Download The Earth`s structure

The Earth's structure
Earth’s inside structure is quite different to its hard, crusty shell. We
sometimes get a glimpse of Earth’s interior through the action of active volcanoes.
Earth’s rocky crust is by no means stationary and we regularly see evidence of crust
movement in the form of earthquakes. Earthquakes in ocean regions produce
destructive ocean waves called ‘tsunamis’. The universal acceptance of plate
tectonic theory is recognized as a major milestone in the earth sciences. It is
comparable
to
the
revolution
caused
by
Darwin’s theory of evolution or Einstein’s theories about motion and gravity. Plate
tectonics provide a framework for interpreting the composition, structure and
internal processes of Earth on a global scale.
The Earth has a layered structure made up of the core, mantle and
crust. The lithosphere - the crust and upper part of the mantle - is broken
into large pieces called tectonic plates. These move slowly over the
mantle.
The Earth is almost a sphere. These are its main layers, starting with the outermost:
1.
Crust - The Earth’s crust is the outermost layer, consisting mainly of the
chemical elements silicon and aluminum. The crust has two types: a continental
crust that varies in thickness between 20 km and 90 km, and an oceanic crust
that varies in thickness between 5 km and 10 km. The oceanic crust is denser
than the continental crust.
2.
Mantle -The mantle is the thickest of Earth’s layers and takes up 83% of
Earth’s volume. It extends down to about 2900 km from the crust to Earth’s core
and is largely composed of a dark, dense, igneous rock called ‘peridotite’,
containing iron and magnesium. The mantle has three distinct layers: a lower,
solid layer; the asthenosphere, which behaves plastically and flows slowly; and a
solid upper layer. Partial melting within the asthenosphere generates magma
(molten material), some of which rises to the surface because it is less dense
than the surrounding material. The upper mantle and the crust make up the
lithosphere, which is broken up into pieces called ‘plates’, which move over the
asthenosphere. The interaction of these plates is responsible for earthquakes,
volcanic eruptions and the formation of mountain ranges and ocean basins. The
section on plate tectonic theory later in this topic explains the occurrence of
these events further.
3.
outer core - made from liquid nickel and iron
4.
inner core - made from solid nickel and iron
The lithosphere consists of the crust and outer part of the mantle. It is the
relatively cold outer part of the Earth’s structure.
Movement of Earth’s crust
Plate tectonic theory
Plate tectonic theory is a theory developed in the 1910s by a German
meteorologist, Alfred Wegener, who amassed a tremendous amount of geological,
paleontological and climatological data that indicated continents moved through
time. He proposed the hypothesis of ‘continental drift’ to explain his data. However,
Wegener’s theory was not accepted at the time because it could not account for a
mechanism by which the huge continental masses move; evidence of a possible
mechanism was not found until the 1950s and 1960s. Plate tectonic theory is now
universally accepted. The significance of this theory is enormous when you consider
that it can account for many seemingly unrelated geological features and events.
According to plate tectonic theory, the lithosphere is divided into about a
dozen rigid sections, called ‘plates’,
which move over the asthenosphere,
the part of the mantle that behaves
plastically and flows slowly (imagine
bricks moving over freshly laid mortar).
Firstly, there are three types of
plate boundary, each related to the
movement seen along the boundary.
•
Divergent boundaries are
where plates move away from
each other
•
Convergent boundaries are
where the plates move towards each other
•
Transform boundaries are where the plates slide past each other.
Divergent Boundaries
•
Divergent boundary is a linear feature that exists between two tectonic
plates that are moving away from each other. These areas can form in the
middle of continents or on the ocean floor.
•
As the plates pull apart, hot molten material can rise up this newly formed
pathway to the surface - causing volcanic activity.
•
Iceland is located right on top of a divergent boundary. In fact, the island
exists because of this feature.
•
As the North American and Eurasian plates were pulled apart (see map)
volcanic activity occurred along the cracks and fissures (see photographs).
•
With many eruptions over time the island grew out of the sea!
•
Question: Why don’t we have islands like Iceland where ever we get an
Ocean Ridge?
