This is a volcanic "cone".
Click on image for full size
Courtesy of USGS
There are several ways in which a volcano can form, just as there are several different kinds of volcanoes. Many of the volcanoes on the Earth's surface are part of the "Pacific Rim of Fire".
During the eruption of a volcano, flowing lava and ashharden around the opening out of which they came, forming a large cone. This cone is what we know as a volcano.
Among the different kinds of volcanos are:
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