Earth's crust is both the outermost and the thinnest of the planet's layers, is composed mostly of oxygen and silicon, and is where the most ancient rock samples in the world have been found. Overall our planet's crust ranges from 3 to 44 miles in thickness, extending to as much as 60 miles deep in some places.
Just 11 elements make up more than 99 percent of the crust's material. Those elements are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, titanium, hydrogen, and phosphorus. The crust is identified as either continental or oceanic, each of which has its own composition and dynamics.
The crust can be thought of as floating on top of the next, middle layer, known as Earth's mantle. Sections of crust known as plates bump into each other, causing the rocky materials to undergo change. Continental crust is older and thicker and made of sedimentary and lighter-weight igneous rocks that are respectively formed by accumulated debris and by volcano activity. Oceanic crust is younger and thinner and made of heavier igneous rock. Combined the continental and oceanic crust comprise just a little more than 2.18 percent of Earth's mass.
Tectonic activity involves oceanic crust more often than continental crust, as plates collide and rip apart at mid-ocean ridges. Because the continental crust is spared from much tectonic activity, some rocks of the continental crust are almost as old as Earth itself. The oldest known rocks in the world are in the Jack Hills of western Australia and have been dated at 4.4 billion years age compared with the 4.6 billion total years of Earth.