Why would a plume of solid silicate rock rising slowly from deep in the mantle begin melting as it neared the base of the lithosphere?
a. the rock heats up and expands at lower pressures, causing it to liquefy
b. temperatures remain high as lowered pressures decrease melting temperatures.
c. the lowered pressures cause rapid heat loss accompanied by melting.
d. none of the above

B. Temperatures remain high as lowered pressures decrease melting temperatures.

Explanation:

Rocks , like ice , contracts on melting so , their melting point decreases on decrease in pressure . When hot solid rocks are pushed towards the surface of the earth, due to decrease in pressure , their melting point decreases , so the solid rocks starts melting without any addition of  heat . Their  present temperature becomes more than their melting point.