Why was aluminum the most common metal in the earths crust?

Answer 1

While aluminum is the most abundant metal in the crust, in terms of the whole composition of Earth, aluminum only comes in fourth place after iron, magnesium, and nickel. Earth is a differentiated object, meaning denser materials are generally found closer to the center. Most of the iron and nickel, which are quite dense, sank to Earth's core. The magnesium-rich and iron-rich minerals, which are generally fairly dense, make up the bulk of the mantle. The less dense aluminum-rich minerals were therefore left closer to the surface and essentially float on the denser inner layers.