Volcanoes and earthquakes mainly occur along plate boundaries where magma can escape from the Earth’s mantle or where stresses build up between 2 plates rubbing together. An exception to this includes Hawaii, which is found in the middle of the Pacific plate over a hot spot.
• Conservative plate boundary - Tectonic plate margin where two tectonic plates slide past each other.
• Constructive plate boundary -Tectonic plate margin where rising magma adds new material to plates that are diverging or moving apart.
• Destructive plate boundary - Tectonic plate margin where two plates are converging or coming together and oceanic plate is subducted. It can be associated with violent earthquakes and explosive volcanoes.
Constructive or Divergent Margins
At this type of plate margin two plates are moving apart (DIVERGE) from each other in opposite directions. Convection currents moving in opposite directions (caused by the intense heat of the Earth's interior) in the mantle move two plates apart. As these plates move apart this leaves cracks and fissures (lines of weakness), that allows magma from the mantle to escape from the highly pressurised interior of the planet. This magma fills the gap and eventually erupts onto the surface and cools as new land. This can create huge ridges of undersea mountains and volcanoes, and where these mountains poke above the level of the sea, islands are created. Both earthquakes and volcanoes can result at these margins, the earthquakes caused by the movement of magma through the crust. A really good example of this is the Mid-Atlantic Ridge, where the Eurasian plate moves away from the North American plate at a rate of around 4cm per year. Iceland owes its existence to this ridge.
At conservative margins mountains are not made, volcanic eruptions do not happen and crust is not destroyed. Instead, 2 plates either slide past each other in opposite directions, or 2 plates slide past each other at different speeds. As they move past each other stress energy builds as the plates snag and grind on one another. When this stress energy is eventually released it sends shock waves through the earth’s crust. We know these shock waves as earthquakes, and a good example of this is the San Andreas Fault in California, where the Pacific plate is moving NW at a faster rate than the North American plate.
Destructive or Convergent Margins
At these margins 2 plates move or CONVERGE together and the destruction of some of the Earth's crust results. An oceanic plate (denser) is pushed towards a continental plate (less dense) by convection currents deep within the Earth's interior. The oceanic plate is subducted (pushed under) the continental plate at what is called a subduction zone, creating a deep ocean trench. It is the Oceanic crust which sinks down into the mantle because it is denser (heavier). As it descends friction, increasing pressure and heat from the mantle melt the plate. Some of this molten material can work its way up through the continental crust through fissures and cracks in the crust to collect in magma chambers. This is often some distance from the margin where magma can eventually re-emerge at the surface to create a range of mountains. The movement of the plates grinding past one another can create earthquakes, when one plate eventually slips past the other releasing seismic energy. There are several really good examples of destructive plate margins, including along the West coast of the Americas and Japan, where the Philippines sea plate is pushed under the Eurasian plate.