Volcanoes occur when molten magma reaches the Earth’s surface due to vents or fissures in the crust. As convection currents force the magma into these fissures, gas bubbles force it towards the surface. When pressure from the rising gas and magma becomes too high, lava is released onto the surface. This is called a volcanic eruption. It is estimated that between 30 and 60 volcanoes erupt each year. Across the globe, there are more than 10000 volcanoes that are capable of erupting some time in the future.
GEO DICTIONARY
Fissure: a narrow crack in the Earth’s crust
: Fig. 3.1 The global distribution of active volcanoes 3.1 Categories of Volcanoes
The following diagram shows the different parts of a volcano.
Ash cloud
Crater Vent
Secondary vent
Ash layer
( Fig. 3.2 The parts of a volcano : Fig. 3.3 Slemish in Co. Antrim is an extinct volcano.
Magma chamber Magma PATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT 37 Volcano bombs Cone
Volcanoes are divided into one of three general categories based on their level of activity:
