Final answer:
The upper mantle is rich in ultramafic rock peridotite, which contains iron- and magnesium-rich silicate minerals. Samples of upper mantle material can be occasionally found and analyzed. The density in the mantle increases downward due to compression.
Step-by-step explanation:
The upper mantle is rich in ultramafic rock peridotite which contains iron- and magnesium-rich silicate minerals.
Peridotite is rarely found at Earth's surface but samples of upper mantle material are occasionally ejected from volcanoes, providing an opportunity for analysis.
The density in the mantle increases downward as a result of compression produced by the weight of overlying material.