Unit 8:  Internal dynamics of the Earth                                    1   2   3

3. Internal geological processes

The internal dynamics of the Earth has as a result different geological phenomena known as internal geological processes. The main ones are: volcanism, seismicity, mountain formation and rocks deformation.

3.1. Volcanoes

Volcanoes are openings in the Earth’s crust through which magma flows

      from inside the planet to the surface. Magma is an extremely hot mixture

      of molten minerals which contains different amounts of water, gases and

      small pieces of solid rock.

      Although in the mantle the temperature is extremely hot, the high pressure

      keeps rocks in solid state.  However in some places the temperature is

      slightly higher or the crust is thinner and exerts less pressure.

      In these conditions, rocks start to melt and magma forms.

      Melted rocks are lighter that solid rocks and they tend to rise up.

      If this magma finds an exit to the surface, a volcanic eruption occurs.

a) Parts of a volcano

b) Volcanic materials

     When a volcano erupts, it expels different kind of materials:

     - Gaseous materials. Gases are released from magma when it reaches the surface.

       The main volcanic gases are water vapour, carbon dioxide

       and sulphides of hydrogen and nitrogen.

     - Liquid materials. The magma without gases is called lava.

     - Solid materials. They are known as pyroclasts. They are solid fragments

       wrenched from the volcanic cone during the eruption. They can be very large

       (volcanic bombs and blocks), gravel sized (lapilli) and dust sized (ashes).

Animation: A volcanic eruption (PBS)

c) Types of volcanic eruptions

     Magma is classified according to the amount of gases and the type of minerals

     it contains.

     The type of magma determines the type of eruption and the type of volcanic cone

     that is formed as a result.

    - Effusive eruptions

        In this kind of eruptions, the magma is very fluid and forms extensive lava flows.

        The accumulated gases easily escape from it and eruptions are very mild.

        They do not produce pyroclasts.

        The crater of these volcanoes can be:

              - a typical opening. They are known as hawaiian volcanoes.

                (E.g. Kilauea, Hawaii Islands)

              - a large longitudinal fissure, as in the case of oceanic ridges. 

                They are known as icelandic volcanoes (E.g. Laki, Iceland).

    - Explosive eruptions

        In this case, the magma is viscous.

        Gases cannot escape easily from it and eruptions are violent, with explosions

        that produce a large amount of pyroclasts, gases clouds and lava flows.

        In the most extreme eruptions, very destructive clouds, known as

        pyroclastic flows, may form close to the ground and move at high speeds.

        There are different types of explosive volcanoes depending on the viscosity

        of the magma and their explosiveness   

        In order of increasing level they are: strombolian (E.g. Stromboli, Italy),

        vulcanian (E.g. Mont Vulcano, Italy), and plinian (E.g. Vesuvius, Italy)

        and pelean (Mont Peleé, Martinique Island).

d) Low-intensity volcanism

     In areas of volcanic activity near active volcanoes, it is possible these phenomena

     occur:

     - Smoking volcanoes (fumarolas). They are cracks that emit gases

       at high temperature.

     - Hot springs. They are springs that emit hot water which is rich in minerals salts.

     - Geysers. They are cracks that emit from time to time eruptions

        of hot mineralized water.

e) Dangers of volcanoes

    Volcanic activity can cause very serious catastrophes: lava flows, clouds of burning

    rocks and gases can destroy everything in their path, gas emission can be toxic,

    piroclasts and ash emission can destroy vegetation, intense heat can melt

    frozen areas around them causing floods and rivers of mud.

    Although it is impossible predict accurately when a volcanic eruption is going to be,

    there are some signs that indicate it is near to occur. For example, cracks

    appearing through which gases escape, increase of the soil temperature,

    tremors, etc.

    To prevent disasters, it is important to take the correct prevention measures:

     - Inform to people about the volcanic risks and what to do in case of eruption.

     - Establish civil protection programmes.

     - Build dykes to change the course of rivers of lava or mud.

3.1. Answer these questions:

a. What are the factors that determine the explosiveness of a volcanic eruption?

b. What is the difference between a fumarole and a geyser?

c. What criterion is used to classify the pyroclastic materials?

d. Is it the same magma than lava? Why?

3.2. Identify and order the following pictures from lower violent eruption

        to higher one.

3.3. Listen and identify the defined term:

Volcanoes (linguaframe)