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15.2. Volcanoes


volcano is a geological structure through which incandescent materials, magma (lava and gases) come out from the interior of the Earth. The internal energy of the Earth causes the temperature and pressure to increase with depth, reaching a temperature of about 5000 ºC in the Earth´s core.

When these materials subjected to high pressure and temperature meet an area where the pressure is lower (some fracture), they melt, the magma rises to the surface and the volcano erupts. When the lava goes outside, it cools and solidifies, increasing each time the volcanic cone.

Volcanoes, generally, are found on the limits of tectonic plates (for example, in the ridges), which is where the magma has more facility to go to the outside. There are also volcanoes that appear inside the tectonic plates in the spots hot or hot spots, with rising materials from the lower mantle, like the Hawaiian Islands, which leaves the lava at about 1200 °C.

Volcanic eruptions vary greatly in intensity, duration, and frequency, and can be gentle lava flows or explosive eruptions. Volcanoes can be located above sea level or be underwater.

Structure of a volcano

The following parts of a volcano are distinguished:

  • Volcanic chimney: conduit through which the magma from the magma chamber comes out.
  • Magmatic chamber: place inside the earth's crust where magma accumulates.
  • Crater: exit hole for volcanic products located at the end of the chimney.
  • Volcanic cone: mound formed by the accumulation of volcanic products (lava flows  and pyroclasts) from previous eruptions. There may be small volcanic cones associated with secondary vents .
  • Dam or veinfilonian rocks that have solidified inside a fracture through which the magma rose but without going outside.
  • Lava flow. Rivers of lava coming out of the crater.
  • Eruptive column. Volcanic gases and ash coming out of the crater.

Partes de un volcán

By This SVG image was created by Medium69.Cette image SVG a été créée par Medium69.Please credit this : William Crochot (File:Spaccato vulcano.png) [CC BY-SA 4.0], via Wikimedia Commons

Interactive activity: Parts of a volcano.

Interactive activity: Learn the parts of a volcano.

Volcanic products

When magma rises to the surface, it may not come out and solidify as plutonic rock. But if the magma reaches the outside, the volcano erupts and the following volcanic products are obtained:

  • Solid or pyroclastic products. They are fragments of lava or fiery rock that are thrown to the outside with force by the pressure of the gases that accumulate inside the volcano. According to their size, they are distinguished:
    • Volcanic ash. Their size is less than 2 mm, and they can remain in the atmosphere for quite a long time.
    • Lapillis. Its size is gravel, between 2 and 64 mm.
    • Volcanic bombs. Greater than 64 mm, even more than 1 meter .
  • Molten or lava products. The lava flows are burning rivers that descend the slope of the volcano. Two types of lavas are distinguished:
    • Acid lavas. They contain more than 70% silica (SiO2) and their temperature is less than 1000 ºC. They are viscous lavas that cause violent and explosive eruptions.
    • Basic lavas. Its silica content is less than 50%. Its temperature is 1000-1200 ºC. They are very fluid lavas, which are gently expelled outside, without violent eruptions.
  • Gases. The most common gases released from a volcano are water vapor, carbon dioxide (CO2), carbon monoxide (CO), and sulfur compounds. Gases are what allow magma to rise and eruptions occur. The gases are dissolved in magma, but when there is less pressure, they separate and reach the surface .

Some volcanoes at rest produce fumaroles, by which they emit gases related to volcanism.

The fiery clouds are fly ash mixed with gas at several hundred degrees temperature burn and devastate everything in their path.

Answer in your notebook

15.2.- What types of lava are most dangerous?


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.