Chemical and Physical Properties of Earth's Layers and the Tectonic Plates (Geology)

To determine the chemical composition inside the Earth indirect measurements are used. One way to determine composition is to measure how the density of rock changes with depth below the Earth’s surface. We can do this by measuring the speeds, because they move more quickly through dense rocks than through less dense rocks. From such measurements we know that density increases with depth, but not evenly. Knowing these different densities, we can estimate what the composition of the different layers must be. The layers of different chemical composition outward from the centre of the Earth are as follows: core, mantle, and crust.

1. Soran University
Faculty of Engineering
Petroleum engineering department







Geology


Prepared by: Raboon Redar Supervised by: Mr. Arkan
Mansur Nidhal



2019

2. Exercise 1

Chemical and Physical Properties of Earth

 Layers of Differing Chemical Composition

To determine the chemical composition inside the Earth indirect measurements
are used. One way to determine composition is to measure how the density of
rock changes with depth below the Earth’s surface. We can do this by measuring
the speeds, because they move more quickly through dense rocks than through
less dense rocks. From such measurements we know that density increases with
depth, but not evenly. Knowing these different densities, we can estimate what
the composition of the different layers must be. The layers of different chemical
composition outward from the centre of the Earth are as follows: core, mantle,
and crust.

The Core

At the Centre is the densest of the three layers, the core. It is mostly metallic iron
with small amounts of nickel and other elements.

The Mantle

The thick shell of the rocky matter that surrounds the core is called the mantle.
The mantle consists of iron-magnesium-silicates and it is less dense than the core
but denser than the outermost compositional layer, which also consists of rocky
matter.

3. The Crust

The outermost compositional unit is the crust. While the core and the mantle
have nearly constant thicknesses, the thickness of the crust is different in
different places. The continental crust (30-70) is much more thicker than the
oceanic crust (8km). The oceanic crust is basalt while the continental crust has a
granitic composition.

 Layers of Differing Physical Properties

In addition to compositional layering, other changes occur within the earth. Most
important, there are changes in physical properties such as rock strength and
solid versus liquid. The layers of different physical properties outward from the
centre of the Earth are as follows: inner core, outer core, mesosphere,
asthenosphere, and lithosphere.

The Inner and Outer Core

Within the core an inner region exists where pressures are so great that iron is
solid despite its high temperature. The solid centre of the Earth is in the inner
core. Surrounding the inner core is the outer core, a zone where temperature and
pressure are so balanced that the iron is molten and exists as a liquid. The
difference between the inner and outer cores is not one of the composition (the
compositions are believed to be the same). Instead, the difference lies in the
physical states of the two: one is a solid, the other is a liquid

4. The Mesosphere

The strength of a solid is controlled by both temperature and pressure. When a
solid is heated, it loses strength. When it is compressed, it gains strength.
Differences in temperature and pressure divide the mantle and crust into three
strength regions. In the lower part of the mantle, the rock is so highly compressed
that it has considerable strength even though the temperature is very high. Thus,
a solid region of high temperature but also relatively high strength exists within
the mantle from the core-mantle boundary (lower mantle) which is called
Mesosphere.

The Asthenosphere

Within the upper mantle bellow the earth's surface. is a region called the
asthenosphere ("weak sphere"), where the balance between temperature and
pressure is such that rocks have little strength. Instead of being strong, like the
rocks in the mesosphere, rocks in the asthenosphere are weak and easily
deformed, like butter or warm tar. As far as geologists can tell, the compositions
of the mesosphere and the asthenosphere are the same. The difference between
them is one of physical properties; in this case the property that changes is
strength.

The Lithosphere

Above the asthenosphere is the outermost strength zone, a region where rocks
are cooler, stronger, and more rigid than those in the plastic asthenosphere. This
hard outer region, which includes the uppermost mantle and all of the crust, is
called the lithosphere ("rock sphere"). It is important to remember that despite

5. the fact that the crust and mantle differ in composition, it is rock strength, not
rock composition, that differentiates the lithosphere from the asthenosphere.

