The Pulse of the Earth,
Orogenies & Transgressions


Xenophanes [of Colophon] thinks that a mixture of the earth with the sea is going on, and that in time the earth is dissolved by the moist. He says that he has demonstrations of the following kind:  shells are found inland, and in the mountains, and in the quarries in Syracuse he says that an impression of a fish and of seaweed has been found, while an impression of a bayleaf was found in Paros in the depth of the rock, and in Malta flat shapes of all marine objects. These, he says, were produced when everything was long ago covered with mud, and the impression was dried in the mud. All mankind is destroyed whenever the earth is carried down into the sea and becomes mud; then there is another beginning of coming-to-be [genesis], and this foundation happens for all the worlds.

Hippolytus, Refutation of all Heresies, G.S. Kirk & J.E. Raven, The Presocratic Philosophers, Cambridge, 1957, 1964, p.177

Literary immortality is a point in geological time.

Will Durant



The most dramatic changes in the shape and appearance of the earth over geological time involve the growth and movement of continents (plate tectonics), the creation (orogenies) and destruction of mountain ranges, which are really part of that process, and the advance (transgressions) and retreats (regressions) of the oceans across the continents. The mechanism of mountain building could not be understood until the theory of plate tectonics became established and it could be seen that plates moving together were responsible. The real mechanism for transgressions and regressions is still not understood.

These processes operate at very different time scales. Mountain building is a slow rhythm that leaves evidence in hundreds of square miles of granite and metamorphic rock. The process can thus be dated as far back as a stable crust allowed rock to survive. For North America, we see approximately six great mountain building episodes over the known course of geological time. Only two of these are in Phanerozoic time, that is, since the beginning of the Cambrian Period. During the Paleozoic Era, North America was moving east up against Europe and Africa. This at first gave rise to more than one orogeny, like the Taconian in the Orodivian Period and the Arcadian in the Devonian, in which island arcs not unlike modern Japan were created. In the climax of the process, however, the Appalachian Orogeny, in the Carboniferous and Permian, the seaway (what is now called the "Iapetus" ocean, named after the father of Atlas, which formed at the breakup of the Precambrian supercontinent "Pannotia") was closed and the continents collided, probably raising up Himalayan sized mountains and a Tibetan-like plateau, in the middle of the great "Pangaea" supercontinent. The modern Appalachians are merely the hard, gnarled roots on the old mountains.

In the Mesozoic, North American began to head west, and eventually separated from Africa and Europe. The consequent Cordilleran Orogeny, on the west coast, never did involve hitting another continent, but smaller bits of continental crust, "terranes," were smeared against the side of the older continent, as described in John McPhee's splendid Assembling California [1993, The Noonday Press, New York, 1994]. The deformation of the continent, however, extended all the way to Colorado, involving processes that are still poorly understood. These effects were on such a large scale, that a very large terrane must have been involved. McPhee entertains speculation that this may actually have been Alaska, which hit North America the way India is now hitting Asia. Before completely fusing with the west coast, Alaska got moved off to its present location. The granite batholith of the Sierra Nevada was formed deep under the surface in the Mesozoic, perhaps with volcanoes erupting above, and has since floated up. There are still volcanoes erupting, of course, in the Cascade Range, where an oceanic plate is still being subducted. Meanwhile, California has overridden the East Pacific Rise seafloor spreading center. The effect of basaltic lavas coming up under the continental crust is to spread and thin it out, with pieces dropping down. This produces the areas below sea level, like the Salton Sea and Death Valley, which will eventually add extensions to the Gulf of California. The same effect of faulting and dropping, however, can be seen in the Basin and Range area of Nevada, as described, again, by John McPhee in, of all things, Basin and Range [1980, The Noonday Press, New York, 1990]. When the Gulf of California breaks through into the Imperial Valley, it is liable to be with the kind of drama and disaster described in a science fiction story by Robert Heinlein.

The brown silhouette chart of mountain-building is adapted from Evolution of the Earth, by Robert H. Dott, Jr. and Roger L. Batten [McGraw-Hill Book Company, 1971], p. 160. The silhouette ends before about 3.6 billion years (Gy, gigayears) because crust is mostly not preserved from that early. The oldest known rocks, about 3.9 Gy, mark the beginning the Archaean Aeon. Prior to that it is not clear whether the earth was mostly ocean, was older rocks that have been destroyed, or was "Hadean" in some more literal, molten and volcanic, sense. The absolute age of the earth, at 5 billion years or so, is speculative, and goes with estimates of the age of the Sun and of the Solar System.

