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)
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.
Iain Stewart explains how the Earth's crust is divided.
Dr Iain Stewart visits the Mid-Atlantic Ridge in Thingvellir, Iceland, a place where it's possible to see the divide between two of the Earth's plates on land.
Iain Stewart visits New Zealand to explain how mountains form.
Dr Iain Stewart visits New Zealand to explain how the collision of two plates results in mountain ranges forming over millions of years.
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.
Shifting continents caused the Mediterranean to dry out.
Dr Iain Stewart explains how, six million years ago, the continents of Europe and Africa moved together and cut off the Mediterranean Sea. He also explains how rivers erode the land and bring salt to the oceans.
Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory describing the large-scale motion of 7 large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, over the last hundreds of millions of years. The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken up into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the 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 relative movement of the plates typically ranges 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 lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.
Tectonic plates are able to move because the Earth's lithosphere has greater strength 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, with downward suction, at the subduction zones. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.