Crusts

Crusts are the outermost solid layers of a planetary body, as well as the edible outer layer of bread, pies, and pastries. In geology, the crust forms the thin shell above the mantle and is distinguished by composition and thickness. In cooking, crusts are created by heat, drying, and browning that affect texture and flavor.

Earth's crust is divided into continental crust, which is granitic and less dense, and oceanic crust, which is basaltic and more dense. Continental crust averages about 30-50 km in thickness, oceanic crust about 5-10 km. The crust lies above the mantle, and their boundary is the Mohorovičić discontinuity. The crust, along with the lithosphere, participates in plate tectonics. Oceanic crust is continually created at mid-ocean ridges and recycled into the mantle at subduction zones. Continental crust is older and evolves through tectonics, erosion, and magmatic activity.

Bread crust forms where the dough surface is exposed to heat, producing caramelization and the Maillard reaction, resulting in a hard, flavored exterior. The texture and color vary with ingredients, fermentation, and baking conditions. Pie and pastry crusts consist of fat-rich doughs that yield flaky or crumbly textures through mechanical layering and fat distribution. Edible crusts are also a subject of culinary study for texture, nutrition, and consumer preference.

Most planetary bodies have crusts with diverse compositions. The study of crusts, through seismology, volcanism, and sample analysis, informs models of planetary formation, interior structure, and surface evolution.