Minor partial melting in the asthenosphere causes liquid magma to form. If the molten rock is able to move at all, it will leave behind the heavier solid rock that did not melt. In what direction is the molten rock most likely to move through the viscous asthenosphere?

What is asthenosphere: The asthenosphere is the highly viscous, mechanically weak and ductile region of the upper mantle of the Earth. Any rock will melt if its temperature is raised high enough. However, the melting point of any rock or mineral is also a characteristics of the pressure exerted on the rock or mineral. Generally, as the pressure is increased on a material, the melting point increases. The temperature of the rocks that constitute the asthenosphere is below their melting point. As temperature or pressure increases, the material tends to deform and flow. If the pressure reduced, the melting point will also reduced, so the material may begin to melt, however the melting point and pressure balance in the asthenosphere have led some certain geologists to assume that as much as 10% of the asthenospheric material may be molten. The rest is so close to being molten that relatively modest changes in pressure or temperature may cause further melting.

What is a Molten rock: When the high pressure changes the rock into a viscous semisolid called magma

N:B Molten rock is also known as Magma

Theories has been developed by Geologist to explain the changes that take place in the asthenosphere when plates begin to diverge or converge. If the lithosphere has not separated, the asthenosphere cools as it rises and becomes part of the lithosphere. If there is a certain leak in the lithosphere, magma (Molten rock) may escape and flow outward. Depending on the temperature and pressure in the region, that outflow of material (magma) may occur in a violent volcanic eruption or a quiescent lava flow. In either case, the plates spread apart in a process known as rifting.

In zones of convergence, where two plates are moving toward each other, the asthenosphere may be exposed to increased pressure and begin to flow in downward direction. In this case, the lighter of the two colliding plates slides up and over the heavier plate, which is subucted into the asthenosphere. Because the cooler and denser lithosphere is more rigid than the asthenosphere, the asthenosphere is pushed outward and upward. During subduction, the downward moving plate is heated, melting occurs, and molten rock flows upward to Earth’s surface.