geography
Explain the distribution of the world’s major geophysical hazards. Some parts of the world are more often and more severely afflicted by natural hazards than others due to the geological structure of the Earth. The earth’s crust is made up of seven major plates and a number of minor plates. The plates move because of the convection currents in the mantle caused by temperature differences within the mantle, leading to volcanoes and earthquakes, the world’s major geophysical hazards. The movement of plates due to convection currents results in a variety of types of plate boundary, each of which gives rise to particular types of hazards.
Constructive boundaries occur where two plates are moving apart. Basalt rises to the surface and forms new crust. The majority of constructive zones occur under the oceans, e.g. the Mid-Atlantic Ridge. The constructive boundaries are associated with numerous shallow focused, low magnitude earthquakes, which due to their location pose only little hazard. Similarly, many of the volcanoes at such margins are submarine, but they do emerge in islands such as Iceland, forming volcanoes such as Heimaey and Surtsey. Volcanoes at constructive margins tend to erupt relatively gently as gases can easily escape from the basalt lava which is at extremely high temperatures and very free-flowing. The main hazards are therefrom from lava flows such as that at Heimaey in 1972.
Destructive boundaries occur where two plates are moving towards each other. Where two crusts are moving towards each other, the denser crust(eg. the oceanic crust)is subducted under the less dense one (eg. the continental crust). The oceanic crust is heated by friction and contact with the upper mantel, which melts it into magma. As magma is less dense than the continental crust above, thus it will rise to the surface to form volcanoes. For example, the Pacific plat is subducting beneath the Eurasian plate at the Japan Trench. The long arc of islands that make up
Constructive boundaries occur where two plates are moving apart. Basalt rises to the surface and forms new crust. The majority of constructive zones occur under the oceans, e.g. the Mid-Atlantic Ridge. The constructive boundaries are associated with numerous shallow focused, low magnitude earthquakes, which due to their location pose only little hazard. Similarly, many of the volcanoes at such margins are submarine, but they do emerge in islands such as Iceland, forming volcanoes such as Heimaey and Surtsey. Volcanoes at constructive margins tend to erupt relatively gently as gases can easily escape from the basalt lava which is at extremely high temperatures and very free-flowing. The main hazards are therefrom from lava flows such as that at Heimaey in 1972.
Destructive boundaries occur where two plates are moving towards each other. Where two crusts are moving towards each other, the denser crust(eg. the oceanic crust)is subducted under the less dense one (eg. the continental crust). The oceanic crust is heated by friction and contact with the upper mantel, which melts it into magma. As magma is less dense than the continental crust above, thus it will rise to the surface to form volcanoes. For example, the Pacific plat is subducting beneath the Eurasian plate at the Japan Trench. The long arc of islands that make up