Principally it refers to the movement and interaction of the earth's lithosphere. A cross section of the earth reveals a crust with a complex pattern of interlocked and interactive plates. These plates derive their name from the predominant continental or oceanic environment that they underlay.
The division is based on Plate tectonics theory in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily.
In terms of heat transfer, the lithosphere loses heat by conductionwhereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.
This division should not be confused with the chemical subdivision of these same layers into the mantle comprising both the asthenosphere and the mantle portion of the lithosphere and the crust: The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plateswhich ride on the fluid-like visco-elastic solid asthenosphere.
Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed.
The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountainsvolcanoesmid-ocean ridgesand oceanic trenches.
The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Plate tectonics theory of Fire being the most active and widely known today. These boundaries are discussed in further detail below.
Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation  and to mantle plumes.
As explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans.
The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents.
Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements " mafic " than continental crust " felsic ".
Types of plate boundaries Main article: List of tectonic plate interactions Three types of plate boundaries exist,  with a fourth, mixed type, characterized by the way the plates move relative to each other.
They are associated with different types of surface phenomena.
The different types of plate boundaries are: The relative motion of the two plates is either sinistral left side toward the observer or dextral right side toward the observer. Transform faults occur across a spreading center. Strong earthquakes can occur along a fault.
The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. Divergent boundaries Constructive occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin.
At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges e. Convergent boundaries Destructive or active margins occur where two plates slide toward each other to form either a subduction zone one plate moving underneath the other or a continental collision.
At zones of ocean-to-continent subduction e. Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous mineralsinto the surrounding mantle.
The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt. The magma that results typically leads to volcanism. Aleutian islandsMariana Islandsand the Japanese island arcolder, cooler, denser crust slips beneath less dense crust.
This motion causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands.Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and billion years ago.
But then in , along came a scientist named Arthur Holmes who didn't think that Wegener's theory of continental drift was too farfetched.
SEA-FLOOR SPREADING CHAPTER 8: PLATE TECTONICS Sea-floor Spreading In Wegener’s time, the world’ s ocean floors were largely unexplored. Mapping the sea floor provided more important evidence for the theory of plate tectonics.
The Plate Tectonics Teaching Packet is available for purchase here. Click here for Table of Contents. Over pages of ready-to-run materials covering: The Theory of Plate Tectonics.
Plate tectonics is the unifying theory of Earth science and explains many of the major features of how our planet operates. It accounts for Earth's history including . Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake.
The drawing above is a cross section of the earth showing the .