The largest active one is Mauna Loa, in Hawaii. It has a volume of about 40,000 cu km. The area above sea level occupies 5,125 sq km. The most active volcano is Kilauea, also in Hawaii, which has been in near- continuous eruption since 1983.
Most of the earth's volcanoes are located around the Pacific Ocean because that is where most of the Earth's subduction zones are found. A subduction zone is a place where one "plate" of the earth's crust is pushed under another plate. The lower plate eventually starts to melt, and the material rises up through the upper plate, forming a mountain chain of volcanoes, such as the Andes, or a chain of volcanic islands such as the Marianas or Aleutians.
When hot rocks, called magma, and gases break through the planet's crust, an eruption occurs. The buildup of ash and lava flows around the eruption hole (or vent) makes a volcano. Some erupt for only a few days or weeks and never do so again. Large volcanoes can erupt many thousands of times over millions of years.
Lava is not the only threat from volcanoes. Mammoth Mountain in eastern California is currently emitting dangerous levels of carbon dioxide, which has forced the US Forest Service to close a nearby campsite because it could kill people. Trees are being killed by the emissions. Tsunami - tidal waves - can kill people on shorelines near erupting undersea volcanoes.
The last volcano was active in the UK about 50 million years ago, during the Tertiary period, when volcanic activity affected northwest Scotland and north Wales. Since Britain is now far away from any subduction zone, it is unlikely to have any volcanic activity for millions of years.
The temperature of a volcano depends on its activity. Most volcanoes are inactive most of the time, and thus are no warmer than non-volcanic mountains. The most common type of lava, basalts, usually have eruption temperatures of about 1150o-1200oC.
The youngest volcano is usually considered to be Paricutin in Mexico, which grew out of a corn field in 1943 and erupted for almost nine years.
The average abount of material thrown out of an erupting volcano is estimated to be about 10 million cubic metres.
The Volcanic Explosivity Index (VEI), in use since 1982, grades eruptions from 0 (gentle) to 8 (cataclysmic). The biggest eruption in recorded history was from the Tambora volcano in Indonesia in 1815 (VEI 7). The explosion produced 40 cu km of ash and killed 10,000 people. Another 80,000 died from the ensuing famine. There has been no VEI 8 eruption for 10,000 years.
About 70,000 people have died since 1902 through eruptions and their related impacts, such as tsunami, starvation, avalanches and disease in evacuation camps.
Mount Pinatubo, which erupted in the Philippines in 1991, is thought to have cooled world temperatures by an average of 0.5oC over three years because dust it threw up reflected some of the sun's light.