We used to have a general idea of how and why magma plumes will erupt from the sea floor in certain areas creating Hotspots such as Hawaii. But we never had much more than theories and little evidence based off the study of physics of magma flow in lab models, which is insufficient to make any definite claim. The short answer is always, a combination of temperatures, pressures, and densities, but the exact mechanisms are sometimes poorly understood in complex geologic processes.