What is the place where an unusually hot part of the mantle rises through the crust causing volcanic activity called?

Hot spots are the volcanic regions fed by an underlying mantle that is extremely hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. They become volcanic when lava pushes up from the mantle and creates a volcano.

Hotspot is on a location on the earth's surface that has experienced active volcanism for an extended period. Hot spots are believed to be precipitated by a narrow stream of hot mantle-core boundary called a mantle plume.