Massive Meteorites in Geological History

The impact of asteroids or comets striking the earth may have cracked the planet's crust, causing cataclysmic lava flows, environmental upheaval, mass extinctions, and plate boundaries.

A look at pictures of the moon reveals a surface pocked with thousands of impact craters that are just open holes, while a few of the larger craters have walls that rise steeply above flat floors covered with dark lava flows. In the midst of this saturated jumble of craters are smooth regions, the maria, which are actually the sites of the largest craters. These craters have been completely filled with lave flows, which in places spill beyond their edges into nearby low areas. Those are the dark patches that can be seen without the help of a telescope. Some of the Apollo missions landed on the maria floors, and their astronauts collected samples of the lava, called basalt.(Basalt is a type of lava that crystallizes to from coal black rock).

Craters are the dominant feature on the surface of the moon, because cratering is virtually the only process that operates there. In contrast, craters are only minor feature in landscapes of the earth because many complexly interactive geologic processes operate on the surface of the earth. Even so, geologists have now recognized dozens of impact craters on the earth, and the list grows.

The Manicouagan Crater of Quebec (diameter about 40 miles) and the Popigai Crater of Siberia (diameter about 60 miles) are the largest now generally recognized. Both have volcanic rocks in their floors.

A comparison of known impact craters on the earth and on the moon reveals a surprising anomaly. While the moon contains several craters with a diameter of more than about 120 miles, the earth contains none with a diameter of more than about 60 miles. Furthermore, while the moon contains several extensive maria, no maria have previously been identified on the earth.

If unrecognized maria exist on the earth, they may mask sites where massive meteorites have struck the earth.

Asteroids and Comets

Astronomers have so far found more than 80 asteroids with orbits that cross the earth's orbit, and the rate of discovery suggests that 10 times that many may finally be found. In addition, an unknown number of comets, probably hundreds, cross the earth's orbit. Anything in an orbit that crosses the earth's can collide with the earth.

The chance that an asteroid or comet will hit the earth this year is probably about the same as the chance that the particular person will win the grand jackpot in the state lottery on a single ticket-not very likely. However, if that same person were to buy 10 million tickets, his chances of winning the lottery would become quite good. So too with the earth and the orbiting asteroids and comets. They have plenty of time to wait for their rendezvous in space, as many million years as they need. Given enough time, such collisions are inevitable.

Exploding Rocks

The crash, when it comes, is catastrophic. Asteroids that strike the earth are just large meteorites- rocks or chunks or metal the size of a small mountain. Resting motionless on a museum shelf, meteorites are perfectly harmless, but when they are hurtling through space at speeds measured in tens of miles per second they carry enormous energy of motion, more energy per pound than a chemical high explosive. As orbiting asteroid has energy of motion equivalent to millions of tons of high explosive. Comets are probably balls of ice with rocks embedded in them, but most of them move even faster than asteroids and are probably just as energetic. Over the long term, the Solar System is an extremely dangerous place for a planet, and our earth's atmosphere is completely inadequate as a protective shield against an oncoming asteroid or comet.

When an asteroid or comet hits the earth, its energy of motion is instantly converted into enough heat to transform it into white-hot vapor, dust, and small fragments. The sudden transformation of an asteroid from speeding rock to an enormous fireball is an explosion that makes a hydrogen bomb look like one of the lesser varieties of firecracker. Some asteroids and comets carry enough energy of motion to open craters much bigger than the largest now generally recognized. Surely over more than four billion years some very large asteroids must have collided with the earth. If this is the case, why have these impact sites remained undiscovered? Clues to answering this question have been found through efforts to find the cause of the sudden demise of the dinosaurs.

Dinosaurs and Boundary Clay

After having dominated life on the continents for some 140 millions years, the dinosaurs abruptly vanished 65 million years ago. Besides dinosaurs, all sorts of animals also became extinct at that time. The list of victims includes animals that lived in the oceans and flew through the air, as well as many of the microscopic animals that drifted in the surface waters of the oceans. Some paleontologists estimate that the great extinction eliminated about 65 percent of all the animal species then living.

