what is a circulating current of magma

Question

It is boiling hot melted rock, moving about in some circular flow.

Apparently there are places underground where magma (molten basalt above 1000 C) circulates in a flow loop, up and down, without reaching the surface. One such place is below a volcano in Nicaragua. There may be others below Flagstaff, AZ, Yellowstone, Hawaii, and the active and dormant Cascade volcanoes of the western US (Lassen; St. Helens; Baker). There is interest in exploring these regions for geothermal energy.

recommended reading:
http://www.osti.gov/bridge/servlets/purl/5205879-LM85Nb/native/5205879.pdf

i have heard that there is exactly 5 in Flagstaff, AZ