Tectonic Plate
A tectonic plate is a large, rigid slab of Earth's lithosphere that moves slowly over the asthenosphere and interacts with neighboring plates.
Definition
A tectonic plate is one of the giant pieces of Earth's rocky outer shell; the plates fit together like a puzzle and move very slowly, carrying continents and oceans with them. Each is a rigid segment of lithosphere, roughly $100\ \text{km}$ thick, that moves as a unit over the asthenosphere. There are seven or eight major plates (Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, South American) and many smaller ones such as the Juan de Fuca and Caribbean; plates can be oceanic, continental, or a mix, and their edges are where most geologic activity happens. Because a plate behaves rigidly on geologic timescales, relative motion between any two plates can be described by a single angular velocity vector about an Euler pole. Plates range from the Pacific at over $100$ million $\text{km}^2$ to microplates of a few thousand, and triple junctions where three plates meet are stable only for certain geometries. Plate age, thickness, and density are coupled: oceanic lithosphere thickens and grows denser with age, and its subduction is the primary driver of plate motion.
Example
North Carolina rides on the North American Plate, which also carries Canada, Greenland, and half of the Atlantic Ocean floor. The Pacific Plate is the largest and is almost entirely oceanic, while the Juan de Fuca Plate off Washington and Oregon is small and is being pushed under North America, feeding the Cascade volcanoes. The Indian Plate moved north at up to $15$ to $20\ \text{cm/yr}$ during the Cretaceous, among the fastest rates known, before colliding with Eurasia about $50$ million years ago to raise the Himalaya. Passive margins such as the U.S. East Coast lie within a plate, where continental and oceanic lithosphere are welded together.
Key Insight
Plates are not the same as continents, since most plates carry both land and ocean floor. The east coast of North America is not a plate boundary; the edge of the North American Plate is in the middle of the Atlantic, which is why the Carolinas have few earthquakes. Rigid plates are an approximation. Deformation zones such as the Basin and Range or the Tibetan Plateau are hundreds of kilometers wide, and plate-boundary width, not a sharp line, is the reality in many continental settings.