Volcanoes
ICSE Class 9 · Geography · Chapter 7
What this chapter covers
- Meaning of a volcano
- Parts of a volcano and the materials it ejects
- Types of volcanoes: active, dormant and extinct
- Constructive effects of volcanoes
- Destructive effects of volcanoes
- Important volcanic zones of the world
Key points
- A volcano is a vent in the crust through which magma, gases and rock fragments reach the surface; the word also means the cone built around it.
- Magma is molten rock below the surface; lava is the same material at the surface.
- Magma rises because it is lighter than solid rock and its gases expand, and it escapes along weak zones of the crust.
- Acid lava is sticky and gives explosive eruptions and steep cones; basic lava is runny and gives quiet eruptions and broad cones or plateaus.
- The parts of a volcano are the magma chamber, pipe, vent, crater and cone; a caldera is a huge collapsed crater.
- A volcano ejects gases (mostly water vapour), lava and pyroclasts (bombs, lapilli, ash and dust).
- Active volcanoes erupt often (Barren Island, Etna, Stromboli); dormant ones are sleeping (Fuji, Kilimanjaro, Narcondam); extinct ones are dead (Mount Kenya, Dhinodhar).
- The classification is not permanent: Vesuvius and Pinatubo erupted after long quiet periods.
- Constructive effects: new mountains, plateaus and islands, fertile soils, minerals, geothermal energy and tourism.
- Destructive effects: pyroclastic flows, lava flows, ash falls, lahars, tsunamis, acid rain, climate cooling and disruption of transport.
- The main volcanic zones are the Circum-Pacific Belt (Ring of Fire), the Mid-Atlantic Belt, the Mid-Continental Belt and the East African Rift Belt, with hotspot volcanoes such as Hawaii.
- India's only active volcano is Barren Island in the Andaman Sea; the Deccan Traps record a huge fissure eruption about 66 million years ago.
Questions answered
Define a volcano.
A volcano is an opening or vent in the earth's crust through which molten rock, gases, ash and rock fragments from the interior are forced out onto the surface. The word is also used for the hill or mountain built around the vent.
Distinguish between magma and lava.
Magma is molten rock with dissolved gases lying below the earth's surface. Lava is the same material after it has reached the surface. The difference is only one of position.
Give two reasons why magma rises towards the surface.
Magma is less dense than the solid rock around it, so it tends to rise; and its dissolved gases expand as pressure falls, pushing it upwards. It rises along weak zones of the crust.
Name the parts of a volcano from the magma chamber to the summit.
Magma chamber, pipe or conduit, vent, crater, with the cone built around the vent.
What is a caldera? How does it differ from a crater?
A caldera is a large, roughly circular depression formed when the summit of a volcano collapses into its emptied magma chamber or is blown away. It is far larger than a crater, which is the small bowl-shaped hollow around a vent.
Name three kinds of pyroclasts in order of size.
Volcanic bombs (largest), lapilli, and volcanic ash or dust (smallest).
Why do eruptions of acid lava tend to be explosive?
Acid lava is rich in silica, thick and sticky, so the gases in it cannot escape easily. Pressure builds up until it is released in a violent explosion.
Define an active volcano and give two examples.
An active volcano is one that erupts frequently or has erupted in recent times and still shows signs of activity. Examples are Barren Island and Mount Etna.
Past board exam questions
ICSE 2019 sample paper · 2 marks
Name any two types of volcanoes giving one example of each type.
Show answer
Any two: Active volcano - Mount Etna (Italy) / Barren Island (India); Dormant volcano - Mount Vesuvius (Italy) / Mount Fujiyama (Japan); Extinct volcano - Mount Kilimanjaro (Tanzania) / Mount Popa (Myanmar).