10

SHOCK AND AWE

On a recent trip to Greece, I was occasionally humbled by the impressive English vocabulary of some of the locals I met there, who would at times say a word that I—a native speaker—might hesitate to use. I remarked on this when someone spoke the word eponymous, which my conversational partner reminded me originated as Greek. As did many of our words, of course.

The word crater is among them. Apparently the ancient Greeks, though big wine drinkers, appreciated moderation too. Unless revelry was in store, wine would be mixed with three times its volume in water, and a krater was the designated blending vessel. A krater has a large round opening—a bit similar in shape to the huge gaping regions on Earth and the Moon that share their name. But the geological feature with a similar name can run up to 200 kilometers across, and the surrounding disturbed region can be even larger.

Some craters are formed here on Earth by volcanoes, without any outside assistance. On Tenerife on the Canary Islands, for example, you can see a few fantastic craters in the big lava field of the El Teide volcano—evidence of turmoil beneath the Earth’s surface that occasionally bubbles out. This is also where I learned that caldera was the Spanish word for “cauldron,” teaching me that the term we use for a volcanic depression has an origin similar to that of the word “crater.” Impact craters, on the other hand, are formed in isolation and—more significantly—only with an extraterrestrial contribution.

Most meteoroid hits—including all the big ones—happened well before people were around to watch, never mind record, them. An impact crater is the remarkable calling card that a speeding meteoroid that descended to Earth leaves in its wake. The craters or depressions and the material in and around them often are the sole surviving records of the disorderly visitors that wreaked havoc upon arrival. The scars, rock types, and chemical abundances buried in the debris provide our most reliable information about these long-ago events.

Impact craters serve as extraordinary evidence of the Earth’s lasting connection to its environment—namely the Solar System. Understanding the formation, shape, and characteristics of impact craters helps us determine how often rocks of various sizes hit the Earth, as well as debate the possible role of meteoroids in extinctions in a more informed manner. In this chapter I’ll explain why and how those awe-inspiring craters formed in the first place—and what distinguishes impact craters from the terrestrially induced depressions from volcanoes. I’ll also comment on the list of objects that struck with enough force to make a lasting impression, which are nicely cataloged in the Earth Impact Data Base you can pull up on the Internet. These observations will be critical later on when I consider dark matter’s role in triggering meteoroid impacts.

THE METEOR CRATER

Before diving into impact crater formation and the full list of those here on Earth, let’s take a moment to reflect on the first one found—one of the earliest discoveries to tie together objects from the sky with the surface of the Earth. (See Figure 22.) Although the name is a little off—remember, “meteor” is the streak in the air—the Meteor Crater was at least formed by a meteoroid, as, by definition, are all impact craters. This particular crater is located near Flagstaff, Arizona. Its name correlates with a nearby post office, in accordance with a meteoroid naming convention. Theodore Roosevelt established the post office in 1906 when his friend Daniel Barringer, a mining engineer and businessman, started investigating the mysterious crater’s contents and origin. Geologists were initially skeptical of his proposal, but Barringer ultimately showed that the crater originated in a meteoroid. The depression is also known as the Barringer Crater in recognition of his contribution.

[FIGURE 22] The roughly kilometer-wide Meteor (Barringer) Crater located in Arizona. (Aerial image courtesy of D. Roddy.)

Though bigger impact structures exist, the crater is among the largest in the United States—measuring some 1,200 meters across and 170 meters deep, with a rim that rises about 45 meters. It is about 50,000 years old, and you can see it right on the surface of the Earth. If it wasn’t obvious from a map, you can tell the crater is in America since, like many things American, it is privately owned. The Barringer family holds the title through the Barringer Crater Company and currently charge 16 dollars to see it. The ownership was secured in 1903, when Daniel Barringer staked a claim along with the mathematician and physicist Benjamin Chew Tilghman, which was soon afterward signed by the president. The company staking the claim—the Standard Iron Company—was given permission to mine with a 640-acre land patent.

Because it is private property, the crater cannot be part of the national park system. Only federally owned land can house a national monument, so it is merely a national natural landmark. The good part of that is that it doesn’t get closed when the government shuts down, as occurred in 2013 when I started writing this chapter. The other good thing about private ownership is that the Barringers have a vested interest in preserving the crater, and it is indeed considered the world’s best-preserved meteor impact site—though its relatively recent origin helps a lot too.

The meteorite associated with the crater is called the Diablo meteorite, named after the ghost town of Canyon Diablo, located along the canyon sporting the same name. The associated 50-meter-wide meteoroid, composed of almost pure iron and nickel, probably hit the ground at about 13 kilometers per second, generating at least two megatons of TNT worth of energy—a few times as much as Chelyabinsk or roughly the energy of a hydrogen bomb. Most of the initial object was vaporized, making fragments difficult to find. Pieces that have been located are on display in the museum there, and some are even for sale.

