ABSTRACT
Extrusive igneous rocks are erupted from volcanoes, which are the sites where molten rock (magma) from deep within the Earth’s interior is erupted through conduits and vents onto the land or sea floor. There are about 500 active volcanoes today of which only about 25 are active in any year. For example, Mount Etna in Sicily (Figures 6.1 and 6.5b) has recently been active with the outpourings of lava and has a long documented history of eruptions over the last thousand years. Their distribution is controlled by major weaknesses in the crust (Figure 2.1) that developed as a result of the movements of tectonic plates (see Chapter 2) and are most common along plate boundaries. However, some volcanoes also occur within continental plates across the world, for example the Mount Suswa (Figure 6.5a) in the East African Rift Valley, and in oceans initially on the sea floor but with time forming new islands, for example the Hawaiian island chain in the Pacific Ocean (Figure 6.3). Eruptions may be violent and short lived, for example Mount Mayon in the Philippines (Figure 6.2), whose most violent eruptions occurred over a few months, or be relatively non-violent and long lived, for example Mount Kilauea (Figure 6.3). Volcanic deposits are commonly interbedded with sedimentary rocks, which reflect times of low or no eruptive activity within a volcanic cycle, when sediments derived from the volcanic edifice accumulated on the sea or lake floor.
