Plate tectonics
Plate tectonics is an important theory developed in the 1960s to explain how the continents move across the Earth's surface.
Early 20th century geologist Alfred Wegener realised that the puzzle-like fit of many the continents was more than a coincidence, but he couldn't correctly explain what powered their movement.
Geologists now know that the Earth's outermost layer, the lithosphere, is divided into independently moving plates into which the continents are embedded. The plates "float" on a layer called the athenosphere.
There are different types of plate boundary. Spreading centres at mid-ocean ridges are where undersea volcanoes create new plate material. Subduction zones are where one plate sinks below another, causing volcanic eruptions and earthquakes and, sometimes, building mountains.
Image: Earth's tectonic plates with arrows indicating motion (credit: Gary Hincks/SPL)
Introduction
Plate tectonics
TV clips (9)
play
Danakil Depression
Aerials of Africa's lowest land point and time-lapse of its most active volcano, Erta Ale.
Wearing gas masks to protect against constant noxious fumes, the crew endured blistering day temperatures of more than 40 degrees. To capture an unusual perspective on this extremely hostile environment, the camera was extended out above the springs on a Jimmy Jib crane. The ever-moving lava lake at Erta Ale was recorded at night using 35mm time-lapse.
play
Earth's moving plates
Plate boundaries are places of chaos and mineral wealth.
Professor Iain Stewart explains how hotspots are a good demonstration of Earth's system of tectonic plates. As the plates move across the Earth's surface, they interact with one another at plate boundaries, which are places where earthquakes and volcanoes are common. Typically, plate boundaries are also places of great mineral wealth.
play
Megathrust earthquakes
Earthquakes are common at subduction zones, points where one plate moves below another.
Earthquakes are common at subductions zones, points where the Earth's plates meet and one plate moves below the other. The powerful earthquakes that are caused by this type of plate movement are known as megathrust earthquakes. The 2004 Boxing Day earthquake was a megathrust quake that triggered the subsequent deadly tsunami.
play
Earthquakes in California and Japan
Why are some parts of the world so earthquake prone?
Scientists explain how the movement of the Earth's plates creates faults in the Earth's crust where the plates meet. Earthquakes are caused by movement along these networks of faults and the movement of the plates themselves.
play
A new ocean
A continent is breaking apart at Ethiopia's Afar Depression.
Dr Iain Stewart explains how new oceans are created when continents are broken apart by the Earth's moving plates.
Plate tectonics
Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory that describes the large-scale motions of Earth's lithosphere. The model builds on the concepts of continental drift, developed during the first decades of the 20th century. It was accepted by the geoscientific community after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.
Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) proposed gradual shrinking (contraction) or gradual expansion of the globe.
Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result in differences in gravitational forces) and drag, downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of the Sun and the Moon. The relative importance of each of these factors is unclear, and is still subject to debate (see also below).
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