Final answer:
The Earth's mantle is not motionless; it is solid rock that can flow due to mantle convection, which moves the tectonic plates. Its density varies with depth, and samples have been brought to the surface by volcanic activities.
Step-by-step explanation:
which of the following is true of the Earth's mantle is: it is thought to convect, contributing to the motion of Earth's tectonic plates. The Earth's mantle is composed of solid rock that, due to high temperatures and pressures, can deform and flow slowly. This behavior facilitates mantle convection, a key process that drives the movement of tectonic plates. Contrary to being motionless or entirely liquid, the mantle’s solid rock gets more dense with depth, from about 3.5 g/cm³ to over 5 g/cm³. Also, mantle materials have been brought to the surface by volcanic activities, disproving the notion that no mantle samples have ever been observed at the surface.
The Earth's mantle is not motionless and is, in fact, thought to convect, contributing to the motion of Earth's tectonic plates. This convection is responsible for the movement of the crustal plates on the Earth's surface. The mantle is composed of rock that has on average the same density and composition as the rocks of the Earth's crust. It is not entirely liquid, but more or less solid, although it can deform and flow slowly at the high temperatures and pressures found there. While samples of upper mantle material are occasionally ejected from volcanoes, permitting a detailed analysis of its chemistry, no samples of the entire mantle have ever been brought to the Earth's surface by any geologic process.