Describe one form of minor extrusive activity and explain its formation.

Geysers are intermittent discharges of turbulent water from the Earth's surface. One of the most famous examples is Old Faithful in Yellowstone National Park, North America.

There are several factors needed for a geyser to occur. There needs to be a water source, in the form of underground waterflow. There also needs to be a heat source, which is found in the form of lava beneath the Earth's surface. This water then needs to be contained in a sealed area, in the form of an underground chamber.

Geyser activity is caused when underground flow is heated by lava beneath the surface, and this water then flows into a tightly contained underground chamber so that it is put under high pressure. The high pressure means that even if the water is over boiling point, it remains in liquid form as it has become "superheated water". However, as pressure increases with increased flow, small amounts of water at the top of the chamber may evaporate, and escape from small holes in the chamber. This process will keep happening over time, until the decreased volume of water means there is a such a low pressure in the chamber that the water is no longer superheated. This means the water will evaporate and turn to a gas, causing it to expand and explode out of the chamber onto the surface, forming the intermittent turbulent discharge we recognise as a geyser.