Download History of the Earth and its structure

Earth’s Structure and Surface
Structure of the Earth
The earth is thought have originated
about 4.5 billion years ago from a
cloud or clouds of dust. The dust
was the remains of a huge cosmic
explosion which is estimated to
have occurred about 15 billion
years ago.
As the particles collided they
continually formed larger
and larger rocks until
eventually forming the earth
and other planets.
Density
On a young earth, most matter was
molten. This allowed for heavier
objects to sink and lighter objects to
settle on the surface.
The light material cooled to make a
thin crust. Eventually the atmosphere
and oceans began to form.
Earth’s Surface
More than 70% is covered by water.
Water distribution:
Pacific Ocean = 52%
Atlantic Ocean = 25%
Indian Ocean = 20%
Ice = 2%
Lakes, Rivers, Atmosphere = 1%
Earth’s Surface
Average Depth of the oceans = 3800m
*deepest is Mariana Trench = 11035m
Average Elevation of land = 840m
*highest area is Mt. Everest = 8848m
Internal Structures
Core: Estimated to be over
4,000°C (7,200°F)
The innermost
layer of the
earth, the
core, is
comprised of
mostly
alloys of
iron.
Mantle
The layer outside of
the core is called
the mantle.
Though most is
thought to be solid
it is very hot, near
the melting point
of the rocks.
Because of this the
mantle flows like a
liquid but much
slower.
Crust
The outermost layer, and most familiar, the
crust is relatively very thin like a rigid
floating on the mantle.
The composition of
the crusts greatly
differ greatly
between the
oceans and the
continents.
Continental & Oceanic Crusts
 Differences
between the two crusts are
caused by the physical and chemical
differences in the rocks themselves.
 Oceanic crust consists of minerals
called basalt while the continental
rocks consist of granite.
 This results in a several characteristic
differences
Continental vs. Oceanic Crusts
Continental
Density: 2.7 g/cm3
20 to 50 km thick
Can be very old
Light in color
Rich in Na, Ca, & Al
Oceanic
Density: 3 g/cm3
0-10 km thick
Geologically young
Dark in color
Rich in Fe and Mg
Theory of Plate Tectonics
 about
55% of the mass of the
continents is submerged into the upper
mantle.
 however, because the crust cannot
react to changes in its weight, when
forces of uplift or downbending exceed
mechanical strength, the rock fractures
along a plane of weakness (fault).
Origin and Structure of the
Ocean Basins
As early as 1620
Sir Francis Bacon
noted that the
coasts of
continents on
opposite sides of
the Atlantic fit
together like a
puzzle.
Pangaea (225 MYA)
Evidence mounted to support this observation.
Coal deposits matched and fossils could be
collected and matched from each side.
In 1912, Alfred
Wegener
suggested that all
the continents
had once been
joined in a single
“supercontinent.”
Theory of Plate Tectonics
 Wegener’s
theory was not widely
accepted because he could not supply a
mechanism that accounted for the
motion of the continents.
 In the 1950/60s the evidence came
together and scientists concluded that
the continents did drift. This process is
called plate tectonics.
Theory of Plate
Tectonics
 much
material is
turned aside under
the crust and
moves laterally,
causing seafloor spreading which provides
the force to cause continental drift.
proof for sea-floor spreading and
plate tectonics
 3 sources:
1) steadily increasing age of the
sedimentary rocks which rest on the
crust with increasing distance from the
ridges (radiometric dating)
 most rocks have small amounts of
radioactive elements (uranium,
thorium, potassium) which decay into
atoms of other elements
proof for sea-floor spreading and
plate tectonics
2) paleomagnetization patterns in
the basaltic rocks - crust retains the
magnetic pattern (like a tape recording)
on either side of the ridge.
- can date these patterns to the timescale of geomagnetic reversals, which
occur at irregular intervals of every
300,000 to 500,000 years
proof for sea-floor spreading and
plate tectonics
3) polar wandering: if the direction of
magnetic north is plotted for each
continent and it is assumed that the
continents remained stationary
throughout time, then it appears that the
earth's magnetic poles have wandered
from the present locations relative to the
earth's north-south axis (path plotted is
called the polar wandering curve)
proof for sea-floor spreading and
plate tectonics
However, because rocks of the same
age point to two different locations
for magnetic north and because all
evidence points to the magnetic
poles remaining close to the
geographic poles, then it is the
continents that have moved.
Provinces of the Ocean
The sea floor is divided into two main
categories: The deep-sea floor and
the continental margins.
Continental margins are the boundaries
between continental crusts and
oceanic crusts.
Continental Margin
Continental Margin
Generally consists of a gently
sloping shelf, a shelf break, a
stepper continental slope,
another gently sloping region
called the continental rise at
the base of the continental
slope.
Margins
Passive
Active
Passive Margins
Relatively inactive margins that
typically have flat coastal
plains, wide shelves, and
gradual continental slopes.
Active Margins
Active Margins are regions where
the continental margins are
highly active with volcano and
earthquakes. They are
identified by little to no shelf
and a steep slope ending in a
trench, no rise.
Mid-Ocean Ridge
In the years after World War II, sonar surveys of
the sea floor resulted in the discovery of the
mid-ocean ridge system.
This system is a
continuous chain of
volcanic submarine
mountains that
encircles the earth like
the seams on a
baseball.

When these
under water
volcanoes occur
in the middle of a
oceanic plate
they are called a
“hot spot” and
are responsible
for island chains
like Hawaii.
Comprehension Check
1)
2)
3)
4)
5)
The three main divisions of the Earth’s
interior are __, __, and __.
A long chain of submarine volcanic
mountains that runs throughout the world
ocean is called the ___ system.
A single super continent called __ began to
break up onto the present-day continents
about __ years ago.
The continental margins consists of __, __,
__, and __.
The average depth of the ocean is __m.