Note: The difference in strength between rock in the lithosphere and rock in the
asthenosphere is a function of temperature and pressure. At a temperature of
1300°C and the pressure reached at a depth of 100 km, rocks of all kinds lose
strength and become readily deformable.

Exercise 2
Why are plate tectonics important?

The process is actually very important to life on Earth. Several billion years ago,
the surface of our Earth began forming into puzzle pieces called plates. Plates
cover the entire Earth, and their boundaries play an important role in geologic
happenings. Plates crash together to make mountains, such as the Himalayas.
They leave trenches where one slips beneath the other. They make giant rift
valleys and ridges when going their separate ways. This process trapped our
atmospheric carbon dioxide into rocks and stabilized our climate, making Earth
habitable.

 There are 3 main types of plate boundaries

1. Convergent boundaries - where two plates are colliding.

Subduction zones occur when one or both of the tectonic plates are composed of
oceanic crust. The denser plate is subducted underneath the less dense plate. The
plate being forced under is eventually melted and destroyed. There are three
types of meetings.

6. • Where oceanic crust meets ocean crust
• Where oceanic crust meets continental crust
• Where continental crust meets continental crust

2. Divergent boundaries - where two plates are moving apart.

The space created can also fill with new crustal material sourced from molten
magma that forms below. Divergent boundaries can form within continents but
will eventually open up and become ocean basins.

• On land
• Under the sea

3. Transform boundaries - where plates slide passed each other.

The relative motion of the plates is horizontal. They can occur underwater or on
land, and crust is neither destroyed nor created.

Because of friction, the plates cannot simply glide past each other. Rather, stress
builds up in both plates and when it exceeds the threshold of the rocks, the
energy is released causing earthquakes.

7. Exercise 3
 Natural Disasters Caused by Plate Tectonics

Plate tectonics are among the most influential forces that shape Earth. Most of
the time, these plates move slowly and only create changes over the course of
millions of years. Sometimes, however, two plates move abruptly with respect to
each other. When that happens, the Earth’s surface is subject to natural disasters.
Events such as earthquakes, volcanoes and seismic waves (tsunamis) all result
because of plate tectonics.

Rocks that Roll: Earthquakes

Most earthquakes occur as the result of the sudden movement along a fault line
between two adjacent tectonic plates. The famous San Andreas fault in California
marks the location where the North American plate and the Pacific plate slide
past each other. The two plates move at a rate of about 6 cm per year, causing
hundreds of tiny earthquakes yearly and the occasional major earthquake.

Erupting Volcanoes

In general, volcanoes occur either along plate boundaries or over “hot spots.”
When a plate moves over the top of another plate, the energy and friction melt
the rock and push the magma upwards. The world's biggest volcano, also an
enormous ocean volcano Mauna Loa is one of five volcanoes on the Big Island of
Hawaii. Its most recent eruption was in 1984, and Mauna Loa has spewed lava 33
times in the past 170 years.

8. Seismic Sea Waves

Plate tectonics indirectly cause seismic sea waves, better known as tsunamis.
When a major seismic tremor shifts the crust underneath a body of water, the
energy from that tremor transfers into the surrounding liquid. The energy spreads
out from its original site, traveling through the water in the form of a wave. The
devastating tsunami of December 2004, which killed more than 300,000 people
around the edges of the Indian Ocean, emanated from an extremely powerful
earthquake (magnitude, 9.2) on the ocean floor near Indonesia.

References

Anon., 2018. Lumen Physical Geography. [Online]
Available at: https://courses.lumenlearning.com/geophysical/chapter/the-composition-and-structure-
of-earth/

Brown, L., April 19, 2018. Sciencing. [Online]
Available at: https://sciencing.com/natural-disasters-caused-plate-tectonics-5516200.html

Duarte, J. C., 2016. Earth and Space Science News. [Online]
Available at: https://eos.org/editors-vox/plate-boundaries-and-natural-hazards

Duarte, J. C., and W. P. Schellart (2016), Plate boundaries and natural hazards, Eos, 97