What is earth's lower mantle made of?
Iron and nickel, with a few lighter elements like silicon or oxygen.
The inner core is a solid ball of mostly metal. It is solid because of the pressure of the rest of the Earth around it, even though it is at 5700K and should be liquid if it were at normal pressure. Its pressure is actually about 3,500,000 atmospheres.
Scientists have tested the density of the core by firing waves at it and measuring their reaction, and found that actually a pure nickel-iron compound is more dense than the core, meaning that the core has lighter elements in it, probably carbon, oxygen or silicon.
It's made of silicates and oxides. The most abundant elements there are oxygen, silicon, magnesium, iron and calcium.
The lower mantle is not to be confused with the core; it's made of silicate and oxide rocks rather than metal. The lower mantle rock differs in two ways from what we see in the crust and upper mantle:
1) Elemental composition: rock in the mantle (both upper and lower) has more iron and magnesium, and less calcium and aluminum, than what we see in the crust. Calcium and aluminum tend to form lower-density silicates than magnesium and iron, so the former silicates tend to float up to the crust.
2) Mineral structure: this applies especially to silicates, which are "perovskite silicates" instead of ordinary silicates. In ordinary silicates we see silicon form four covalent bonds to oxygen, filling up the usual "octet" of valence electron states. Under the high pressure inside the lower mantle, however, the silicon atoms will form six bonds making
A detailed discussion of Earth's mantle is described here: