Where did Earth's (interior) heat come from? a. volcanoes b. accretion and differentiation as Earth formed c. radioactivity d. all of the above e. b. & c.

e. b. & c. (accretion and differentiation as Earth formed and radioactivity)

Explanation:

There are two sources of earth's interior heat. The first one is the left-over heat from the planet earth's original formation as a result of accretion and differentiation. Gravitational attraction of material in the vicinity of the solar disk and forming the earth caused some impact. The heat was a result of the release of energy during impact of these accreting material. Now as it's gravitational field increased as a result of the forming earth's growing size and mass, the earth began to compact with this compaction also producing heat. The second one is the heat caused by the decay of radioactive elements. By nuclear fission radioactive atoms of the elements spontaneously disintegrate thereby releasing energy in the form of heat.