not all plates collide,
some “shear” past each
other (San Andreas
Fault). Sudden, jerky
slippages of these faults
cause earthquakes.
Some plates move
apart (as in the Atlantic
Ocean).
In these cases, the
mantle material wells
up forming ridges
(Mid-Atlantic Ridge).
What moves the
plates? Probably
convection currents
in the mantle.
Continental drift was proposed in
1912 by Alfred Wegener.
What evidence
is there for
continental
drift?
1. Geographical evidence the continents fit together.
2. Fossil evidence Fossils of an extinct reptile
are found only on the
Brazilian coast and the
west coast of Africa.
3. Ocean floor age The ocean floor close to
the Mid-Atlantic Ridge
is young, farther away
on either side gradually
gets older.
4. Paleomagnetism the Mid-Atlantic Ridge
shows matching
reversals of the Earth’s
magnetic field on either
side of the ridge.