Marking this fatal event in the geological record is the boundary clay, a nondescript layer or rock an inch or two thick. Below the boundary clay are layered rocks containing the fossil remains of the dinosaurs and the rich assortment of other animals that populated the earth with them. Rocks immediately above the boundary clay contain a pathetically impoverished fauna, most notably primitive mammals about the size of rats that are thought to have served as the progenitors of the diverse mammal species that soon came to dominate the continents.

Geologists have found the boundary clay in more than 60 localities all over the world-on land and in the ocean floor. A deposit of fine dust that widespread must have fallen from the sky like winter snow. In many places, the boundary clay is full of soot-clear evidence that great fires raged across the continents as the dinosaurs met their fate.

Iridium and Shocked Quartz

In 1980, a group at the University of California in Berkeley, led by physics Nobel laureate Luis Alvarez and his son, Walter found that the boundary clay contains remarkably high concentrations of iridium. This element is so rare that it hardly exists in ordinary rock or in comets, but it is fairly abundant in most kinds of meteorites. The Berkeley group interpreted the iridium as evidence that a large asteroid struck the earth 65 million years ago, filling the sky with dust that settled to become the boundary clay. Although the idea was exciting, it needed independent confirmation to be persuasive.

That confirmation came six years later when researchers from the U.S. Geological Survey found grains of "shocked" quartz in the boundary layer. Common quartz is an abundant mineral found occasionally in the form of large distinctive crystals, but also widespread as sand and as a component of many rocks. Looked at under a petrographic microscope, common quartz is bland, with little to distinguish it. In contrast, shocked quartz shows a distinctive, microscopic pattern of fine lines. Quartz acquires this pattern only under conditions of extreme shock such as is produced by meteorite impact or nuclear explosion. Even the force of volcanic eruption is insufficient to produce the fine line pattern in quartz. Thus the "shocked" quartz in the boundary layer proves beyond any reasonable doubt that an enormous explosion did indeed rock the earth 65 million years ago. The abundance of iridium suggests that the object was probably as asteroid, instead of a comet.

The Search for an Impact Site

Neither meteorites nor the oceanic crust contain quartz, but it abounds in the continental crust. So the quartz in the boundary clay means that the asteroid of 65 million years struck a continent. Where?

At a minimum, the crater must be big enough to contain all the boundary clay. The Manson Crater of northwestern Iowa is the right age, and several others may be, but they are all much too small. None of the larger known craters are even close to the right age. Yet in these days worldwide satellite imagery no open crater big enough to contain the boundary clay could possible exist undetected.

Another possibility is that the impact site is no longer an open crater, but may by now have become some other kind of geological feature. Whatever it is, the impact site must be very large, a conspicuous patch of color on the geologic map. The problem, then, may not so much be to find the impact site as to recognize it and offer a better interpretation of something already well known.

Geologic maps reveal only one plausible candidate: the Deccan Plateau of western India. It is a flood basalt province; a vast volcanic field composed primarily of enormous basalt lava flows, many of which cover tens of thousands of square miles. The Deccan flows are exactly the same age as the boundary clay, and no other large continental feature is even approximately the right age.

If the effects of 65 million years of erosion could be erased from the Deccan Plateau and that resorted region transported to the moon, few geologists would hesitate to include it among the lunar maria. Neither would they hesitate to interpret it as the scar of an enormous impact. Nevertheless, most geologists find it difficult to interpret the Deccan Plateau as an impact scar on the earth.

Modern geology developed from the premise that small forces and slow processes continued over a long time can produce large results. That deep intellectual tradition makes it difficult for most geologists to accept the idea that at least some of the earth's large features formed in sudden and devastating catastrophes. Nevertheless, those vagrant asteroids and comets do turn in their earth-crossing orbits and must occasionally strike the earth, making great catastrophes inevitable.

