Extinct volcano: A volcano that has not erupted in recorded history, for example Slemish in County Antrim. Over time, the magma beneath the earth’s surface may cool down or move away.
This can reduce volcanic activity on the surface, leaving areas with hot springs and geysers. Yellowstone National Park in the United States and Bath in England are such areas.
3.4 Products of volcanic eruptions
Volcanic eruptions produce the following substances: 1. Lava 2. Pyroclasts and pyroclastic flows 3. Poisonous gases 4. Ash, pumice and dust (tephra) 5. Water vapour
1. Lava
There are two types of lava. They each have different compositions. (a) Acidic lava
This type of lava is produced at destructive plate boundaries at subduction zones. Acidic lava has a silica content of 70% or more. It is sticky and moves slowly. Because it is so sticky, gases are trapped and great pressure can build up within the magma chamber. As the magma rises to the surface huge domes are formed within the crater of the volcano. At a critical point the magma can no longer hold its gases and a massive explosion occurs. Acidic lava produces steeply convex volcanic cones called dome volcanoes. Le Puys in France and Mount St Helens in the USA are typical.
(b) Basic lava
This type of lava is produced at constructive plate boundaries and at hotspots. Basic lava has a silica content of 55% or less which allows gases to escape easily forming a runny type of lava. When it erupts the lava flows like a fountain into the air forming a wide, gently sloping volcanic cone known as a shield volcano. The Hawaiian island of Mauna Loa is a good example. Ninety to sixty-five million years ago
County Antrim was the site of spectacular fissure eruptions that produced massive amounts of basic lava. This lava ran over the landscape and created the Antrim Plateau.
Fig. 4 Castle Geyser spews water and steam at Yellowstone National Park, Wyoming, USA.