internaldynamics4eso

Biology And Geology 4th ESO Marta García Toledano

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UNITS 8 - 9. THE INTERNAL DYNAMICS OF THE EARTH

1) EARTH’S INTERNAL LAYERS.

The interior of the Earth can be divided into layers, according to two criteria: its chemical composition, and its dynamic behaviour.

a) Compositional Layers: in order of increasing density:

- Crust: made up of less dense rocks, rich in silicon and aluminium. - Mantle: made up of denser rocks, like peridotite and magnesium. - Core: principally made up of iron.

b) Physical Layers: - Lithosphere: is the rigid surface layer. It means the crust and the uppest and solid part of the

mantle. - Asthenosphere: is the area under the lithosfere, where the mantle is ductile and partially molten

in some places. - Lower Mantle: is solid, but still flexible and ductile. - Core: consists of a molten outer layer and a solid inner part.

The melting point of materials inside the Earth increases with depth, despite the rise in temperatura. This is due to the effect of pressure. For ex. Iron, which melts at 1550ºC on the surface, is found in a solid state in the centre of the Earth, at a temperature of 6000 ºC.


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2) SEISMIC DISCONTINUITIES.

The interior of the Earth is studied by scientists using different methods, such as drilling and mines. They also get information from the study of rocks, meteorites and the seismic waves produced in earthquakes.

There are two types of seismic waves:

a) Internal: are transmitted in the interior of the Earth. There are two types:

- Primary Waves (P): are the fastest ones, and the first to reach the surface, and also seismic stations. They move through solids and liquids.

- Secondary Waves (S): are slower, and so registered after P waves. Thay can only travel through solid materials.

b) External: they are transmitted on the surface, producing the damages.

Seismic waves’ travels are recorded using a seismometer, and sudden changes in their speed indicate a change in the composition (and physical state) of the material they are travelling through. From this analysis, we can distinguish three main discontinuity boundaries:

a) Mohorovicic Discontinuity: marks the boundary between the crust and the mantle. It is 30-40 Km below the continents, and less than 10 Km below the oceans.

b) Gutenberg Discontinuity: found at a depth of 2900 Km. There, S waves cannot travel any further and disspaear, while P waves are strongly refracted and slow down suddenly. This boundary marks the limit between the mantle and the molten external part of the core.

c) Lehman Discontinuity: aproximately 5100 Km under the surface, P waves suffer a significant increase in their speed, which means they are reaching a solid structure. It is the begining of the solid inner core.

3) THE MOVEMENT OF CONTINENTS:

During the 19th and 20th century there was controversy among scientists about the geological history of the continents. Some believed their position was fixed, while others proposed that they had move great distances over long periods of time.

Alfred Wegener was the first to collect evidences to demonstrate continents had previously been joined.


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In 1912, he set up the Continental Drift Theory:

According to this theory, around 200 millions of years ago, the continents were joined as one, which he called “Pangaea” (a word that means “the whole earth”). After that time, they moved further away each other, until they got their current position. He collected different evidences to prove the existence of this supercontinent:

a) Paleobiological Evidences: identical fossils of land-based organisms such as reptils and plants were found in continents situated far apart. These organisms could never have crossed the oceans that now separate them.

b) Geological Evidences: the continents fit together along their coastlines and continental shelves. Besides, rocks of the same type, age and structure appear on each side of the line when they are joined.

c) Paleoclimatical Evidences: the continents which were situated in the South Pole of Pangaea have glacial moraines from the same age. The present Northen Hemisphere continents have coal deposits which indicate they were once covered in vast tropical forest, which corresponds to the equatorial position they would have had in Pangaea.


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4) THE PLATE TECTONICS THEORY.

But Wegener’s opponents had a very strong argument against his theory: how was this movement posible? No known force could have move continents over the solid mantle, considering their weigh and the fritcional forces.

The answer came with the increasing knowledge about the interior of our planet:

The lithosphere is not continue, but broken into fragments called “Lithospheric Plates”. Each plate is separated from the next one by a tectonic boundary. Lithospheric plates can be classified into three types depending on the type of lithosphere they are composed of:

- Continental Plates: composed of continental lithosphere.

- Oceanic Plates: composed of oceanic lithosphere.

- Mixed Plate: composed of both oceanic and continental lithosphere.

The internal heat of the Earth (residual heat from its formation, and heat released in radiactive desintegrations in the core) allows the mantle to be partially molten, so currents can be generated inside. As lithsopheric plates are floating on the asthenosphere, these current (called Convection Currents) could actually move them, as a piece of cork on a pond. This is the present explanation to how continents can drif.

So, plates are moved by convection currents in different ways, causing different types of boundaries between plates.

a) Divergent Boundaries: zones where two plates move apart.

b) Convergent Boundaries: two plates move together and finally collide.

c) Transform Fault Boundaries: fractures where two plates slide horizontally against each other.

Plate tectonics theory is also known as Global Tectonics because it explains the relationships between many geological phenomena, past and present, which previously didn’t seem to have a common origin: volcanic activity, earthquakes, the formation of mountains ranges, the formation and destruction of the ocean floor, the location of mineral deposits and fossil fuels, etc.


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5) RISKS ASSOCIATED WITH INTERNAL GEOLOGICAL ACTIVITY.

The Earth’s internal energy is responsable for sudden geologic events such as earthquakes, volcanic eruptions or tsunamis, which can be catastrophic for humans. The boundaries of the plates are usually the zones with the greates seismic and volcanic risk.

a) Seismic Risk:

Earthquakes are sudden tremblings of the Earth’s crust, produced by the movements of the lithospheric plates. All types of boundaries mean a risk of tremblings, but not in the same degree.

b) Volcanic Risk:

The danger and violence of a volcanic eruption depend mostly on the viscosity of the magma (which is related to its silca content) and on the quantity of gases it contains. There are three main types of eruptions:

- Hawaiian Eruptions: the viscosity and the gas content are very low, so the lava flows are mostly fluid. Enormous shield volcanoes are formed.

- Vulcanian Eruptions: the viscosity is intermediate, and there are alternating lava flows and piroclasts (solid material ejections).

- Plinian Eruptions: viscosity and gas content are very high. There are frequent emissions of ash and cloud of gas.

The Iberian Peninsula is near the bounaries of the African and Eurasian Plates, although is not a very active zone. As a consequence, the southern part of the península has a moderate seismic risk.

The Canary Islands are Spain’s only active volcanic areas. The most recent eruption occurred in October 2011, in El Hierro island.

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