Final answer:
The heat source for active tectonics comes from Earth's interior, specifically through mantle convection and mantle plumes which lead to hotspots, driving the movement of tectonic plates and associated geological phenomena such as volcanism.
Step-by-step explanation:
The heat source that drives active tectonics on Earth is primarily the mantle convection, which is the slow movement of Earth's mantle caused by the circulation of heat from the deep interior to the surface. This process of mantle convection facilitates the movement of tectonic plates that fit together like a jigsaw puzzle and causes the gradual drifting of continents.
Additionally, mantle plumes, which are thought to originate from the core-mantle boundary, can create hotspots. These hotspots can occur anywhere and are not reliant on plate boundaries. Hotspots result in localized areas of melting in the lithosphere leading to volcanism, which is independent of but sometimes related to the movement of tectonic plates. As such, they can form volcanic islands like Hawaii, and create geothermal features such as those found at Yellowstone National Park.
In conclusion, the heat from the Earth's interior, descending from mantle plumes and through the process of mantle convection, is the fundamental driver of active tectonics and associated geological phenomena, including volcanism and mountain-building.