At divergent boundaries, hot mantle rock rises and occurs.

volcanoes

Plate tectonics refers to the movement of the lithospheric plates horizontally over the asthenosphere. In divergent plate tectonics two plates slide apart from each other. In a divergent plate boundaries, the separation of plates along with the eruption of hot mantle resulting in the volcanic eruption.

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.