UNIT 2: Internal Geological Agents

UNIT 2: Internal geological agents

1- Internal geological agents Internal geological agents: They are the movements of the litosferic plates, earthquakes and volcanic eruptions. These agents build the Earth’s relief. The energy resource that lets them work is the heat stored inside the Earth. Proof of this heat is the geothermal gradient defined as the increase of temperature with depth. This heat is due to: - the collision of the planetesimals during the Earth’s formation - the disintegration of radioactive elements

2- Movement of the litosferic plates The Earth can be divided into three layers (from a dynamic point of view): - Lithosphere: is the rigid outermost layer. It is composed of the crust and a little portion of the upper mantle (from 0 to 100 Km). - Mesosphere: is the rest of the mantle. It is a semiliquid layer. - Endosphere: consists of the core (outer and inner core).

Litosphere is not a continuous layer, but is divided into pieces just like a puzzle, forming the lithosphere plates. There are three types of lithosphere plates depending on the kind of crust they are formed by:  Continental plates: they are formed by the continental crust and a portion of the upper mantle (Arabian plate).  Oceanic plates: they are formed by the oceanic crust and a portion of the upper mantle (Pacific plate, Nazca plate).  Continental/Oceanic plates: they are formed by both continental and oceanic crust and a portion of the upper mantle (Indoaustralian plate, Eurasian plate, North American plate, African plate,…).

The places where the lithosphere plates contact each other are called plate boundaries, and there are three types:

A)- Divergent Boundary: the two plates move away from each other. Volcanic activity is common in these areas since mantle easily moves to the surface through the thin, fractured rock as it separates. This happens all along the mid-ocean ridge where magma is constantly streaming to the surface, creating new ocean floor as the plates separate (The Atlantic Ridge is between the American plate and African and Eurasian plates). Oceanic crust is created.

B)- Convergent Boundary: When both plates are pushing into each other. Depending on the kind of crust colliding, two different things can happen:

- Oceanic plate-Continental plate: As oceanic crust is more dense than continental crust, the former slides down (causing earthquakes because of friction) under the latter, and it is melted. The melted materials rise and a line of volcanoes are formed on the continental crust. (i.e. when the Nazca plate collided with the South American plate forming the Andes ranges in South America). The places where these two types of plates collide are called subduction zones. In these zones, a narrow and deep trench are typical (Mariana Trench). Oceanic crust is destroyed. - Continental plate-Continental plate: high mountains are formed in these areas (i.e. the Himalayas were formed when the Indoaustralian plate collided with the Eurasian plate).

C)-Transform boundaries: Where plates slide past each other causing earthquakes. Crust is neither destroyed nor created (i.e. the San Andreas fault was caused by friction of the Nazca plate and the North Amerian plate).

Apart from these three types of boundaries, we can find another event that causes magmatic activity: Hotspots. They are places far from the plate boundaries where magma rises from the mantle to the surface forming volcanoes (i.e. the Canary islands or Hawaiian islands)

Why do lithospheric plates move? Plates move by floating on top of the liquid mantle by convection currents within the mantle. Convection currents are formed by hot magma near the core rising towards the surface, while cooler magma near the crust sinks, provoking a current that causes the plates to move.

3- Earthquakes They are movements of the ground due to the relesaing of energy from fractured rocksin the crust when the plates move. This energy is released as tremors called seismic waves. The hypocenter is the point within the earth where an earthquake starts. The epicenter is the point at the surface of the Earth directly above the hypocenter Seismographs are the principal instruments of the researchers who study earthquakes. The graphs made by seismographs are known as a seismograms.

Sometimes, an underwater earthquake causes a tsunami (is a series of waves).

3- Volcanoes A volcano is a fracture in the crust that allows magma to escape to the surface. Magma is a mixture of melted rocks, solid fragments and dissolved gases Parts of a volcano: There are three different materials that can erupt from an active volcano:

 Lava: is magma erupting from a volcano. It can be fluid (it causes flows of lava, and therefore, not very explosive volcanoes) or viscous (as lava solidifies quickly in the vent it usually form a dome which obstructs the crater generating a high pressure, and consequently, very dangerous explosive volcanoes).

 Pyroclasts: are fragments of rock ejected from the volcano. When pyroclasts accumulate around the crater, the volcanic cone is formed. Pyroclasts are classified according to size in bombs (the largest), lapilli and ash (the finest).

 Gases: The main gases are water vapour, carbondioxide, sulfuric gases,etc.They are very toxic.

Volcanoes can be classified according to their eruption type:

1. Hawaiian: Lava is very fluid and flows of lava slide down the sides of the volcano. Gases are released easily. 2. Strombolian: Lava is not very fluid. Gases are not easily released. Eruptions have a medium level of explosivity. 3. Vulcanian: Lava is viscous. . Gases are not easily released. Eruptions have a high level of explosivity. 4. Plinian: Lava is very viscous (forming domes). Gases are difficultly released. Eruptions are extremily violent.

Lahars are volcanic mudflows created when water (from rain or meltes snow from glaciers) and ash mix. This combination can have devastating results on the surrounding area.

4- Seismic and vocanic risk A geological risk is any geologic event capable of causing damage or loss of property and life.

The risk depends on: Danger=is the power and frequency of a given phenomenon. Exposure= is the amount of people or belongings that can be affected. Vulnerability=indicates how susceptible of destruction a place is.

Risks=Danger x Expoure x Vulnerability

It is really important to predict (prediction) where and when a geologic event is going to happen, and prevent (prevention) its consequences.

4.1.Seismic risk Seismologists study two parameters:  Magnitude of an earthquake: is the amount of energy released by the earthquake. It is measured by the Richter scale. The magnitude does not vary with the distance from the epicenter.  Intensity of an earthquake: Is the destructive effect of an earthquake. It is measured by the Mercalli scale ranged from I (people do not feel the earthquake) to XII (total destruction). The intensity varies with the distance from the epicenter (the nearer to the epicenter, the more destructives effects).

It is not possible to predict an earthquake. Nevertheless, seismologists count on seismic hazard maps for this purpose. For prevention, three different things should be taken into account: -To construct structures that will not be damaged by earthquakes. -To inform people what to do in case of an earthquake. -Not to build in zones where the danger of having earthquakes is high.

Earthquakes can mainly be found in: convergent boundaries and transform boundaries.

4.2.Volcanic risk Volcanologists study a series of factors to predict when a volcano is about to erupt:

-The level of gases increase. -The ground elevates due to the accumulation of lava inside the volcano. -The composition and temperature of the water in nearby lagoons varies. -There are more and more earthquakes.

The volcano explosivity index indicates the magnitude of a volcanic eruption. It depends on the volume of ash and the height of the cloud.

In areas close to volcanoes, a series of rules should exist to prevent the effects of an eruption: -Evacuation plan to avoid human casualties. -Give people gas masks. -cool flows of lava with water. -make trenches to modify the lava-flows course.

Volcanoes can mainly be found in: Divergents boundaries (Ridges. Not very dangerous for populations), convergent boundaries and hotspots.