Volcanoes General

There are a few historic examples of simultaneous eruptions from volcanoes or vents located within about 10 km of each other, but it's very difficult to determine whether one might have caused the other.
The violent separation of gas from lava may produce rock froth called pumice.
Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Because it is lighter than the solid rock around it, magma rises and collects in magma chambers.
There are about 1500 potentially active volcanoes worldwide, aside from the continuous belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time.
More than 80 percent of the Earth's surface -- above and below sea level -- is of volcanic origin.
Igneous rocks (from the Greek word for fire) form from when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.
Volcanoes that produced exceedingly voluminous pyroclastic eruptions and formed large calderas in the past 2 million years would include Yellowstone, Long Valley in eastern California, Toba in Indonesia, and Taupo in New Zealand.
Over the long term and geologic time, volcanic eruptions and related processes have directly and indirectly benefited mankind.
A Quick Reference Guide For Volcano Types Cinder Cones and Scoria Cones