HKDSE Geography/M1/Extrusive Vulcanicity

Process

1. The magma is under great pressure.
2. Plate movements and tectonic processes produce lines of weakness in the crust.
3. The lines of weakness extend downwards and to the magma chamber below.
4. The magma pressure is released.
5. Magma and gases in the mantle rise along the cracks.
6. They extrude onto the earth's surface (extrusive vulcanicity).
7. Rocks formed by the cooling of lava above the surface are called Igneous rocks

There are two main types of openings through which the magma may extrude: Vents and fissures.

Fissure Eruptions and Lava Plateau

Fissures are very fine lines of weakness in the crust. Very basic magma (we'll look at that term later) can rise along fissures to the earth's surface. Fissure are very gentle.

Lava flows as sheets after fissure eruptions. They form a lava flow. Continuous flow accumulates sheets of lava on the earth's surface to form a lava plateau, which can be hundreds of metres thick. The Deccan Plateau in India is an example.

Vent Eruptions and Volcanoes

When magma is ejected from a vent through violent eruption, extrusive vulcanicity occurs.

Formation

1. The magma is under great pressure.
2. Plate movements and tectonic processes produce lines of weakness in the crust.
3. The lines of weakness extend downwards and to the magma chamber below.
4. The magma pressure is released.
5. Magma and gases in the mantle rise along the cracks.
6. They extrude onto the earth's surface (extrusive vulcanicity) through a vent.
7. The lava and ash cool down and solidify.
8. After a series of successive eruptions, materials accumulate to form a volcano.

Ejected Materials

These materials are ejected from volcanoes:

• Lava: Magma that reaches the earth's surface is called lava. It may be acidic or basic. Acidic lava is viscous, is lighter in colour and has higher silica content. Basic lava is non-viscous, is darker in colour and has lower silica content.
• Gases: Gaseous compounds of sulphur and hydrogen, carbon dioxide, carbon monoxide and steam are released.
• Pyroclasts: Solid materials like lava particles (called volcanic bombs), rock fragments and stones, volcanic ash and cinder are ejected from the bent.

Classification

Volcanoes are classified according to the ejected materials.

• Shield volcanoes or basic lava cones eject basic lava. As basic lava is non-viscous, it can reach very far from the vent before it finally cools down and solidifies. Therefore, basic lava cones have short height, concave slopes and large base area.
• Acidic lava cones eject acidic lava. As acidic lava is viscous, it cannot reach very far from the vent before it cools down and solidifies. Therefore, acidic lava cones have greater height, convex slopes and narrower base area.
• Ash and cinder cones eject lava and rock fragments which are broken into ash and cinder. They are high, conical and steep-sided.
• Composite volcanoes or stratovolcanoes eject both lava and ash, and alternate between quiet lava outflows and violent eruptions. A stratified structure is formed in the volcano, with alternating layers of ash and lava. These volcanoes have higher height, broad bases and steep, convex sides.

Here's a handy table:

	Shield volcanoes/Basic lava cones	Acidic lava cones	Ash and cinder cones	Composite volcanoes
Materials ejected	Lava	Lava	Ash and cinder	Ash and lava alternatively
Viscosity of lava	Non-viscous	Viscous	N/A
Acidity of lava	Basic	Acidic	Acidic/Basic	Acidic/Basic
Height	Low	High	High	High
Base area	Broad	Narrow	Narrow	Broad
Slope shape	Concave	Convex	Conical	Convex
Slope gradient	Gentle	Steep	Steep	Steep

Life Cycle

• Active volcanoes erupt frequently.
• Dormant volcanoes have not erupted for a long time.
• Extinct volcanoes have not erupted for a very long time. The possibility of future eruptions is almost nil.

Volcanic Features

These are not important definitions, but you should understand what the terms mean.

• The vent is the central passageway by which magma passes
• The crater is the central depression above the vent from which the lava is thrown out.
• The slopes are usually symmetrical.
• A volcanic eruption may create cracks in the volcano. Lava may rise through these cracks to cause a small eruption. This forms a small volcanic cone called a parasitic cone on the side of the principal one.
• When the crater of a volcano is filled with water, a crater lake is formed.
• When small-scale eruptions occur at the vent, they form secondary cones or conelets in the middle of the crater.
• If a secondary cone/conelet appears in the middle of a crater lake, it is a volcanic island.
• A plug dome is a volcano ejecting very viscous lava that hardly flows. It expands from the insides and the outer layer is worn down very quickly by erosion. They are often secondary or parasitic cones.
• Calderas are large, circular, basin-shaped hollows on the top of a volcano, and are formed under several situations
  • A very violent eruption blows off the upper part of the volcano.
  • As the magma chamber emptied during the eruption, the top part of the cone lacked support, subsided and collapsed. This formed a caldera.
  • Dormant and extinct volcanoes may very worn away by denudation, forming a caldera.

Other Associated Landforms

• Hot springs are formed when superheated water flows onto the earth's surface by vulcanicity.
• Geysers are intermittent hot springs in which superheated water and steam are ejected to the earth's surface at regular intervals.
• Fumaroles are vents that eject steam and gases from the earth's crust.
  • Solfataras are fumaroles that eject sulphur-rich gases.