A tectonic plate or a lithospheric plate is an enormous and irregular shaped slab of solid rock which is composed of continental and oceanic lithosphere. Its extension range from 9 miles to thousands of miles. 

Experts estimate tectonic plates were developed early in the Earth’s 4.6-billion-year history. Through all this time, they have been slowly moving, clustering together, and then separating from each other.

The tectonic composition is the reason why these massive slabs can float despite its weight. 

While continental crust is made of granitic rocks that come from lightweight minerals, oceanic crust is composed of a more substantial and denser material as basaltic rocks. So variations in thickness are a way in which nature compensates for the imbalance between these two types of crust. 

Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 62 miles). In contrast, the crust under the oceans is generally only about 3 miles thick. Like icebergs, of which only the tips are visible above water, continents have deep “roots” to support their elevations.

Tectonic plates are relevant for us because, despite most of the boundaries between them are not visible, those regions concentrate a significant earthquake and volcanic activity. 

The United States lies on the North American plate, which is one of the major tectonic plates on the planet. Though smaller in size, the minors are no less important, for example, the tiny Juan de Fuca plate is mostly responsible for the volcanoes that dot the Pacific Northwest of the United States.

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