What is the lithosphere made of? How does it compare to other planets?
The lithosphere, the outer rigid layer of Earth's rocky matter consisting of the crust and upper mantle, is made mostly of silicates. Rocky bodies at least throughout our Solar System are similar.
The silicate composition is inherent to the abundance and chemical reactivity of elements. According to http://www.knowledgedoor.com/2/elements_handbook/element_abundances_in_the_solar_system.html, the ten most common elements in the Solar System are:
Rocks are formed by the reaction of oxygen with other elements to form solids. The most stable such solids, from the elements listed above, are silicates. So that is what we see in rocky matter everywhere in the Solar System, not just Earth.