Plate Tectonics - The Australian Museum

Plate Tectonic processes

Since the 1950s, several discoveries have led to a new understanding of how the Earth works. This includes Plate Tectonics, which explains the structure of the Earth's lithosphere (outer shell) and the forces that drive changes in its structure.

Tectonics comes from the Greek word tekton, meaning builder. The first scientist to propose that continents drift (a key to later Plate Tectonics) was the German meteorologist, astronomer and geophysicist, Alfred Wegener in 1912. Other important geologists who helped develop present theories were the two South Africans Alex du Toit and Lester King, and the Australian Samuel Carey. The theory of sea-floor spreading was outlined by Harry Hess of Princeton University, USA, and confirmed by F.J. Vine and D.H. Mathews from the UK.

Evidence for Plate Tectonics

When geologists in the late 1800s began to explore different areas of the Earth, they learnt that many stratigraphic successions had similar ock types, ages, fossils and depositional settings. After the 1950s, geologists realised that this also applied to the oldest parts on Earth that formed the stable cratons and metamorphic rock belts. Evidence of rocks forming in the same place and time and then being widely split apart, led to the ideas of cycles of super continents and new oceans. Evidence for ocean-spreading episodes includes symmetrical magnetic anomalies parallel to mid-ocean ridges, zones of large earthquakes, active volcanoes at some ocean margins, and the distribution of animals in the world (e.g. the restriction of certain mammal groups to Australia).

Based on mechanical properties, especially resistance to shearing force, the Earth can be subdivided into three main outer layers:

• Lithosphere: is the colder rigid, more resistant outer shell of the Earth. If it wasn't rigid, then mountains would simply level themselves out. It is made of both the crust and the uppermost part of the upper mantle. Under oceanic crust it extends down to 70 km, while under continental crust it extends down to 150 km. The word lithos is Greek for rock.
• Asthenosphere: this starts with a sharp decrease in shear strength known as the Low Velocity Zone (LVZ). The lower strength in Earth's mechanical properties means that this layer can flow more under stress. Within the asthenosphere, the Low Velocity Zone extends down to 250 km, below which shear strength increases progressively as a result of compression, until at 400 km depth there is a sudden rapid increase. The base of the asthenosphere is defined by the deepest known earthquakes (approximately 700 km) and is where descending lithospheric plates most likely bottom. The word astheno comes from the Greek combination of a - (means without) and stheno (which means strength).
• Mesosphere: Below the asthenosphere is the mesosphere which extends down to the outer core (at a depth of 2900 km). The mesosphere is composed of strong dense material and shear wave velocities increase rapidly with depth.

What are plates?

The lithosphere consists of rigid plates on a non-rigid subsurface (from approximately 70 km - 700 km in depth). Tectonic processes mostly take place at the plate edges. A plate moves as a single entity along the surface of the Earth over a plastic mantle. There are two types of plate:

• Oceanic plates form at the mid-ocean ridges. They thicken as they move away (at about 1 km for every million years) and have a basaltic composition.
• Continental plates are much more complex as their rocks vary in composition and thickness.

The differences between oceanic and continental plates go down to a few hundred kilometres.

The plates move under horizontal forces that cause them to collide, combine, break up, or, in the case of oceanic plates, to be drawn down (subducted). The forces that act on or near mid-oceanic ridges are called ridge-push force. It is driven by upwelling magmas from mantle regions that drive the plates away on either side of the ridge. Another driving force (particularly where oceanic plates are being subducted beneath continental plates) is slab-pull. This is produced by oceanic plates being significantly cooler and hence denser than the underlying mantle. They get pulled below the adjoining continental plate, dragging the remaining ocean plate after it. At the base of the plate, drag occurs as it moves over the underlying mantle.

Continental land masses and major ocean floors contrast strongly. For example, Mount Everest, the highest bit of continent, is 8 848 m, yet the Marianas Trench with a depth of 10 912 m, is vertically greater.