What is a Tectonic Plate

From GeoClasses

modified from  Kios and Tilling, 1996, Dynamic Earth, USGS

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.

There are two very different types of plates.

Continental plates/crust are composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar.
Oceanic plates/crust are composed of basaltic rocks, which are much denser and heavier than continental crust.

Size and shape of plates

Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest.
Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interior parts of North and South America).

How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the composition of the rocks.

Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations. The variations in plate thickness are nature's way of partly compensating for the imbalance in the weight and density of the two types of crust (continental and oceanic).

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