4. Which conditions normally can be found in Earth’s asthenosphere, producing a partial melting of ultramafic rock?

A) Temperature = 1,000°C; Pressure = 10 million atmospheres

B) Temperature = 2,000°C; Pressure = 0.1 million atmospheres

C) Temperature = 3,500°C; Pressure = 0.5 million atmospheres

D) Temperature = 6,000°C; Pressure = 4 million atmospheres

Correct Answer: Option B - Temperature = 2,000°C; Pressure = 0.1 million atmospheres

The asthenosphere is the mechanically weak ductily-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~ 62 and 124 miles) below the surface, but perhaps extending as deep as 400 km (~ 249 miles).The material of which the asthenosphere is composed can be described as plastic-like, with much less rigidity than the lithosphere above it. This property is caused by the interaction of temperature and pressure on asthenospheric materials. Any rock will melt if its temperature is raised to a high enough temperature. However, the melting point of any rock is also a function of the pressure exerted on the rock. In general, as the pressure is increased on a material, its melting point increases. Materials that make up the asthenosphere tend to be slightly cooler than their melting point. This gives them a plastic-like quality that can be compared to glass. As the temperature of the material increases or as the pressure exerted on the material increases, the material tends to deform and flow. If the pressure on the material is sharply reduced, so will be its melting point, and the material may begin to melt quickly.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, creating the movement of crustal plates described by Plate tectonics theory.

Hence, options A, C and D are incorrect.