The paucity of fragments made it difficult at first to ascertain that the crater was in fact formed by an extraterrestrial object, rather than by a volcano, as the European settlers who first ran across it in the nineteenth century had assumed. This wasn’t an unreasonable hypothesis at the time given how exotic an extraterrestrial explanation must have seemed, and how misleading the proximity of the San Francisco volcano field—only 40 miles west—must have been.

In an illuminating story of science gone wrong—and only later put right—the U.S. Geological Survey’s chief geologist, Grove Karl Gilbert, made his official determination that it was a volcano in 1891. Gilbert had heard about the crater from the Philadelphia mineral dealer Arthur Foote, who was interested in the iron that shepherds in 1887 had found nearby. Foote had recognized the extraterrestrial origin of the metal and visited the site to see what else he could dig up. In addition to iron, he found microscopic diamonds. These had been formed on impact but Foote—not knowing this—had incorrectly thought the object that had hit had been as large as the Moon. Foote made a further mistake in that he didn’t associate the crater with the meteorite material he was investigating. Despite his acceptance of the extraterrestrial origin of the material on the ground—in his mind, the nearby crater was a separate phenomenon that had been created by volcanic activity.

On the other hand, Gilbert, who had learned about the crater from Foote, was one of the first to propose that it had originated from a meteoroid. But in his attempt to scientifically verify his claim, Gilbert too came to the wrong conclusion. Since no one yet understood the morphology of impact craters, he incorrectly ruled out his impact hypothesis because the mass in the rim did not agree with the missing mass from the crater and also because the shape was circular and not elliptical as he would have predicted for an impact arriving from a particular direction. Furthermore, no one found any magnetic evidence for a difference in iron content that would have indicated something extraterrestrial. Given the lack of evidence of a meteoroid, Gilbert was compelled by his own methodology—which neglected the more subtle elements of impact crater formation that I will soon describe—to mistakenly conclude that volcanic activity and not an impact had been responsible for the crater.

The crater’s origin was correctly pinned down at last when Barringer and Tilghman published a couple of extraordinary papers in the 1905 Proceedings of the Academy of the Natural Sciences of Philadelphia, in which they demonstrated that the Meteor Crater did indeed result from an extraterrestrial impact. Their evidence included the overturned rim strata, which I’m told is truly spectacular to see, and nickel oxide in the sediment. However, the thirty tons of oxidized iron meteorite fragments around the crater led Barringer to make a different and costly mistake. Barringer thought most of the rest of the iron would be buried underground and spent 27 years drilling to find it. Its discovery would have been another bonanza for the man who in 1894 had made fifteen million dollars (more than a billion in today’s dollars) from the Commonwealth silver mine, which was also in Arizona.

But the meteorite was smaller than Barringer had thought and anyway most of a meteorite vaporizes on impact. So he didn’t make any money or successfully convince many people of the crater’s origin, even after his excavation was complete. Barringer died of a heart attack a few months after the president of the Meteor Crater Exploration and Mining Company—a venture Barringer had helped create—closed down operations. Barringer and his company lost $600,000 on prospecting the crater, but at least Barringer did live long enough to vindicate his hypothesis.

As planetary science advanced and people finally began to more fully understand crater formation, more scientists were won over to Barringer’s deduction. The final confirmation came in 1960, when Eugene Merle Shoemaker—a key player in scientific understanding of impacts—found rare forms of silica at the crater that could only have arisen from rocks containing quartz that were severely shocked by impact pressure. Aside from a nuclear explosion—unlikely 50,000 years in the past—a meteoroid impact is the only possible known cause.

Indeed, Shoemaker carefully mapped the crater and showed the similarity between its geology and that produced around nuclear explosion craters in Nevada. His analysis legitimized the concept of an extraterrestrial impact on Earth and marked a milestone in Earth science’s absorbing the significance of the Earth’s interaction with its cosmic environment.

IMPACT CRATER FORMATION

My joy in rock climbing comes in no small part from my pleasure in examining the material, texture, and density of rocks—closely inspecting the surfaces to identify the safest and most efficient route up. But the real treasure buried in rocks is their long history. Along with the evidence of tectonic plate movement that they display, rocks’ morphology and composition provide a trove of information for geologists to evaluate. Paleontologists too learn a great deal from the Earth’s embedded fossils and its terrain.

