The causes of tectonic activity.

What causes tectonic activity? The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. The plates move because of convection currents in the Earth's mantle. Meeting plates Plate Tectonics Crust & Mantle Why do they move? Where tectonic plates meet, the Earth's crust becomes unstable. (BBC 2014)

Causes of Earthquakes Build-up of stress. Pressure becomes too much. The pressure is then suddenly released. Parts of the surface experience a shaking motion. This is an earthquake (Holmes & Warn 2008)

Mohorovicic discontinuity The Mohorovicic Discontinuity, or "Moho", is the boundary between the crust and the mantle. The red line in the drawing at right shows its location. In geology the word "discontinuity" is used for a surface at which seismic waves change velocity. One of these surfaces exists at an average depth of 8 kilometres beneath the ocean basin and at an average depth of about 32 kilometres beneath the continents. At this discontinuity, seismic waves accelerate.

Subduction Zones are Potential Tsunami Locations Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck". Accumulated Seismic Energy As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plate. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries. Earthquake Causes Tsunami Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered. Tsunami Races Away From the Epicenter The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline. IMPORTANT!! Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.

A volcano is a rupture on the crust of the Earth Earth's volcanoes occur because the planet's crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in the Earth's mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. so-called " hotspots ", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another.