By Mayo Clinic Staff. Erectile dysfunction â the inability to get and keep an erection firm enough for sex â is a common problem. You've likely seen advertisements for erectile dysfunction herbs or supplements to increase your sexual performance..
plate tectonics. In geology, a theory that the Earth's lithosphere (crust and upper mantle) is divided into a number of large, plate-like sections that move as distinct masses. See Notes at fault, Gondwanaland, See more at tectonic boundary.
Plate tectonics is the scientific theory that Earth's outer layer is made up of plates or blocks that move, which accounts for why mountain and volcanoes form and earthquakes happen. The surface of Europa -- one of Jupiter's four largest moons and slightly smaller than Earth's moon -- is riddled with cracks and ridges.