What is the second most abundant element in Earth's crust?

Silicon, after oxygen - and the two are mostly found together. Nearly all common rock is silicate, built from a tetrahedron of one silicon atom and four oxygens, linked into chains, sheets or frameworks depending on the mineral. Quartz, feldspar, mica and clay are all variations on how those units join.

Silicon is directly below carbon in the periodic table and shares its four bonding sites, which is why silicon-based life is a durable science fiction idea. The chemistry does not really cooperate: silicon bonds to itself weakly and does not form the long stable chains carbon does, and its equivalent of carbon dioxide is sand rather than a gas, so a silicon-based creature could not exhale.

What it does instead is run everything. Ultra-pure silicon - refined to something like one impurity atom in ten billion, then deliberately contaminated with precise traces of other elements - is the substrate of every microchip, and the entire semiconductor industry exists because the second most common element in the ground happens to sit exactly on the boundary between conductor and insulator.