The precise association in time between the shocked quartz in the boundary clay and eruption of the Deccan basalts strongly suggests that the Deccan Plateau is the impact site. It is easy to understand how an enormous caters could develop into a flood basalt province indeed, that transition seems inevitable.

Crater Basin Volcanoes

The lithosphere or crust is the earth's rigid outer rind. Yet it is not nearly strong enough to support a hole as deep as the initial crater excavated in the impact explosion of a large asteroid. It has been estimated that the asteroid whose impact caused the formation of the boundary clay layer would have formed a crater 9-12 miles deep. The sides of such a crater would immediately collapse, converting it into a much broader and shallower crater basin. That happens for essentially the same reason that a soft chocolate pudding collapses into the hole your spoon leaves.

Underlying the earth's crust, at an average depth between about 60 and 150 miles and partially molten is the part of the earth's mantle called the asthenosphere. Earthquake waves that pass through it return to the surface showing the distinctive effects of passage through a partially liquid zone. The only thing that keeps the asthenosphere from melting completely is the enormous pressure of the rocks above it; rocks expand as they melt, so pressure tends to keep them solid. If a deep crater at the surface reduces the pressure on the asthenosphere, the rocks there will partly melt, producing basalt magma.

Imagine the molten magma rising, filling the crater basin to make an enormous lave lake, then overflowing in enormous floods of basalt that spill across low areas of the surrounding countryside. Each overflow again relieves the pressure on the rocks at depth, causing more melting and more floods of basalt-the eruption lays the conditions for its successor. Meanwhile, a column of hot rock, called a mantle plume, begins rising from greater depths in the mantle to replace all the basalt erupted at the surface.

Hotspot Volcanoes

The earth's crust, above the soft asthenosphere, consists of a mosaic of more than a dozen large and small pieces called plates that fit together to cover the earth's surface in about the way bones fit together to make a skull. All the plates move on the partially molten and soft asthenosphere beneath, not in concert but more or less randomly.

The series of enormous flood basalt lava flows that built the Deccan Plateau ended as the moving lithosphere carried the impact site away from the rising plume in the mantle, probably within a couple of million years. Eruptions then continued on a much-reduced scale to create a volcanic hotspot track.

India rides a plate that moves almost directly north. The rising plume of hot rock in the mantle beneath melts the lithosphere moving across it, fueling a long chain of volcanoes that started at the Deccan Plateau about 65 million years ago and continued south through the Laccadive and Maldives Islands. The chain then jumped west across the Carlsberg oceanic ridge into another row of islands. Each of those islands is an extinct volcano. They become progressively younger southward to Reunion Volcano, which is active, the hotspot. It erupts directly above the part of the asthenosphere that was beneath the Deccan Plateau 65 million years ago.

Oceanic Ridges

Oceanic ridges exist where two plates move away from each other. As the plates separate, they reduce the pressure on the partially molten rock in the asthenosphere, generating basalt magma, which erupts through the opening gap between them. The basalt lava flows make new oceanic crust, which rides away from the oceanic ridge on the separating plates.

The Carlsberg oceanic ridge began to generate new oceanic crust while the Deccan flood basalt lava flows were erupting, and has since opened most of the board expanse of ocean between India and Madagascar. It is reasonable to conclude that the heavy blow of the asteroid impact cracked the plate, opening a rift that developed into the Carlsberg Ridge.

The relevant geography of 65 million years ago can be reconstructed by cutting all the oceanic crust generated at the Carlsberg Ridge out of the map, then sliding the pieces together where they fit can reconstruct the relevant geography of 65 million years ago. This simple procedure will bring the Amirante Arc and Seychelles Islands from their present location near Madagascar onto the west coast of India.

