Geology's second great paradigm shift
(the first was the discovery of Geologic time)
Today, we consider this question: If the Earth is as old as it seems to be, why are there any mountains left?
Volcanoes like Cotopaxi (right) makes some sense, as they are still forming in the present day.
But what about others like Mt. McKinley (right). Why hasn't erosion worn them flat?
The resolution of this simple sounding question has a complex and momentous history.
In Darwin and Lyell's time, the only rocks to which people had direct access were the crustal rocks of the continents. People expected that the topography and composition of the ocean's floor should resemble that on land. The first practical test of that hypothesis occurred in 1872 when the British government sponsored the first interdisciplinary research expedition to expore the world's oceans - the four-year voyage of the H. M. S. Challenger
. The deep oceans defied expectations:
Clearly the geology of the oceans was unlike that of the continents. WTF?
Continental Drift - the beginnings of an answer:
The German meteorologist and geophysicist Alfred Wegener (1880-1930) performed field work in Greenland, covered by a continental ice sheet. There, he had ample opportunity to observe the behavior of glaciers. He observed that ice, when greatly compressed, flowed plasticly, allowing the ice sheet to glide slowly across the underlying rock, and apparently began wondering if rock did the same thing on a larger scale. He noticed the following patterns:
- Shorelines of continents separated by oceans tend to match up geometrically.
- Continents separated by oceans tended to have matching types of metamorphic and sedimentary rocks.
- Mountain belts are linear and tend to occur on one side of continents.
- Continents separated by oceans tended to have matching types of fossil plants and animals, many of which (E.G. Lystrosaurus could probably not have crossed oceans.
- Continents separated by oceans tended to have matching records of continental glaciers.
"Continental Drift:" To explain this, Wegener proposed the hypothesis of "continental drift:" i.e. that the location of continents was not fixed, and that they had "drifted" across the globe. Specifically, Wegener thought that the continental crust slid over the oceanic crust like glacial ice sliding over bedrock.|
- The matches in shorelines, geology, and paleontology between continents occurred because these continents had once been joined in an hourglass-haped supercontinent called Pangaea (also spelled "Pangea"). Wegener particularly noted the similarities in India, Africa, and South America, and correctly predicted the discovery of similar rocks in Antarctica.
- Linear mountain ranges were formed by the "bow shock" of a continent plowing across the earth.
Problem: While Wegener was a genius at making observations and recognizing patterns, he was not able to provide a theory to explain and predict the movements of continents, i.e. to say how it happened. Of course, no one who favored stable continents could begin to explain either why the patterns Wegener saw existed or where mountains came from, but that didn't really matter. The Geological profession didn't like amateurs claiming to solve puzzles that had defied them for 40 years. In 1926, Wegner proposed as motivating forces:
- Tides in the earth
- The Earth's rotation.
In 1928, Harold Jeffries published an analysis demonstrating that neither of these forces could begin to propel continents around. Continental drift was relegated to the idiot fringe. No US textbook mentioned it until 1960. In 1930 Wegener died in a freak storm while doing field work in Greenland.
New evidence and reconsideration
Geologists didn't entirely forget Wegener's evidence for moving continents. In the southern hemisphere (where much of the best evidence originated) his ideas never really went out of style. In 1931, the prescient Arthur Holmes (of radiometric dating fame) proposed seafloor spreading that slow convection currents in the mantle carried the overlying crust with them, and caused the formation of new seafloor. Continental crust would be carried passively along on top of the oceanic crust. Elegant speculation that lacked only evidence.
Arthur Holmes' conceptual sketch of seafloor spreading.
Paleomagnetism After WWII, several lines of evidence from the study of the intrinsic magnetic fields of igneous rocks came together to support Holmes' notion.
- Paleomagnetism on land: Igneous rocks, when they solidify, preserve a record of their magnetic environment at that time. Since the magnetic poles wander, geologists thought that a neat way to track the ancient movement (right) of the poles would be to read the record preserved in igneous rocks' intrinsic magnetic fields. Should have been easy except for a huge problem:
Diffrent continents tell contradictory histories. If you assumed that the continents were stationary, then the rocks of different continents told radically different stories. A creeping suspicion developed that Wegener's hypothesis described these observations as well as anyone's. As of 1960, academic opinion was split 50-50.
