Earth’s Dynamic Processes: Structure, Interior, and Plate Tectonics
The Dynamics of Earth’s Processes
We can consider the Earth as a system in which all natural elements and phenomena establish interdependent relationships.
Geological processes are a further example of this interaction between factors which imply a constant flow of energy and matter.
Our planet is a closed system with regards to matter (matter neither goes out of it or into it) and an open system with regards to energy (energy comes into it from the Sun).
Previous Concepts About Geology
- The structure of the Earth
The Earth has three layers based on its composition:
a. Crust
The outermost layer of Earth.
It is made up of different material (igneous rocks, sedimentary rocks, and metamorphic rocks).
The thickness of Earth’s crust varies:
- Continental crust: 30-80 km
- Oceanic crust: 6-12 km
b. Mantle
The largest layer of Earth.
It is made up of igneous rocks.
c. Core
The deepest layer of Earth.
It is made up of an alloy (78% iron and 10% nickel).
- The Earth has five layers based on behaviour:
- the lithosphere.
- the asthenosphere.
- the mesosphere.
- the outer core.
- the inner core.
a. Lithosphere
The rigid outer layer of the Earth.
It includes the upper, solid part of the mantle and the crust.
The thickness of the lithosphere varies:
Continental lithosphere: 150-300 km
Oceanic lithosphere: 50-150 km.
b. Asthenosphere
It is a hot, malleable semi-liquid zone in the upper mantle.
Its behaviour is like a ductile plastic.
c. Mesosphere
The inner-most layer of the mantle.
It is made up of strong, solid rock.
d. Outer core
Liquid layer of the core.
The outer core is so hot that the metals in it are all in the liquid state.
e. Inner core
The inner core is solid.
Studying the Interior of the Earth
Geophysical methods are used to study the Earth’s interior.
The most widely used geophysical methods are:
Electrical: behavior of electricity when it is transmitted through rocks.
Geothermal: measure the anomalies in temperatures in the rocks of the Earth’s surface.
Magnetic: record the local variations in the intensity of the Earth’s magnetic field.
Gravimetrical: compare the value of the gravitational field recorded at each point on the surface to detect the changes in density of the rocky masses.
Seismic: based on the propagation in the interior of the Earth of seismic waves produced by earthquakes.
Seismographs or Seismometers (from Greek Seism – “the shakes” – and Metro – “I measure”) are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions and other seismic sources.
Alfred Lothar Wegener (1880-1930) was a German scientist and meteorologist. He is most notable for his theory of continental drift (Kontinentalverschiebung), proposed in 1912, which hypothesized that the continents were slowly drifting around the Earth.
Continental Drift
200 million years ago the continents that we know today were joined together and they formed a super-continent which is called Pangea. As a result of significant ruptures, Pangea divided successively into fragments (the actual continents) which began to separate from each other. After a time, some of the drifting continents collided and gave rise to the large mountain ranges. To elaborate his theory, Wegener based his studies on geographical, paleontological and tectonic data.
- Geographic data. Wegener noticed that the current continents, especially Africa and South America, join together like the pieces of a giant jigsaw puzzle.
- Paleontological data. Fossils suggest that 350 million years ago the fauna and the flora of Africa and South America, of India and Australia were the same. This means that, at that time, these continents were joined together.
- Geological and tectonic data. On the two sides of the Atlantic Ocean large geological units and the axes of mountain chains coincide. This indicates that the continents were at one time joined together: Africa was joined to South America and Europe to North America.
Tectonic Plates
The theory that the surface of the Earth is made of lithospheric plates, which have moved throughout geological time resulting in the present-day position of the continents. The theory explains the location of mountain building as well as earthquakes and volcanoes. The rigid lithosphere plates consist of continental and oceanic crust together with the upper mantle, which lie above the weaker plastic asthenosphere.
These plates move relative to each other across the earth. Six major plates (Eurasian, American, African, Pacific, Indian and Antarctic) are recognized, together with a number of smaller ones. The plates margins coincide with zones of seismic and volcanic activity.
A constructive (or divergent) plate margin occurs when two plates move away from each other.
The production of new crust at constructive plate margins is compensated by the destruction of material along a destructive (or convergent) plate margin.
A third type of plate margin – the transformation plate margin – occurs where two plates are slipping past each other.
The Boundaries of the Plates
The boundaries between the plates are very important because this is where most of the internal processes of Earth takes place (volcanoes, earthquakes and the formation of mountain ranges). The movement between the plates has a speed of between 1 and 12 cm per year.