Plate Tectonics II

Harry Hess from USGS
By the 1950s, paleomagnetic evidence made it difficult to deny that Albert Wegener had been right about moving continents. But how? Answering that question was the transformational work of a professional group collaborating over a decade. The result was a theoretical basis for understanding how this movement was possible: the theory of plate tectonics.

Harry Hess : Hess was an igneous rock geologist (a "hard-rock" man) who had participated in sea floor geologic surveys in the 1950s and early 60s. At the beginning of the sixties, although not solely responsible, he was preeminent among a group of researchers who finally put the pieces together:

Plate Tectonics:

The result is the Theory of Plate Tectonics, which has become the unifying theory of modern Geology. Between 1960 and 1970, the academic community was won over to it. Here is its essence:

But note: Plate tectonics was only a partial vindication of Wegener's continental drift. It was different in that the continents are merely passengers riding on mobile plates, and do not drift across the earth's crust by themselves.

Three types of plate boundaries:

Additional information!

Plate tectonics' explanatory power:

Hot spots:

Emissaries from below the realm of plate tectonics.

Hawaii from The New York Times
There are places where volcanoes erupt not in a plate-boundary subduction-zone volcanic arc, but at a single point. Examples are

These are called hot spots. Why are they there?

The asthenosphere extends no deeper than 400 km. Except for subducting lithospheric plates, the deeper mantle is not directly involved in the processes of plate tectonics. Instead, the deep mantle is the source of mantle plumes - columns of hot rising rock that are the source of the magma for hot spots.

Hawaii hot-spot schematic from
In contrast to the volcanic arcs (lines of active volcanoes) that occur at subduction zones, active hot-spot volcanoes tend to be single or tightly clustered, Nevertheless, they often sit at the end of long strings of extinct volcanoes. Hawaii contains the youngest volcanoes in the Emperor Seamount chain. These chains form as a plate moves over the mantle plume, continually carrying away the older volcanoes and allowing new ones to form. Similar to the effect of allowing an old vinyls LP to rotate over a bunsen burner flame.

Volcanoes of Mars' Tharsis region from Wikipedia
Beginnings and Endings: Plate tectonics are not forever. When that happens, the only volcanoes will be hot-spot volcanoes like those of Mars (right) and Venus.

Key concepts and vocabulary:

Additional reading: