What layer of earth is the source of lava that erupts from volcanoes

The layer of Earth that is a source of lava is the core.

Explanation:

The core is nothing but a ball of fire

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.