Rock formations always tell a story, and some locations are particularly spectacular in this respect. On a recent university visit to the Basque country in Bilbao, Spain, I was very lucky to have a physics colleague tell me about the Flysch Geoparque in the nearby town of Zumaia. The Geoparque is an active ecotourism site that features an incredible limestone outcrop representing millions of years of geological history—fascinating both for its use of the geological treasures there to provide sustainable economic development and for its diverse scientific activity and discoveries. When I visited the Geoparque, the scientific director there pointed out to me the 60-million-year span of rock layers readily visible across the vertical cliff, which was wonderfully situated along a stunning beach. (See Figure 23.) He described the cliff as an open book with every page visible at the same time. The K-T boundary (now officially known as the K-Pg boundary, which I’ll return to later) separates the layer of white rock with fossils from the grayer layer above without them. This line that marks the last major extinction is well preserved in this quiet site in the Basque country.

But such magnificent layers of rock aren’t the only way to learn about the past. Impact craters, which are some of the most remarkable formations on the Earth’s surface, constitute a very different wellspring of information. Despite our limited knowledge of how and when meteoroids will strike, scientists understand quite a lot about impact craters’ geology. A crater’s shape, rock morphology, and composition provide clues that help to distinguish impact craters from calderas or other round depressions. And, since impact craters’ distinctive appearance and composition can be understood in large part from their origin, the depressions and special rock types where meteoroids made groundfall tell us a lot about the events that created the craters in the first place.

If the words hadn’t already been corrupted by a spectacularly unsuccessful military policy, “shock and awe” would probably be the most cogent description of impact craters’ formation. Impact craters are the result of extraterrestrial objects hitting the Earth with sufficient energy to create a shock wave that excavates a circular crater—which is awesome indeed. The shock wave—not the direct impact—is responsible for the circular shape of impact craters. A more direct excavation would instead produce a depression with a preferred orientation that would reflect the impactor’s initial direction—not something that looks the same all around. This was the red herring that misdirected Gilbert’s Barringer Crater analysis. But the crater can’t simply be understood as an impactor pushing down on rock. The crater is created when the impactor pushes down so much on the Earth that the compressed region acts like a piston, which rapidly decompresses to release stress, rebounding from the initial impact and ejecting material. The release of the pressure through the hemispheric pattern of the shock wave is the actual explosion that creates the crater. This subsurface explosion gives rise to the impact crater’s distinctive circular form.

[FIGURE 23] The 60 million years of history visible in the rock of the Flysch Geopaque on Itzurun Beach in Zumaia, Spain. (Jon Urrestilla)

Objects that form impact craters usually hit the ground at speeds up to eight times the Earth’s escape velocity, which is 11 km/sec, with roughly 20–25 km/sec most typical. For larger objects, this speed—many times the speed of sound—guarantees that an enormous amount of kinetic energy gets released, since kinetic energy grows not only with the mass but with the square of the speed. An impact on solid rock, which can be comparable to a nuclear blast, produces shock waves that compress both the object from space and the surface on Earth. The shock released on impact heats up the material it encounters and almost always melts and vaporizes the entering meteoroid, and—with sufficiently big meteoroids—regions of the target too.

The expanding supersonic wave creates stress levels far in excess of the local material’s strength. This creates rare crystalline structures, such as shocked quartz, that are found only in impact craters—and in the blast region of nuclear explosions. (See Figure 24.) Other distinctive features include shatter cones in rock, which are conically shaped structures whose apexes point to the collision point, as illustrated in Figure 25. Shatter cones are also clear evidence of a high-pressure event that again can be accounted for only by impacts or nuclear events. Shatter cones are interesting in that they range from millimeters to meters in length, thereby providing a macroscopic scale effect in the material. Along with crystal deformations and evidence of melted rock, shatter cones help distinguish craters that truly represent impact events.

[FIGURE 24] The distinctive criss-cross deformation pattern in shocked quartz indicates a high-impact meteoroid origin.

Other rock forms that are characteristic of impacts are those formed at high temperatures. Known as tektites and impact melt spherules, these are glassy materials that have their origin in molten rock. As these are generated by high temperature and not necessarily high pressure, they can conceivably also originate in volcanoes—generally the chief contenders to impacts for crater formation. But impact craters generally have a different chemical composition, including metals and other materials—such as nickel, platinum, iridium, and cobalt—that are rare on the Earth’s surface. These additional clues help corroborate an impact origin.

The chemical composition of the impactor can have other distinctive features as well. For example, particular isotopes—atoms with the same charge but different numbers of neutrons—might be more typical of extraterrestrial formations, though this will be useful only for a small percentage of the remaining material since most of the original matter is vaporized

[FIGURE 25] The notable conical shapes of different sizes that occur many times in the same rock is a macroscopic indication of the rock structure’s high-pressure formation.

