Continental crust is made primarily of silica-rich rocks like granite and gneiss, with an overall silica content of about 64%. It’s lighter and thicker than the oceanic crust beneath the sea floor, which is why continents sit high above sea level. But the full picture goes deeper than “it’s granite,” because continental crust changes dramatically from top to bottom.
The Dominant Minerals
More than 90% of the continental crust is built from silicate minerals, compounds structured around silicon and oxygen. Feldspars alone make up over half the crust, split between plagioclase feldspar (39%) and alkali feldspar (12%). Quartz accounts for another 12%. After that come pyroxenes (11%), amphiboles (5%), micas (5%), and clay minerals (5%). The remaining 8% is non-silicate material: carbonates, metal oxides, and sulfides.
If you’ve ever picked up a piece of granite and noticed the glassy, milky, and pinkish grains packed together, you were looking at quartz and two types of feldspar. That combination is essentially the recipe for the upper continental crust.
Chemical Makeup
When geologists melt down a representative sample of continental crust and measure its chemistry, the result is roughly 64% silicon dioxide (silica) by weight, on a volatile-free basis. Aluminum oxide comes in second, followed by smaller amounts of iron oxide, calcium oxide, and magnesium oxide. This silica-heavy profile is what earns continental crust the label “felsic,” a shorthand for rocks rich in feldspar and silica. The older name for continental crust, “sial,” comes from the same idea: silicates plus aluminum.
That composition stands in sharp contrast to oceanic crust, which is basaltic and richer in iron and magnesium. Oceanic crust is denser as a result, around 2.9 to 3.0 grams per cubic centimeter, while continental crust averages about 2.7 grams per cubic centimeter. That density difference is the reason continents float higher on the underlying mantle, much like a block of pine floats higher in water than a block of oak.
How the Crust Changes With Depth
Continental crust isn’t uniform. It’s layered, and each layer has a different character.
The upper crust is the most familiar part. It’s dominated by granite and similar felsic rocks, rich in silica, potassium, and heat-producing elements like thorium and uranium. These elements concentrate near the surface because they don’t fit easily into the dense mineral structures that form deeper down. The upper crust is also the main reservoir for what geologists call incompatible elements: atoms that get pushed upward during melting and crystallization because they’re chemically excluded from deeper minerals.
The middle crust sits in the amphibolite zone, where temperatures and pressures have transformed the original rocks into intermediate compositions. It still contains significant amounts of potassium, thorium, and uranium, but less than the surface layer.
The lower crust is a different world. Rocks here have been cooked at high enough temperatures and pressures to reach the granulite state, a dense, dehydrated form. The average composition is mafic, closer to a primitive basalt derived from the mantle, though it varies from region to region. Some areas have lower crust that’s intermediate rather than fully mafic. Overall, the lower crust at about 58% silica is noticeably less silica-rich than the upper crust, and it contains far fewer heat-producing elements.
How Continental Crust Forms
Continental crust doesn’t come out of the mantle ready-made. The process starts at subduction zones, where one tectonic plate dives beneath another. Water released from the sinking plate triggers partial melting in the mantle above it, producing basaltic magma. That basalt rises into volcanic arcs, the chains of volcanoes you see along plate boundaries like the Andes and the Cascades.
Those initial additions to the crust are basaltic, not granitic. The transformation happens through a process called crustal differentiation. When basaltic rock partially melts again inside the crust, the liquid that separates out is richer in silica than the original rock. That silica-rich melt migrates upward and crystallizes at shallower levels, building the evolved, granitic upper crust. Meanwhile, the dense, iron-and-magnesium-rich residue stays behind and becomes part of the lower crust. Over billions of years, this internal recycling has sorted the continental crust into its layered structure: light and silica-rich on top, dense and mafic at the bottom.
This process also explains why the continental crust carries a distinctive chemical fingerprint. The trace element signatures found in all continental crust trace back to arc magmatism, modified by repeated rounds of internal melting and separation.
The Oldest Crust on Earth
Continental crust is ancient. The oldest known continental rocks, found in cratons in Canada and Greenland, are about 4 billion years old. That’s nearly as old as the Earth itself, which formed around 4.5 billion years ago. Oceanic crust, by comparison, is constantly recycled at subduction zones and rarely survives past 200 million years.
This longevity is a direct consequence of buoyancy. Because continental crust is too light to be pulled down into the mantle at subduction zones, it accumulates over geologic time. Continents get scraped, stretched, and rearranged, but the material persists. The granite beneath your feet may have been through multiple mountain-building events, partially melted and reassembled, yet it remains part of the continental crust rather than being swallowed back into the Earth’s interior.
Continental vs. Oceanic Crust
- Composition: Continental crust is felsic (granite, gneiss, rich in silica and aluminum). Oceanic crust is mafic (basalt, rich in iron and magnesium).
- Density: Continental crust averages 2.7 g/cm³. Oceanic crust runs 2.9 to 3.0 g/cm³.
- Age: Continental crust can exceed 4 billion years. Oceanic crust is rarely older than 200 million years.
- Thickness: Continental crust is thicker, especially under mountain ranges. Oceanic crust is thinner and more uniform.
- Behavior: Continental crust resists subduction because of its buoyancy. Oceanic crust gets recycled back into the mantle.
These differences are why collisions between two continental plates build massive mountain ranges like the Himalayas. Neither plate is dense enough to sink, so the crust crumples and thickens instead.