Wednesday, 11 April 2012

Lecture - Origin of the Earths Crust

Cafe Scientifique had another in their series of sciency talks recently, this one being presented by Prof Hugh Rollison from the University of Derby.

The talk was focused on the origin of the Earths crust, a topic which Hugh said had “occupied my mind since I was a doctoral student some 30 years ago”.

He began by explaining that the Earth is different from Mars and Venus in that it has a continental crust (together with ocean basins) and that these two basic types of crust had very different characteristics

Comparison of Ocean and Continental Crust Properties
Ocean CrustContinental Crust
Density ~2.9g/ccDensity ~2.7g/cc
Lower altitudeHigher altitude
7kn thick40km thick
60-100Million Yr old~2200 Million Yr old

The lower density of continental crust means that, when oceanic and continental crust are pushed together, it is generally the oceanic crust that is submerged below the continental.

The amount of continental crust on the earth has not been constant, but has instead increased over time, with particular spurts of activity 2.7, 1.9 and 1.2 billion years ago and it is noticeable that the oldest continental crust has a different composition to that of more modern crustal rock

There are two main models for the mechanism of how the continental crust formed, with both camps being able to muster evidence for their point of view.

Following the talk, BFTF did a little digging on the Interweb and was surprised by how recent it was that geologists understand the main processes that have shaped to location and nature of the continents.

Today, it is common knowledge that the earth’s crust is composed of a number of plates that are slowly moving around the earths surface, with new crust being formed at mid-ocean ridges, and old crust being lost at areas of subduction (where one plate is forced under another) - but this only became widely accepted in the 1960s and is known as the theory of "plate tectonics".

Everything you need to know about Plate Tectonics (see here for big version)

Plate Boundaries around the World (see here for big version)

Prior to Plate Tectonics, however, a number of theories were prominent at various times, their popularity waxing and waning as science developed and more data became available. Some of these are listed below. . .

Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past. Uniformitarianism was formulated by Scottish naturalists in the late 18th century, starting with the work of the geologist James Hutton. Hutton then sought evidence to support his idea that there must have been repeated cycles, each involving deposition on the seabed, uplift with tilting and erosion, and then moving undersea again for further layers to be deposited.

Perhaps most surprisingly, there was support for a theory that the mountain ranges around the world had been formed due to the earths surface crumpling as the earth shrank due (usually) to cooling. Indeed, support for the theory was strong even in the early 1900s when Prof Robert T. Hill suggested that evidence that there "was no doubt that earthquakes are diminishing." By suggesting that the displacement of the 1906 San Francisco earthquake only a few feet, while prehistoric earthquakes had made fissures and slides of 20,000 feet.

In contrast, the “Expanding Earth” hypothesis, popular until the 1970s, asserted that the volume of the earth was increasing and that this was what responsible, at least partly, for the positions and movements of the continents.

The Vine–Matthews–Morley hypothesis, developed in the early 1960s, was the first key scientific test of the seafloor spreading theory of continental drift and Plate tectonics. Geophysicist Frederick John Vine and the Canadian geologist Lawrence W. Morley independently realized that if the seafloor spreading theory was correct, then the rocks surrounding the mid-oceanic ridges should show symmetric patterns of magnetization reversals, a record of the Earth's geomagnetic reversals, captured in the cooling volcanic rocks. Later geomagnetic surveys found the patterns are in fact present, providing strong confirmation of the theory.

Age of Sea Floor Rocks, showing young rock close to mid ocean ridges
(see here for big version)

Image Sources Tectonic Boundaries, Plates, Sea Floor