VOLCANIC ERUPTIONS:

Volcanic activity, as is the case with earthquakes, is linked to plate tectonic processes.
Most of the world's active above – sea volcanoes are located near convergent plate boundaries site of subduction e.g, margins of Pacific ("the Pacific Ring of Fire").
About 3/4th of all volcanism on Earth takes place along the oceanic spreading centres, Mid-Atlantic Ridge, East Pacific Rise- beneath the ocean- no direct hazard.

A. Subduction –zone volcanoes are called composite cones or strata-type volcanos, and these typically erupt with explosive force because magma is too stiff to allow easy escape of volcanic gases.
As a consequence, tremendous internal pressures mount as the trapped gases expand during ascent, before the pent-up pressure is suddenly released in a violent eruption.
Examples include Mount St. Helens, Washington State; Mount Pinatubo, Luzon, Philippines.
On June 15, 1991, Mt. Pinatubo spewed ash 40 km into the air and produced huge ash flows (Pyroclastic flows) and mudflows that devastated a large area around the volcano; this eruption is regarded as one of the largest eruptions in this century.
This volcano, located 90 km from Manila, had been dormant for nearly 600 years.
Impact of 1991 Pinatubo eruption was global.
It sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that drifted around the world.
The sulphur dioxide in this cloud, about 22 million tonnes- combined with water to form droplets of sulphuric acid, blocking some of the sunlight reaching the Earth and thereby cooling temperatures in some regions by as much as 0.5 deg. C.
Slightly cooler than normal temperatures recorded worldwide and the brilliant sunsets and sunrises have been attributed to the above processes.
Implications- an eruption such as 1991 Pinatubo could affect the weather for a few years.