Reconstructing the positions of the continents back in the past is straightforward for recent times, because one simply has to take today’s plate motions and run them in reverse. Eventually, how- ever, one reaches a point at which the current continents have amalgamated into two continents, and then even earlier, into one supercontinent. At that point, roughly 200 million years ago, plate motions cease, and the earlier history of plate tec- tonics must be deduced from similarities between ancient rock formations on different continents, as well as the magnetic ori- entations of rocks.
Figure 9.10 shows the history of spreading from 200 mil- lion years ago (Jurassic times) to the present.Pangaea, a name derived from the Greek pan (all) and gaia (earth), was a single Jurassic supercontinent comprising all current landmasses. It was surrounded by a global ocean called Panthalassa “all sea".
It began to break up with North America and Eurasia forming Laurasia, from the Greek laura, meaning passage or channel, and the remaining continents comprisingGondwana, named by
40 Ma Middle Eocene
60 Ma Late Paleocene
125 Ma Early Cretaceous
200 Ma Early Jurassic (a)
(b)
(c)
(d)
Figure 9.10A depiction of the movement of the continents back to (a) 40 million years ago; (b) 60 million years ago; (c) 125 million years ago;
and (d) 200 million years ago.
725 Ma Late Proterozoic
550 Ma Early Cambrian (a)
(b)
Figure 9.11Continental drift from (a) 725 million to (b) 550 million years ago, showing the supercontinent previous to Pangea shown in Figure 9.10.
a contemporary of Wegener, for unknown reasons, for an ancient tribe in India. TheTethysseaway between the two is the ancestral Mediterranean, meaning mid-land, Sea. By 135 million years ago, Gondwana had begun to break up into Africa/South Amer- ica, Antarctica/Australia, and India. At the close of the Creta- ceous, some 65 million years ago, Africa had reconnected with Eurasia, but the major new event was the opening of the Atlantic Ocean, the growth of which continues at present. Extrapolat- ing 50 million years into the future, the Atlantic will widen, a new sea will open in East Africa, Australia will cross the equator, the present southern California coast will shift north of San Francisco and the Gulf of California will merge into the Pacific.
While this scenario generally has been considered sound for about three decades, less certain is the history of the conti- nents prior to 200 million years ago. Detailed detective work on rocks and their magnetic orientations provide some clues.
Rocks from prior to the Mesozoic era in North America and Africa have different magnetic directions, such that they could not be pieced back together into a single Pangaea prior to about 260 million years ago. Evidence showing that ancient ocean basins actually closed to form Pangaea at this time comes from volcanic rocks of ocean crust composition appearing in moun- tain ranges of Argentina, Africa, and parts of the Appalachi- ans, these rocks being forced up onto the continents by the closure.
Reconstructing the continental configurations prior to Pan- gaea raises an important point: because of the continuous process of subduction of ocean crust, there are few seafloor rocks older than roughly 250 million years before present. On the continents, which do not subduct, rocks dating back billions of years are not uncommon. Here and there on the continents, very ancient oceanic crust can be found, where it was pushed upward and sutured onto continents during collisions. These preserved remains of ocean floor, orophiolite suites, stand in stark contrast to the vast bulk of the oceanic crust that is part of a youthful and dynamic conveyor belt of material moving from ridge to trench in a matter of some hundred million years or so.
Expeditions to Antarctica have found sandstones in the moun- tains of that continent from about 700 million years ago that contain fossil remains of worms identical to those in rocks of similar age in Wisconsin and other locations around the world.
This and other resemblances in widely separated rocks of the era push one toward the existence of another supercontinent, named Rodinia, in existence from perhaps as long as 2.2 bil- lion years ago. The break up of that continent about 700 to 800 million years ago and eventual reassembly into Pangaea some half-billion years later illustrates the cyclic nature of the for- mation and break up of supercontinents. (Most reconstructions imply an intermediate supercontinent, Pannotia, came together very briefly about 500 million years ago.) Figure 9.11 illustrates
the possible sequence of the break up of Rodinia and formation of Pangaea, based on computer simulations that account for the rock relationships seen on the continents of today.
Prior to Rodinia the record of continental motions is extremely limited. The fossil record is very poor before 750 million years ago, in part because the variety and abundance of preservable organisms was impoverished prior to that time. The explosion of new and complex biological types is a remarkable water- shed about 600 million years before present that we consider in Chapter 18. The rock record itself is spottier, both fossils and magnetic signatures being more poorly preserved because of subsequent modification of the rocks. Furthermore, the decreas- ing abundance of rocks at earlier and earlier ages is not simply a matter of preservation: the continents themselves have grown over time, and prior to several billion years ago, much of the landmass we see today did not exist (Chapter 16). Thus the his- tory of plate tectonics prior to a billion years before present, including assembly and disruption of supercontinents, is not well understood.
What is now understood from the efforts to track the motions of continents over hundreds of millions of years is that creation and destruction of supercontinents must have had important effects on the climate and biota of Earth. This is not simply due to the changing latitudes of individual localities but more profoundly to the global effects on ocean currents and landmass ice sheets of having continents and oceans in different configu- rations. We discuss these possible effects in Chapter 19.