Also useful in distinguishing craters are impact breccias, which consist of fragments of rock held together by a fine-grained matrix of material—again indicating an impact that shattered what was initially there. Shocked fused glasses are also interesting in that their formation requires both high pressure and elevated temperature. Their unusually high density helps identify them. Another notable feature can be dikes within the crater floor or central sheets that line the floor of complex structures formed from glass particles.

These distinctive shock and melt features are critical to confirming impact events since there is no other way for them to form. However, they aren’t always easy to find as they can be deeply buried under rock fragments and melt. Nonetheless, meteorites abound and many natural history museums display examples. I like the seven-foot-high, 34-ton Ahnighito meteorite at the American Museum of Natural History in New York, which is the largest on display. This enormous rock was a later acquisition, added to the meteorite collection that the museum has housed since its creation in 1869.

Materials help identify impact craters, as do their distinctive shapes. Whereas impact craters are depressions below the surrounding regions, most volcanic craters arise from eruptions and are found above the level of the surrounding terrain. Impact craters also have raised rims—again not typical for volcanic craters.

Another identifying feature is the inverted stratigraphy—the overturned rim strata—in the ejecta blanket, a consequence of material being excavated and then “flipping over” outside the crater—resembling the edge of a stack of large pancakes. The deep, roughly circular depression in the surface of the Earth—or on any planet or moon—with a raised rim and inverted stratigraphy is also clear evidence that a massive body hit the surface at enormous speed.

Though the materials distinguishing impact craters are mostly shaped during the sudden shock release, the crater’s shape relies on the subsequent formation history too. Initially, when hitting the target, the impactor decelerates while the target material accelerates. The hit, compression, decompression, and the outflow of the shock wave all happen within a few tenths of a second. Once the shock wave has passed, changes occur more slowly. The accelerated material that got hit—having been accelerated by the initial wave—keeps moving even after the shock wave has decayed away, but at this stage the motion is subsonic. Even so, the crater continues to form, with the crater rim rising and more material ejected. The crater isn’t yet stable, however, and gravity will make it collapse. For small craters, the rim falls down a bit and debris heads down the walls of the crater, with melted material flowing into the deeper portion of the crater. The end result is still bowl-shaped and looks a lot like the crater that initially formed, but it might be considerably smaller. The Meteor Crater, for example, is half its original size. Afterward, breccias and melted and ejected rock fill in the cavity. The shape of a simple crater is illustrated in Figure 26.

Larger impacts not only displace and eject material, but also vaporize part of the original ground that got hit. This melted material can coat the inside of the cavity, while the vaporized material typically expands away—creating in effect a mushroom cloud. Most of the coarser material will drop down within a few crater radii. But some of the more finely grained matter can be dispersed around the globe.

[FIGURE 26] A simple crater formed from an impact has an excavated bowl-shaped central region covered by relatively flat breccia and a distinctive raised rim.

When the impactor is bigger than a kilometer across, the craters formed will be twenty kilometers or larger. The impactor in this case essentially creates a hole in the atmosphere, and ejecta fill this vacuum—going upward before descending over a wide area. The hottest material can rise above the stratosphere, and the fireball of vaporized material can then be widely dispersed, as happened to the worldwide iridium-rich clay deposited by the K-T impact we will encounter soon.

Larger impacts create a complex crater (see Figure 27), in which the cavity undergoes more extensive changes after the initial crater was established. The central region rises up while the rim partially collapses because as the shock wave propagates through the ground, it interacts with the nonuniform rock to generate a new wave that propagates in the opposite direction of the shock wave and “unloads” the shock. This rarefaction wave pulls deep material to shallow depth and leaves the crust thinned below large-impact craters. The speed at which all this happens is remarkable. Depressions several kilometers deep can be created in seconds, and peaks can rise thousands of meters in minutes.

[FIGURE 27] A complex crater, like a simple crater, a raised rim—but with terraced structure—as well as an inner uplifted region and a greater quantity of collapsed material.

Complex craters have an appearance different from the simple craters formed by smaller impacts. The precise crater shape depends on the size. When a crater is bigger than two kilometers across in layered sedimentary rock, or four kilometers across in stronger igneous or metamorphic crystalline rocks, it generally has a central uplifted region, a broad flat crater floor, and terraced walls. This is what is left after the initial compression, excavation, modification, and collapse.

When the size is larger than 12 kilometers across, an entire plateau or ring might rise up in the center. All these clues are critical information when (metaphorically and sometimes literally) digging up the past. As we will see in Chapter 12, these distinctive features were helpful in the 1980s in identifying the crater in the Yucatán associated with the K-T extinction.

