JONOE LOUIS A. MILLAN
MARCH 23, 2012
MELGIE T. CALIAO
NATSCI-A7
THE LAYERS OF THE EARTH
(PLATE BOUNDARIES)
What is the use of locating seismic discontinuities?
•
Locating these disturbances enable scientists to map the inner regions of the Earth. This science,
known as tomography originates from the knowledge gained from discontinuities.
•
Tomographists have found that this planet is divided into six regions: the inner core, the outer
core, the lower mantle, the upper mantle, the transition region, and the crust (oceanic and
continental).
Here is a brief synopsis of the depths of each layer (in
kilometers):
•
0- 40 Crust
•
40- 400 Upper mantle
•
400- 650 Transition region
•
650-2700 Lower mantle
•
2700-2890 D'' layer
•
2890-5150 Outer core
•
5150-6378 Inner core
About the Crust
•
We know that the crust plunges down to adepth that varies between 5 to 70 km, making it the
thinnest layer. It is mainly composed of rocks, which are subdivided into two groups – sial and
sima.
•
The continental crust is mainly composed of silica and alumina, hence called sial. On the other
hand, oceanic crust, which is mainly composed of silica and magnesium, is called sima.
 Oceanic Crust
•
is the smallest part of Earth, only 0.099% of its mass and reaching a small depth of 0-6 miles (010 kilometers). In the beginning of time, it was possible that this area did not exist for through
frequent volcanic activity does only the crust form.
•
Evidence of this is marked by the oceanic ridge system, which is a 25,000 mile (40,000kilometer) array of many volcanoes which creates layer after layer of new crust at the rate of 17
km3 per year.
•
The ocean floor is covered in basalt originating from volcanic activity and as a matter of fact,
Iceland and Hawaii are two island systems that emerged from the accumulated basalt.
 Continental Crust
•
The second smallest area of the Earth is the Continental crust, making up only 0.374% of the
Earth's mass and extending a short depth of 0 - 31 miles (0-50 kilometers).
•
Looking at the percent by composition, the continental crust makes up only 0.554% of the
mantle-crust mass.
•
The layer is composed primarily of crystalline rocks made of low-density buoyant minerals
dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates).
•
This is the outer part of the Earth composed essentially of crystalline rocks.
•
The continental crust and the oceanic crust are also referred to as the lithosphere because of
the cool and rocky conditions that exist in its chemical composition.
About the Mantle
•
We know that the mantle reaches down to a depth of about 2,890 km. While the crust is the
thinnest layer, the mantle is the thickest.
•
The upper mantle is represented by the asthenosphere, a highly viscous layer that supports the
lithosphere. Like the lithosphere, the asthenosphere plays a significant role in plate tectonics.
•
Despite the extremely high temperatures in the mantle (up to 4,000 degrees Celsius), even
higher than the melting points of the rocks that constitute it, the very high pressures there
prevent melting from taking place.
 Lower Mantle
 its chemical composition includes silicon, magnesium, and oxygen.
 also contains some iron, calcium, and aluminum.
 is comprised of 72.9% of the antle-crust mass, making the Earth abundant in the chemical
elements of silicon, magnesium and oxygen, the layer's primary components.
 Upper Mantle
Through excavations in volcanoes, scientists have found that this part of the crust composes of
15.3% of the total mantle-crust mass and is made of crystalline forms of Olivine (Mg,Fe)2SiO4 and
pyroxene (Mg,Fe)SiO3.
•
The upper mantle makes up 10.3% of the Earth's mass, extending a depth of 6-250 miles (10-400
kilometers).
•
A relatively large portion when compared to the other interior layers.
•
This layer is not completely made of solid minerals for scientists speculate that the
asthenosphere could be partly liquid molten.
 Transition Region
•
The next layer, the Transition region comprises 7.5% of Earth's mass with a depth of 250-406
miles (400-650 kilometers).
•
This layer is also known as the mesosphere and is 11.1% of the mantle-crust. It is made of mainly
basaltic magmas with amounts of calcium, aluminum and garnet (an aluminum-bearing silicate
mineral.
•
The layer becomes dense when the garnet mineral cools but is buoyant and light when subject
to heat due to the low melting points.
About the Inner Core
At the center of the Earth is the inner core, a solid sphere believed to be made up of an iron-nickel alloy.
That the solid inner core is distinct from the liquid outer core was suggested by Inge Lehmann, a
seismologist who based his theories on earthquake-generated seismic waves.
•
The inner core is a solid section of the Earth and is unattached to the mantle, being suspended
by the molten outer core.
•
This solidified state is the result of a very intense pressure-freezing process that occurs in most
liquids when temperature decreases or pressure increases.
About the Outer Core
•
is mainly composed of melted iron and nickel.
•
They are melted because the pressures at these depths are not sufficient to prevent the much
higher temperatures, which range between 4,400 degrees Celsius to 6,100 degrees Celsius, from
melting the rocks there.
•
Convection in the outer core, which move the liquefied metals about, is believed to be the
source of the magnetic field that encapsulates the Earth and protects it from cosmic radiation.
•
The outer core of Earth is a scorching hot, electrically conductive liquid in which convection
takes place.
•
The outer core is in the range of 200 to 300 kilometers (125 to 188 miles) thick and represents
about 4% of the mantle-crust mass.
•
This layer is sometimes identified as part of the lower mantle due to its geographical nature.
•
However, studies on seismic discontinuities suggest that this "D" layer might differ chemically
rom the lower mantle lying above it.
REFERENCES
1. http://library.thinkquest.org/28327/html/universe/solar_system/planets/earth/interior/lay
ers_of_earth.html
2. http://www.universetoday.com/61856/the-earths-layers/