History of plate tectonic theory
In 1912, Alfred Wegener proposed that the continents as they are now used to form a single landmass or supercontinent, which he called Pangaea, to explain the close match of shapes on continental boundaries. He postulated that they were surrounded by a single global ocean, but that Pangea broke apart and the remnants "drifted" to their current locations.
Whilst evidence seemed to support this, Wegener couldn’t produce an acceptable theory to explain the movement of continents. In 1930 Arthur Holmes suggested a mechanism to explain continental drift. Currents of heat and thermal expansion within the Earth's mantle, he propounded, could force the continents toward or away from one another, creating new ocean floor and building mountain ranges carrying the continental fragments on larger sections of the earth’s crust.
Data to back up this theory came with the discovery of the Great Global Rift. A German expedition in 1925, using deep sounding techniques, had determined that a continuous mountain range ran along the middle of the Atlantic Ocean, where it surfaced at Iceland, continued around Africa, through the Indian Ocean, between Australia and Antartica, and north through the Pacific Ocean. This range was called the Mid Ocean Ridge.
In 1953, Americans Maurice Ewing and Bruce Heezen discovered that a deep canyon ran through the Mid Ocean Ridge occasionally coming very close to land. The rift appeared to be breaks in the earth's crust, but perfectly fitted breaks, as if they were joints made by a carpenter. This Great Global Rift outlined sections of the earth's crust, which Ewing and Heezen called tectonic plates (tectonic being derived from the Greek for carpenter.) They found six major plates ( African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American) and seven smaller ones ( Anatolian, Arabian, Carribean, Cocos, Juan de Fuca, Philippine, and Somali). They discovered also that the majority of the world’s volcanic and earthquake activity occurred at the points that plates met – the Pacific plate alone accounting for 80 percent of the earth’s earthquake energy.
The work of Harry Hess and others in the 1960s led to the refining of plate tectonics theory to its current state. As it stands, there are still gaps, but it’s the most complete explanation of global dynamics available.
The Theory
Background
The Earth divides into three chemical layers: core, mantle and crust. The core consists primarily of iron and nickel and although it has been cooling for 4.5 billion years, it’s still extremely hot. It is subdivided into a solid inner core and a liquid outer core. The middle layer of the planet, the mantle, comprises minerals rich in iron, magnesium, silicon, and oxygen. The crust (subdivided into Oceanic and Continental) is made up of rock, rich in the oxygen and silicon with smaller amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. Oceanic crust is made of basalt (the most common rock on earth), while Continental crust consists of lower density materials like granite.
Plate tectonics is concerned with the mantle and the crust.
The outermost part of the mantle and the crust form the rigid outer layer of the earth - the lithosphere - which is the ‘plate’ of tectonic theory. Beneath this is the lower mantle - the asthenosphere – which, although solid, flows (solids can flow – look at toothpaste) . It’s suggested that this flow may be caused by mantle convection which pushes the plate in the same way that hot air rises to be deflected by ceilings, alternatively gravity may exert a stronger force on the older, colder ocean floor pulling it towards the core, with more force than newer lighter material.
Whatever causes the flow, there are four types of boundaries where plate tectonic activity occurs: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where land masses collide; and transform boundaries, where plates slide against each other.
Divergent Boundaries
New ocean floor is continuously created along the Great Global Rift as heated magma rises from the Earth’s core, cooling on contact with the sea.
The speed of this floor creation varies along the ridge. Between North America and Europe, the rate is about 3.6 cm per year. At the East Pacific rise, it’s 12.6 inches 32.2 cm annually.
The newly created floor pushes the plates outward as it forms – a process called sea floor spreading.
Convergent Boundaries
Despite the seafloor spreading described above, Earth retains a constant size – one of the reasons is subduction at convergent boundaries
At these boundaries parts of the crust slide beneath other parts. This invariably occurs where an ocean floor meets a land mass at a boundary, since the land mass is more buoyant, but also happens where two ocean floors meet. Deep trenches are formed at these boundaries, as oceanic plates bend downward toward the core.
At a depth between 300 and 700 kilometers, the rock of the subducting plate becomes molten and some of this molten material rises back to the surface as volcanic activity, though most becomes part of the asthenosphere.
Collisional Boundaries
When two land masses meet neither is able to slide under the other. Instead, they crush together, thrusting material upwards along collisional boundaries, forming mountain ranges. As boundaries move over time new ranges arise, and older ones begin slowly to erode – The Himalayas are still slowly getting higher and some ranges in New Zealand are growing by double-figure centimeters every year.
Transform Boundaries
Transform boundaries don’t create or absorb crust. Instead, the two plates rub against each other, generating tension which is released suddenly and often violently in a forward jerk as they slide past each other – this release leads to earthquakes – transform boundaries are often called fault lines.
The San Andreas Fault is the best known – here the Pacific plate to the west of the fault is moving northwest, while the North American Plate on the east is moving southeast.
This movement is very slow, but undeniable – eventually Los Angeles, on the Pacific plate will be north of San Francisco, on the North American. But don’t hang around waiting – it’s going to take 16 million years.