Final answer:
The upper mantle of Earth is primarily composed of peridotite, an ultramafic rock that is rich in iron and magnesium silicate minerals.
Step-by-step explanation:
The composition of the Earth's upper mantle is thought to be peridotite. Peridotite is an ultramafic rock made of iron- and magnesium-rich silicate minerals. It is rarely found at Earth's surface, but samples can sometimes be ejected from volcanoes, allowing scientists to study its properties. The mantle itself is a massive layer that extends from the base of the crust down to a depth of 2900 kilometers, with temperatures and pressures that allow it to deform and flow slowly. In contrast, the crust is made up primarily of oceanic basalt and continental granite, each with different densities and compositions.