Crust is the outermost solid layer of a rocky planet, the hard surface that forms on baked foods, or the dried material that develops on skin wounds and around the eyes. The word applies across geology, food science, and medicine, and the meaning shifts depending on context. Most people searching this term want to understand Earth’s crust, so that’s where we’ll start before covering the other common uses.
Earth’s Crust: The Planet’s Thin Shell
Earth’s crust is the thin, rigid outermost layer of rock that sits on top of the much thicker, hotter mantle beneath it. Think of it like the skin on an apple. Despite being the ground we stand on and the ocean floor beneath every sea, it makes up less than 1% of the planet’s total volume. Two elements dominate its composition: oxygen at about 46.6% by weight and silicon at roughly 27.7%. These combine with other elements to form silicate minerals, which are the building blocks of nearly all crustal rock.
There are two distinct types of crust, and they differ in thickness, density, and composition.
Continental Crust
Continental crust is the thicker, lighter variety that forms the landmasses and shallow continental shelves. It averages about 30 kilometers thick, but under major mountain ranges like the Alps or the Sierra Nevada it can extend as deep as 100 kilometers. Continental crust is made primarily of granite-type rocks, which are rich in silicon and aluminum. Because it’s relatively buoyant, continental crust “floats” higher on the mantle, which is why dry land exists above sea level at all. Some continental crust is billions of years old, making it the oldest material on Earth’s surface.
Oceanic Crust
Oceanic crust is thinner and denser. Beneath the oceans, it generally extends only about 5 kilometers down. It’s composed mainly of basalt, a darker, heavier rock richer in iron and magnesium. Oceanic crust is constantly being created at mid-ocean ridges, where molten rock rises from below and solidifies, and destroyed at subduction zones, where it dives beneath continental crust and melts back into the mantle. As a result, no oceanic crust on Earth is older than about 200 million years.
Crust vs. Lithosphere
The terms “crust” and “lithosphere” are often confused, but they describe different things. The crust is defined by its chemical composition: it’s a distinct layer of silicate rock that differs from the mantle beneath it. The lithosphere is defined by how rock behaves mechanically. It includes the crust plus the rigid uppermost portion of the mantle, and it’s the layer that breaks into tectonic plates.
The lithosphere averages about 70 kilometers thick but varies widely. Under mid-ocean ridges it can be just a few kilometers thick, while under continental mountain belts it can exceed 150 kilometers. So the lithosphere is often thicker than the crust alone, because it extends deeper into rigid mantle rock.
Crusts on Other Worlds
Earth isn’t the only body with a crust. Mars has a significantly thicker one, averaging 42 to 56 kilometers globally. That’s thicker than both Earth’s and the Moon’s crusts. The Moon’s crust, by contrast, is relatively thin. These differences reflect how each body cooled and differentiated early in its history. A thicker crust on Mars suggests the planet’s interior cooled and solidified more extensively than Earth’s, which still has a vigorously convecting mantle driving plate tectonics.
Bread Crust and the Science of Browning
In food science, crust refers to the firm, browned outer layer that forms on baked goods, roasted meats, and other cooked foods. On bread, crust develops through two chemical processes: the Maillard reaction and caramelization. Both are triggered by heat but work differently.
The Maillard reaction occurs when amino acids (from proteins) react with sugars at high temperatures. It happens in three stages. The early stage is colorless and odorless, involving initial chemical bonding between these molecules. The final stage produces brown-colored compounds called melanoidins, which give bread crust its characteristic golden to dark brown color. Caramelization, meanwhile, involves the direct breakdown of sugars under heat. Together, these reactions create the complex flavors, aromas, and textures that make a good crust appealing. Browning typically kicks in at around 160°C (320°F), which is why the surface of bread browns while the moist interior stays pale and soft.
Baking itself proceeds in three phases: the dough expands, the surface dries out, and then crust browning begins. That surface desiccation is key. The outside of the loaf has to lose enough moisture for temperatures to climb above 100°C, at which point the browning chemistry takes over.
Skin Crusts: What They Are and What They Mean
In medicine, a crust is dried material that forms on the surface of the skin. It’s composed of keratin, serum, cellular debris, and sometimes microorganisms. “Crust” and “scab” are often used interchangeably, though both refer to the same basic process: fluid from a wound or inflamed skin dries and hardens into a protective layer.
Crusts are classified by what they contain. A serous crust is mostly dried serum (the clear fluid from blood). A hemorrhagic crust contains mostly dried blood. Cellular crusts are packed with inflammatory cells, and serocellular crusts are a mix of serum and inflammatory material. The type of crust can hint at what’s happening underneath.
Most skin crusts are harmless, forming naturally as cuts and scrapes heal. But certain patterns are worth paying attention to. Golden or “honey-colored” crusts are a hallmark of impetigo, a bacterial skin infection common in children. Impetigo starts as red, itchy sores that break open and leak clear fluid or pus. After a few days, that fluid dries into the distinctive yellowish crust. The infection is caused by staph or strep bacteria and is treated with antibiotics. The sores typically heal without scarring.
Eye Crust (Sleep or Rheum)
The crusty material you find in the corners of your eyes when you wake up is called rheum, though most people just call it sleep crust or eye gunk. It’s a mix of mucus, exfoliated skin cells, oils, and tears that the eye produces or sheds during sleep. While you’re awake, blinking constantly washes this material away. During sleep, it accumulates and dries at the inner corners of the eyes or along the lash line.
A healthy eye will always produce the mucus, salts, and oils that form sleep crust, though the amount varies from person to person. A small amount of white or cream-colored crust in the morning is completely normal. Large amounts of thick, green, or yellow discharge can signal an eye infection like conjunctivitis.