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=Julie D. per.4=



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Plate tectonics (from the [|Late Latin] //tectonicus//, from the [|Greek]: //τεκτονικός// "pertaining to building") is a [|scientific theory] which describes the large scale motions of [|Earth]'s [|lithosphere]. It is vital for the existence of life on earth because of the role that it plays in the [|global cycle] that maintains the balance of [|carbon] between the [|biosphere], [|pedosphere], [|geosphere], [|hydrosphere], and [|atmosphere].[//[|citation needed]//] The theory builds on the older concepts of [|continental drift], developed during the first decades of the [|20th century] by [|Alfred Wegener], and [|seafloor spreading], developed in the 1960s. The lithosphere is broken up into what are called **tectonic plates**. In the case of Earth, there are currently seven to eight major (depending on how they are defined) and many minor plates ([|see list below]). The lithospheric plates ride on the [|asthenosphere]. These plates move in relation to one another at one of three types of plate boundaries: [|convergent], or collisional boundaries; [|divergent] boundaries, also called spreading centers; and [|transform] boundaries. [|Earthquakes], [|volcanic activity], [|mountain]-building, and [|oceanic trench] formation occur along plate boundaries. The lateral relative movement of the plates varies, though it is typically 0–100 [|mm] annually.[|[1]] Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Their movement is driven by heat dissipation from the [|mantle]. Lateral density variations in the mantle result in [|convection], which is transferred into tectonic plate motion through some combination of [|drag], downward [|suction] at the [|subduction] zones, and variations in topography and density of the crust that result in [|differences in gravitational forces]. The relative importance of each of these factors is unclear. In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, through [|geosynclinal theory]. It was observed as early as 1596 that the opposite [|coasts] of the [|Atlantic Ocean]—or, more precisely, the edges of the [|continental shelves]—have similar shapes and seem to have once fitted together.[|[2]] Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid earth made the various proposals difficult to explain.[|[3]] The discovery of [|radioactivity] and its associated [|heating] properties in 1895 prompted a re-examination of the apparent [|age of the Earth],[|[4]] since this had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a [|black body].[|[5]] Those calculations had implied that, even if it started at [|red heat], the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.