ATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT 3.1 The parts of a volcano
A volcano occurs when molten magma forces its way up to the earth’s surface either through a vent or through a fissure. As magma rises, gas bubbles expand and help force it up to the earth’s surface through the crust. Once the magma reaches the surface the sudden release of pressure causes a volcanic eruption. A vent eruption leads to the formation of the typical cone-shaped mountain. Fissure eruptions lead to the formation of a wider and flatter plateau. Upon reaching the surface the magma is called lava. Depending on the type of lava being ejected, eruptions may be extremely violent or more quiet.
Tephra Vent
Ash Lava flow Parasitic cone
Pyroclasts Crater Lahar Fissure
Magma Magma chamber
Fig. 1 Stromboli volcano erupting in Italy
Fig. 2 Exam Diagram: Parts and products of a typical volcano
NORTH EURASIAN PLATE o
AMERICAN PLATE
3.2 The distribution of volcanic activity
AFRICAN PLATE
INDO-AUSTRALIAN PLATE
PACIFIC PLATE
NAZCA PLATE
SOUTH
AMERICAN PLATE
Volcanoes occur in three types of locations: 1. Mid-ocean ridges (at constructive plate margins).
2. Subduction zones (at destructive plate margins, i.e. oceanic–oceanic and oceanic–continental plate collision zones).
ANTARCTIC PLATE Fig. 3 Global distribution of volcanoes 3. Hotspots in the middle of plates.
The reasons for these locations have been explained in Chapter 1.
A volcano may fall into one of three categories: Active volcano: Continuously erupting, e.g. Stromboli, Italy.
Dormant volcano: This is a volcano which has not erupted for many hundreds of years. Mount St Helens in the US was dormant for 123 years until its spectacular but predicted eruption in 1980, and Mount Pinatubo in the Philippines had not erupted for 600 years before its 1995 eruption.