The Amirante Arc is a semicircular ridge in the floor of the Indian Ocean, about 400 miles in diameter. It very likely outlines the western half of the crater basin that formed as the initial crater collapsed after the impact of 65 million years ago; and if so, the eastern half of the outline is hidden beneath the flood basalt flows of the Deccan Plateau. The entire state of Wyoming would fit into such a crater basin.

The Seychelles Islands are a scrap of continental crust that appears to have been detached from western India. They nestle within the Amirante Arc, and probably consist of rocks that moved into the floor of the crater basin as the initial crater collapsed.

The Signature of Catastrophe

The Deccan Plateau is not unique. In fact, at least 14 flood basalt provinces have formed during the last 250 million years. Most of those are associated with a hotspot track, a new oceanic ridge, or both. Although most are smaller than the Deccan Plateau, the earth's flood basalt provinces seem as alike as the proverbial peas in a pod. If the Deccan Plateau originated in a large impact, then so must the others. It is beyond imagining that such complex arrays of geologic features could originate in more than one way.

Consider a few examples:

The Parana Basalts of South America and the Entedeke Basalts of the West coast of southern Africa are about 130 million years old. They appear to be parts of a single basalt plateau that separated as the South Atlantic Ocean opened between them. The southern part of the mid-Atlantic Ridge is the same age as the basalts. The hotspot track is the Rio Grande-Walvis volcanic ridge, which crosses the ocean from one area of flood basalt flows to the other. Tristan de Cunha is the active volcano.

Flood basalt lava flows in the northern British Isles and eastern Greenland erupted about 63 million years ago, just as the northern part of the mid-Atlantic oceanic ridge began to open the ocean that now separates them. Iceland is the hotspot. It does not move because it is exactly centered on the oceanic ridge, but it has generated a broad ridge of basalt that extends east and west from Iceland to the opposite sides of the Atlantic Ocean. That ridge is the equivalent of a hotspot track.

The Penghu Islands between Taiwan and main land China consist of flood basalt flows, which may be the earth's youngest major impact scar. They probably mark the source of the tektites, black blobs of molten glass that fell like hail all across the western Pacific region from the Philippines to Australia about 700,000 years ago. The Penghu Islands are a small flood basalt province that did not start a new oceanic ridge. The next few million years will reveal whether it will generate a hotspot track.

The Asteroid Impact In The Pacific Northwest

Flood basalts that erupted between 17 and 15 million years ago cover much of eastern Oregon and Washington, as well as nearby areas of northern California, northern Nevada, and western Idaho. They probably spilled from a crater basin volcano that formed in southeastern Oregon after an asteroid struck there about 17 million years ago. Some of the flows covered areas of eastern Washington almost as big as the state of Maine, probably within a few days. A few of those floods of basalt poured into the Pacific Ocean along the coast of Oregon.

The Snake River Plain is a hotspot track, a series of giant volcanoes that become progressively younger as they sweep across the Yellowstone Volcano, which is exactly above the point in the asthenosphere that was beneath southeastern Oregon 17 million years ago. That is the hotspot. The volcanoes become younger to the east because the continent is moving west across the hotspot.

The broad valleys and isolated mountains of the Northern Basin and Range province, which extends south from southeastern Oregon through Nevada and western Utah, are the continental equivalent of an oceanic ridge. A new ocean basin at least as wide as the Red Sea would now exist there if North America were not moving west across the spreading zone in the earth's mantle. That motion distributes the spreading across a wide zone of the continent, instead of concentrating it in a narrow zone.

Asteroids and Plate Tectonics

All of the earth's flood basalt provinces appeared suddenly with in plates, without any recognizable setting in previous events. No evidence suggests that the earth generates them through its own internal processes; they just happen, exactly as you would expect of asteroid or comet impacts.

Most flood basalt provinces are the starting points of oceanic ridges; all oceanic ridges started in flood basalt plateaus. The evidence shows that impacts establish new plate boundaries, thus setting the pattern of plate motion. The earth does not set its own agenda of plate motions; instead, it passively awaits its destiny in random collisions with wandering asteroids or comets.