Megacraters on Earth

Long ago when Pangea alone ruled the earth, a cosmic body of some sort was attracted by gravity toward the sun.  Before plunging into its surface or whizzing off into an open or closed orbit, it felt the urging of another gravitational field.  Earth beckoned it away from its original path into a collision course. Its explosion at high altitude near the present Timbuktu in North Africa created a giant, multi-ring structure 1930 mi in diameter.  The outlying, circular fault from that event carved the great bulge seen today protruding into the Atlantic Ocean as the western Sahara Desert.  Radiating from ground zero was a linear, rift system that fractured Pangea along the straight line of Africa’s west coast..

Three, previous explosions participated in this drama. As the rift lengthened, it encountered the edge of a crater whose evidence at this point is the concave, coastline of Angola. Rather than rigidly adhering to a straight course, it proceeded along a curved path following a trail that had already been weakened by a crater rim that remains.  No portion of this crater remained on the continent. But the author has surveyed the huge, nearly vertical escarpment behind the coastal plain of Angola from a small airplane.

Next, an ancient crater named Angola was circumvented. It had been associated with a massive, lava flow known as Parana-Etendeka Trap.  Most of the latter was sheared off with  the Parana portion moving westward and only a small fraction, Etendeka, remaining in Africa.

Finally, a crater 1180 mi in diameter, called Kalahari, had carved the southeastern corner of Africa that extended westward beyond the rift as shown in Fig. I.1. Beginning The Breakup.  That portion was simply sliced away to join other remnants that eventually drifted about 4300 mi to the present location of South America.

Compounding damage to the crust from three rift systems (Timbuktu, Angola, and Kalahari) probably triggered the eruption of Parana-Etendeka.  Starting 133 My and lasting about 1 million years, the lava spread out over 1.25 million sq mi (Renne 1992).  Opening of the Atlantic Ocean was delayed until nature could provide for disposal of surplus crust.The traditional cause of Pangea’s fragmentation has been continental movement over the Tristan de Cunha hotspot.

  A legitimate question may be raised about the reason, if any, that the rift system from Timbuktu elected to choose its particular course instead of any number of other possibilities. Such systems are, indeed, common features of the largest craters but their directions vary widely. A value of the angle can be established by noting the angle from true north as an azimuth to the right or a negative value to the left. In this case, the angle denoted a is – 20.6º.     Some noteworthy points arise from this examination of a major impact.  In a single stroke it sculpted the west coast of Africa with the round bulge in the north and the linear coast in the south.  The time of the event can be established from radioactive decay of frozen lava and core samples from the ocean floor. Interactions with earlier events is marked by snaking around some older craters or slicing through others. Earth’s crust is most severely damaged when effects of more than one explosion are compounded. Finally, some intelligence may be deciphered from the geometry of linear rifts. This question is probed thoroughly in a later chapter.

Almost everyone has marveled at the beautiful rainbow of the Aleutian Islands illustrated at the top of Fig. 1.19. Bering Twins.  Some authors have suspected that they were left as a scar from an asteroid or comet impact of unprecedented proportions.  The majority of geologists, on the other hand, have rejected that idea. They staunchly defend the notion that the arc was created by tectonic forces related to subduction of the Pacific Plate down the Aleutian Trench.  But that theory carries little weight, is vacuous, and never has been satisfactorily explained. It will be shown here that the islands are, indeed, the consequence of cosmic disruption.  In fact, there were two, closely associated events hence the figure’s title.

Astute observers have pointed out a discontinuity in the curvature of the arc near the island of Unalaska. Actually, the axis of Unalaska is perpendicular to the radius from the site of Aleutian E whereas its neighbor to the west, Umnak, is oriented as a rim of Aleutian W.  These two distinct impacts have respective diameters of 1140 mi and 1340 mi.

Epicenters are notably close together.  From precisely determined coordinates it is a simple matter to calculate their difference in latitude of only 109 mi.  Such a small value would be expected from fragments of a mother comet approaching earth along the same trajectory. The difference in longitude of 9.69 corresponds to 671 mi.  If the two objects traveled together, that spread implies a time delay of 38 min 46 sec.  A real, double whammy!

Noteworthy details on the Russian end are illustrated at the lower left in the figure.  Two islands continue the American chain, Probrazhenskoye and Nikol’skoye whose shapes and orientation are appropriate.  The terminus in Russia is an oddly configured peninsula. An outer rim in this region has avoided subduction for special reasons and preserved evidence as an underwater ridge.  Highest peaks are known as Meiji Guyot and Detroit Seamount.