Evidence for the transgressions and regressions of the ocean is confined, or only well studied, for the Phanerozoic, where sedimentary rocks of marine origin are plentiful. Things have been at a quicker pace, however, than with orogenies, with six transgressions evident, five of which completely covered the transcontinental arch, i.e. the land from Hudson Bay to the Gulf of Mexico, of North America. The orange on the chart indicates the emergent dry land of the continent during regressions. As on any map, East is at right and West at left. The transgressions have been given local North American names. The last time the center of the continent was submerged, therefore, was in the Cretaceous Period, during the Zuñi transgression. Contrary to the image of dinosaurs floundering in swamps, it can be seen that the Jurassic continent was as high and dry as it has ever been. This is consistent with other evidence, e.g. the fossilized stomach contents of sauropods yield pine needles, not aquatic plants.

The chart at right is the same as the one above, at a larger scale, preserving more detail.

When to expect the next transgression is a good question. The timing has been irregular, though the pace seems to have slowed since the Paleozoic. It is not the amount of liquid water available that is a factor, since there were several transgressions during the Paleozoic, when there were glaciers, and few during the Mesozoic, when there weren't. Even the melting of the present Arctic Ocean and the Antarctic icecap will thus not produce "Waterworld." The inundation of coastal areas is one thing, a waterway across the continent is another; and the mountains have always stood dry. The best guess for the process that causes the transgressions is sufficient tectonic activity to raise the seafloor over enough of an area to displace massive amounts of water. But how this really works is yet to be demonstrated.

The chart of transgressions and regressions is adapted from Essentials of Earth History, An Introduction to Historical Geology, by William Lee Stokes [Prenctice-Hall, Inc., Englewood Cliffs, New Jersey, 1973], p. 127.

The chart at right, which I put together for the review of Singer and Avery's Unstoppable Global Warming, adds important data about climate to the information above about transgressions and regressions. What I have added, in a different shade of blue from the ocean color, is an indication of when the earth has had polar caps. This is shown when blue bars extend at each end of the chart -- the "Icehouse" conditions for the Earth. In Phanerozoic time there have been four "Icehouses." In the Paleozoic Era this involved glaciation, principally in the Southern Hemisphere, when many continents and come together around the South Pole. "Ice rafted debris" and "glacial deposits" are found in the strata for the icehouse periods. During the Mesozoic the icehouse is indicated with a different tint of blue because true polar ice caps have not been documented for this period. There is some "ice rafted debris" but no evidence of glaciation. The reason for this is probably that the poles were free of land and open to moderating ocean currents. Ice caps may have existed, but perhaps only seasonally. The long "Greenhouse" episodes remind us that life flourished on the Earth when it was much warmer than it is now, and free of ice. Finally, we are in an Icehouse ourselves, which has persisted most of the Cenozoic Era, with glaciation since the Pleistocene and icecaps and many glaciers persisting (where we find a continent at the South Pole and the North Pole tightly surrounded by continents, limiting circulation). The data for Icehouses is from "Celestial Driver of Phanerozoic Climate?" GSA Today [July 2003, Vol.13, No.7], by Nir J. Shaviv and JŠn Veizer.

I have added a fifth icehouse before the Cambrian. It is now a popular theory about why life only became abundant and varied in the "Cambrian Revolution" that before then we had a "Snowball Earth," with all the oceans frozen and life limited by the darkness of unfrozen, subsurface water. Given the rhythm of the icehouses, it is reasonable that there should be a Precambrian episode anyway. There is actually direct evidence of glaciation from the "Snowball" period, of the same sort as all other evidence of glaciation, i.e. glacial deposits, the striations that are scratches left by glaciers dragging debris against the bedrock, etc. Not only is there such evidence for the immediate Precambrian period, but there is also evidence of glaciation from early in the Proterozoic Aeon and even during the Archaean Aeon. I have added indications of this on the diagrams, with data from Frozen Earth, The Once and Future Story of Ice Ages, by Doug Macdougall [University of California Press, 2004, p.143].


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