- Paleomagnetism at sea: Recall that geomagnetic reversals occur at irregular intervals, causing North and South magnetic poles literally switched places. During WWII, marine magnetometers had been invented to detect submarines. In the late 1950s, geologists sought magnetic information from marine rocks, by towing submarine magnetometers behind research vessels. What they found was spectacular.
- Bands of rocks whose intrinsic magnetic fields had normal magnetic polarity alternated with parallel bands showing reversed polarity. These bands paralleled the mid-ocean ridges. Thus, bands of rock with the same polarity must be of roughly of equal age.
- Furthermore, these bands formed pairs of mirror image counterparts on the opposite side of the ridge. It appeared that sea floor was forming and at the ridge crests and moving apart, as if they were being spooled out.
- To further clinch the argument, by the 1980s, sensitive measurements had been made, actually measuring the rate of sea floor spreading. This is roughly 3 cm/year for the Atlantic. If you project this backwards you get the ocean opening at the time of its oldest sediments.
Harry Hess puts the pieces together: 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, he finally put the pieces together: Wegener had been right to say that the continents moved, but for the wrong reasons. Holmes had understood the true reason, and the paleomagnetic information from the sea floor confirmed his vision. Hess's view is the foundation of 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:
Layers of the upper Earth: Remember the crust and mantle - the compositional zones of the rocky layers of the Earth? Now we break the Earth down into a different set of zones based on physical properties instead of composition.
- Lithosphere: The zone in which rocks are rigid and deform brittlely. Includes the crust and upper mantle. The lithosphere is broken into distinct rigid plates.
- Asthenosphere: A region of the upper mantle beneath the lithosphere in which rock is partially molten and deforms very ductilely. Plate movement occurs when lithospheric plates glide over the ductile asthenosphere.
- Below the asthenosphere, the mantle rock deforms ductilely but contains less liquid and is therefore more stiff than in the asthenosphere.
- New oceanic crust is formed by frequent volcanic eruptions along the length of mid-ocean ridges and is pushed outward from them gradually.
- Old oceanic crust is destroyed when it subducts or dives beneath adjacent plates at subduction zones. Oceanic trenches are the topographic expression of these subduction zones.
- Oceanic crust behaves differently from continental crust, being denser. Whereas sea floor gets subducted, continental rock is light enough not to, although it can be profoundly deformed, when two continents (i.e. blobs of continental crust) collide above a subduction zone.
- Mid-ocean ridges and subduction zones largely divide the oceanic crust and uppermost mantle into rigid plates that glide across a deeper layer of ductile and partially melted rock and move relative to one another.
- There are several major plates and numerous minor ones. You should know the names and locations of the plates in figure 2:29 of your textbook (p. 54):
Thickness: The thickness of the lithosphere varies depending on:
- whether it carries oceanic or continental crust: Continental lithosphere is, on average, 150 km thick, although it can be thicker beneath mountain ranges.
- how far from a mid-oceanic ridge it is. Old oceanic lithosphere is up to 100 km thick, while at the axis of a mid-oceanic ridge, it may be only ten km thick.
: 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.
Key concepts and vocabulary:
- H.M.S. Challenger
- Ocean basin topography
- Young volcanic bedrock
- Thin sediment
- Mid-oceanic ridges
- Ridge axis
- Oceanic trenches
- Oceanic crust (rich in magnesium (Mg) and iron (Fe))
- Continental crust (rich in aluminum (Al) and silicon (Si))
Pangaea (also spelled "Pangea")
Arthur Holmes (again)
Remnant magnetism (aka paleomagnetism)
Geographic geomagnetic reversal pattern in sea floor
Subduction - subduction zones (= trenches and their parallel volcanic arc)
Seafloor spreading centers (=mid-oceanic ridges)
- Shoreline profile matches
- Alignment of matching rock types across oceans
- Linear mountain belts
- Matching fossil distributions
- Continental glacier record