The Earth’s crust is the planet’s thin, solid, outermost layer, acting as a rigid shell that floats atop the far more viscous mantle. Although the source of all accessible natural resources, the crust accounts for less than one percent of the Earth’s total volume. Its thickness varies significantly, ranging from five to ten kilometers beneath the oceans to between twenty and seventy kilometers under the continents. The chemical composition of this layer governs nearly all surface geological processes, from mountain building and volcanism to the formation of soil and mineral deposits.
Primary Elemental Makeup
The crust’s chemical composition is dominated by eight specific elements, which make up over 98% of its total mass. Oxygen is the most abundant element, comprising approximately 46.1% of the crust by weight. This percentage is due to oxygen’s reactivity and its tendency to readily bond with other elements to form stable compounds.
Silicon is the second most abundant element, accounting for roughly 28.2% of the crust’s mass. Together, oxygen and silicon form the foundation of the vast majority of rock-forming minerals. The remaining six of the “Big Eight” include Aluminum (8.2%), Iron (5.6%), Calcium (4.1%), Sodium (2.3%), Magnesium (2.3%), and Potassium (2.1%).
Mineralogical Structure
The abundant elements in the crust rarely exist in a pure, elemental state, instead combining to form structured, naturally occurring compounds known as minerals. The most widespread class of these compounds is the silicates, which constitute over 90% of the crust’s mass. This dominance results directly from the high availability of oxygen and silicon, the two most common elements.
The fundamental building block for all silicate minerals is the silicon-oxygen tetrahedron, a structure consisting of a single silicon atom bonded to four oxygen atoms. These tetrahedra carry a net negative charge, allowing them to link together with positive ions like iron, magnesium, aluminum, and calcium, creating diverse mineral structures. Depending on how these tetrahedra share oxygen atoms, they form complex arrangements like chains, sheets, or three-dimensional frameworks. Other mineral groups like oxides, carbonates, and sulfides also exist for elements that do not readily fit into the silicate structure.
Continental Versus Oceanic Crust
The Earth’s crust is divided into two distinct types with different chemical and physical properties: continental crust and oceanic crust. Continental crust is significantly thicker, ranging up to 70 kilometers in depth, and is characterized by a lower density. Its composition is described as felsic, a term derived from its richness in lighter elements, primarily silicon and aluminum.
The most common rock type in the continental crust is granite, which is rich in light-colored minerals such as quartz and feldspar. This less dense, felsic material allows continental landmasses to float higher on the mantle, keeping them generally above sea level. Continental crust is also older, having undergone complex geological processes over billions of years.
Oceanic crust is thin, typically only six to ten kilometers thick, and has a higher density. Its composition is characterized as mafic, meaning it is rich in the heavier elements magnesium and iron. The primary rock type is basalt, an extrusive igneous rock that forms when mafic magma rises and cools quickly at mid-ocean ridges.
The substantial difference in density means that when a continental plate and an oceanic plate collide, the denser oceanic crust sinks beneath the lighter continental crust in a process called subduction. This compositional distinction controls the movement of tectonic plates and the location of major geological features. The oceanic crust is also relatively young, with the oldest sections dating back only about 200 million years.