Why is the earth's crust composed of 'lighter' elements than the mantle or core?

True, earth's upper crust that is lithosphere is made of lightweight rocks and is thinner as compared to that of the rock that are found in the below layers.

Explanation:

The outer layer covering the planet is made up of lighter and less dense rock as this layer is called the plating layer or the crustal rock layer which is composed of silicon 27.7% and aluminum 8.1 %, Iron 5.0% and Oxygen 46.6% and having light-matter has 1 % of the entire landmass.  Being composed of all types of rocks like sedimentary, metamorphic and igneous rocks. The crust is either oceanic or continental as the continental being the landmass is light and consists of only 29% of the entire earth some area partly submerged. Depth of 700 km. Mantel being the second, layer is composed of heavier elements like the Magnesium, Iron and other dominant metals has an average thickness of 2900 km constitutes 70% of earth mass. Divided into three layers upper, lower and Transition, etc. The outer core starts from 5150 kilometers to 6370 kilometers from outer to inner containing iron, nickel, and sulfur and followed by the transition in between presence due to the liquid boundaries, intensity gravity, and pressure.