ὁ δὲ Ξενοφάνης [ὁ Κολοφώνιος] μίξιν τῆς γῆς πρὸς τὴν θάλασσαν γίνεσθαι δοκεῖ καὶ τῷ χρόνῳ ὑπὸ τοῦ ὑγροῦ λύεσθαι, φάσκων τοιαύτας ἔχειν ἀποδείξεις, ὅτι ἐν μέσῃ γῇ καὶ ὄρεσιν εὑρίσκονται κόγχαι, καὶ ἐν Συρακούσαις δὲ ἐν ταῖς λατομίαις λέγει εὑρίσθαι τύπον ἰχθυος καὶ φυκῶν, ἐν δὲ Πάρῳ τύπον δάφνης ἐν τῷ βάθει τοῦ λίθου, ἐν δὲ Μελίτῃ πλάκας συμπάντων τῶν θαλασσίων. ταῦτα δέ φησι γενέσθαι ὅτε πάντα ἐπηλώθησαν πάλαι, τὸν δὲ τύπον ἐν τῷ πηλῷ ξηρανθῆναι. ἀναιρεῖσθαι δὲ τοὺς ἀνθρώπους πάντας ὅταν ἡ γῆ κατενεχθεῖσα εἰς τὴν θάλασσαν πηλὸς γένηται, εἶτα παλιν ἄρχεσθαι τῆς γενέσεως, καὶ ταύτην πᾶσι τοῖς κόσμοις γίνεσθαι καταβόλην.
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 [θάλασσα, thalassa] and becomes mud; then there is another beginning of coming-to-be [γένεσις, genesis], and this foundation happens for all the worlds [κόσμοι, kósmoi; singular, κόσμος, kósmos].
Hippolytus, Refutation of all Heresies, G.S. Kirk & J.E. Raven, The Presocratic Philosophers, Cambridge, 1957, 1964, p.177; until the 19th century it was argued that shells grew naturally in the rock; but this could not account for fossil fish, seaweed, and leaves. Xenophanes was the first to collect and record this evidence.
Literary immortality is a point in geological time.
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]. Nevada has grown wider as the crust spreads beneath it. 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, who wrote it before anyone knew about plate tectonics. Such breakthroughs of the sea have occurred elsewhere, apparently in both the Black Sea and the entire Mediterranean basin. The Red Sea will eventually extend down into Ethiopia and along the East African rift zone.
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, where 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 chart at right is the same as the one above, at a larger scale, preserving more detail.
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. The more recent Tejas (i.e. Texas) transgression did not completely cover the continental arch.
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. We now see that the Brontosaurus could rear up, all the better to feed off high conifers. It was less clear what good a long neck would do in a swamp. In fact, there are large modern mammals that live in water and swamps, Hippopotami, but they have short legs, not long legs as Elephants and Sauropods do. Also, the modern mammals with long neck, i.e. Giraffes, manifestly use them to eat off high trees. In the Jurassic, grasses and flowering plants did not yet exist for low feeding. Giraffes also offer an example of how a circulatory system can deal with the problems of blood pressure in long necks and high heads. Some paleontologists have trouble believing that sauropods could have had such adaptations -- perhaps just as old geologists had trouble believing that the continents could move.
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," usually invoving, not just ice caps, but extensive glaciation.
In the Paleozoic Era this involved glaciation principally in the Southern Hemisphere, when many continents and come together around the South Pole. These two episodes now seem to get called the "Andean-Saharan" and the "Karoo" Glaciations. "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 or glaciers 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. Nevertheless, the Zuñi Transgression, named after the Zuñi Pueblo in New Mexico, had properly submerged the mid-continent of North America.
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 South Pole, in the very middle of Antarctica, is the coldest place on Earth; yet science teams live there year round.
The data for Icehouses is from "Celestial Driver of Phanerozoic Climate?," by Nir J. Shaviv and JŠn Veizer, GSA Today [July 2003, Vol.13, No.7]. See discussion of our Icehouse at the review of the NOVA episode, "Polar Extremes."
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. Periods of glaciation in the immediate Precambrian have now been given names, "Baykonur, Gaskiers, Marino," and "Sturt," as I have noted on the long charts above. The "Sturt" is the oldest, from 715 My to 680 My.
The evidence for Snowball Earth is now solid enough that a new geological Aeon has been proposed for it, the "Cryogenian," carved out of the Proterozoic between 635 million years ago and 850. The Cambrian Period began between 541 and 590 million years ago, so perhaps we have a little bit of hiatus between the Cyrogenian and the Phanerozoic Aeon.
The last glaciacian before the Cambrian, now called the "Baykonur," is dated between 549 and 530 million years ago. If the Cambrian Period began around 541 My ago, then the Gaskiers (580 My) and the Baykonur (perhaps 547 My) Glaciations are barely older. What is regarded as the last geological Period before the Cambrian is the "Ediacaran," named after a fossil formation in Australia. Where this is still regarded as part of the Proterozoic, the "Cryogenian" may be reduced from Aeon to Period status.
Not only is there such glaciation 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, as the "Huron" and "Pongola" Glaciations, with data from Frozen Earth, The Once and Future Story of Ice Ages, by Doug Macdougall [University of California Press, 2004, p.143]. Macdougall shows phases in the Huron Glaciation that I do not now see in recent terminology. It is extraordinary to imagine glaciation even when the Earth must have been warmer and the area of continental surface smaller that at present.
The chart above I have recently pulled off Wikipedia, adding some notes about the glaciations. It shows actual temperatures, where the previous charts on this page do not. I have also altered the division between "Paleogene" and "Neogene," which I do not like -- they replace the traditional "Tertiary" and "Quaternary" -- in favor of "Cz" for "Cenozoic." The change in terminology obscures the meaning of the "K-T" boundary, i.e. where the Cretaceous meets the Tertiary, and the dinosaurs died, as the attempt is even being made to discontinue the use of "K-T," to the needless confusion of all. In any case, recent glaciations now seem to get called the "Late Cenozoic Ice Age." Also, the Cambrian is shown with its proper symbol, "Ꞓ," the "barred capital C." The symbol for the Triassic ought to be a ligature of "TR," but I have not bothered.
Philosophy of Science