•
Answer: Scientists believe that there is a large mantle plume (an upwelling
of hot mantle material) located right underneath where Iceland has formed.
This would mean that more material would be erupted in the Iceland area
compared with if there was just the divergent boundary without the plume
underneath it.
Convergent Boundaries
•
One plate sinks beneath another plate (called ‘subduction’) along a
subduction zone. The leading edges of the colliding plates may both be
oceanic, or one plate may be oceanic (and will be the sinking plate) and the
other continental, or both plates may be continental. Where oceanic plates
collide, deep trenches in
the ocean occur. As the
plate descends it melts to
generate magma. As this
magma rises, it may
erupt at Earth’s surface,
forming
a
chain
of
volcanoes.
Where
continental plates collide,
mountain ranges such as
the
Himalayas
arise.
Mountain chains are also
formed where one of the
plates is continental and
the other is oceanic.
Example:
•
India used to be an island, but about 15 million years ago it crashed into Asia
(see map).
•
As continental crust was pushing against continental crust the Himalayan
mountain belt was pushed up.
•
“Mountains” were also pushed down into the mantle as the normally 35 km
thick crust is approximately 70 km thick in this region.
•
Mt Everest is the highest altitude mountain on our planet standing 8,840
meters high. This means that below the surface at the foot of the mountain
the crust is a further 61 km deep!!
At a convergent boundary where continental crust pushes against oceanic crust, the
oceanic crust which is thinner
and
denser
than
the
continental crust, sinks below
the continental crust.
•
This
is
called
Subduction Zone.
a
•
The
oceanic
crust
descends
into
the
mantle at a rate of
centimeters per year.
This oceanic crust is
called the “Subducting
Slab” (see diagram).
•
When the subducting
slab reaches a depth of
around 100 kilometres, it dehydrates and releases water into the overlying
mantle wedge (Presenter: explain all of this using the diagram).
•
The addition of water into the mantle wedge changes the melting point of the
molten material there forming new melt which rises up into the overlying
continental crust forming volcanoes.
•
Subduction is a way of recycling the oceanic crust. Eventually the subducting
slab sinks down into the mantle to be recycled. It is for this reason that the
oceanic crust is much younger than the continental crust which is not
recycled.
Transform Boundaries
•
Plates slide sideways past each other. The San Andreas Fault in California is a
transform plate boundary separating the Pacific plate and North American
plate. Sliding plates build up pressure in certain places, causing the sudden
movement of plates to release the pressure. The sudden movements of
plates are earthquake.
Recoverable or Renewable Resources
Renewable resources are natural resources that can be replenished
in a short period of time.
● Solar
● Geothermal
● Wind
● Biomass
● Water
1.
Solar energy is radiant light and heat from the sun harnessed using a range
of
ever-evolving
technologies
such
as solar
heating,
photovoltaics, solar thermal energy, solar
architecture
and
artificial
photosynthesis. It is an important source of renewable energy and its technologies
are broadly characterized as either passive solar or active solar depending on the
way they capture and distribute solar energy or convert it into solar power.
2.
Geothermal energy is the heat from the Earth. It's clean and sustainable. Resources of
geothermal energy range from the shallow ground to hot water and hot rock found a few miles
beneath the Earth's surface, and down even deeper to the extremely high temperatures of
molten rock called magma.
Palinpinon Geothermal Plant,
Negros Philippines.
3.
Wind
energy
or
power
is extracted from air flow using wind
turbines or sails to produce mechanical or electrical power. Windmills are used for
their mechanical power, wind pumps for water pumping, and sails to propel ships.
Wind power as an alternative to fossil fuels, is plentiful, renewable, widely
distributed, clean, produces no greenhouse gas emissions during operation, and
uses little land.
Bangui Wind Farm,
Ilocos Norte Philippines.
4.
Hydroelectric power, or hydroelectricity, is generated by the force of falling
water. (Hydro comes from the Greek word for water.) It’s one of the cleanest sources
of energy, and it’s also the most reliable and costs the least.