CRATERS ON EARTH

Many impact craters have been found in the last half century. By studying their chemical composition as well as those of the scars known as astroblemes—the mostly destroyed craters that still leave identifiable impressions—we can begin to fill in our planet’s visitor record. The guestbook is the Earth Impact Database.

The Earth Impact Database certainly contains some of the more fascinating lists you can find on the Internet. If you check it out, you will find catalogs of the many objects that have hit the Earth and left a scar big enough to be found and identified as an impact crater. This is not a complete list of impacts. Since a lot of the very old craters on Earth were wiped out by geological activity, most of the ones we observe here arose from more recent and less frequent impacts.

Most strikes probably occurred more than 3.9 billion years ago in the early phase of the Solar System when material leftover from the formation of the planets was swept up and moved around. But the Earth, Mars, Venus, and other more geologically active bodies have tended to lose evidence of the craters over time, which is why the geologically passive Moon shows them so much more prominently.

[FIGURE 28] List of known craters on Earth greater than 20 kilometers in diameter created in the last 250 million years, obtained from the Earth impact data base. The sizes represent estimates for the diameter from rim-to-rim of the crater itself, which is smaller than the affected impact region.

Even evidence of more recent impacts is mostly now lost. Though they occur reasonably often, small hits don’t leave a noticeable scar—at least for long. In fact small craters are even less common than you might otherwise expect because of the Earth’s dense atmosphere. Like on Venus and Titan, the atmosphere protects us from the many small impacts that occur much often on Mercury and the Moon, where the atmosphere does not protect them.

Larger impacts occur only rarely—pretty fortunate for the stability of life on the planet. An impact that is violent enough to produce a 20-kilometer-wide crater might occur and cause global damage once every few hundred thousand to a million years. Yet even this rate is not reflected in the Earth Impact Data Base. If you check it out, you will find evidence of only 43 such craters and only 34 within the last 500 million years, 26 within the last 250 million years (See Figure 28.), and only about 200 structures in total.

Several factors account for the paucity of the crater record. The first relevant issue is that 70 percent of the Earth’s surface is covered by oceans. Not only are underwater craters difficult to find, but water can interfere with the formation of a crater in the first place. Furthermore, geological activity in the ocean floor is likely to wipe out all but the more recent of any scars that did in fact form. Seafloor evidence is largely eliminated every 200 million years, since plate tectonics changes the ocean floor in a conveyer-belt-like process of spreading and subduction that covers up any preexisting evidence on this time scale.

Even on the ground, geological activity such as erosion due to wind or water can destroy evidence. This is one reason most craters have been found in the more stable interior regions of continents (and are more likely to be retained on planets with less geological activity, such as Venus). And of course, even if not as inaccessible as four kilometers underwater, meteoroids might land in less accessible regions on land too. Finally, human processes can cover up evidence by changing the Earth’s surface. So in some respects it’s remarkable that the list of craters is as big as it is.

Several standouts are notable for being relatively recent events (on geological time scales). Two 10-kilometer-wide craters were created within the last million years—one in Kazakhstan and one in Ghana. Two other notables are those in South Africa and in Canada—Vredefort and Sudbury. These are even bigger than the Chicxulub crater caused by the impact event that gave rise to the K-T extinction, but were shaped in the far more distant past—a couple of billion years ago. The Sudbury mine in Canada was created to excavate the nickel and copper that became concentrated when the enormous object that created the crater struck and melted the crust. The impactor at Sudbury did not directly deliver most of the metals, but instead melted a huge sea-size volume of the Earth’s crust that took a long time to crystallize. This left enough time for the small amounts of nickel and copper already present in the crust to settle to the bottom of the pool of impact melt. The metals were then further concentrated by hydrothermal activity generated by the hot impact melt sheet to produce economically recoverable ores.

The Sudbury mine is famous among particle physicists because of an underground lab there. Though still an active mine, it is also an active physics experimental site. The Sudbury lab’s deep underground location, two kilometers below the surface, shields detectors inside from cosmic rays, making the lab an ideal location to study neutrinos from the Sun, as it did between 1999 and 2006. It is also an excellent place to search for dark matter, which is the goal of several experiments currently housed inside.

But most impact stories are not quite so rosy. I’ll soon describe the more recent Chicxulub impact event, which demonstrates the tremendous destructive capacity that large impacts can have. However, before presenting this incredible story of the meteoroid that triggered the K-T extinction 66 million years ago, let’s first reflect on the larger tale of the major extinctions of the past half a billion years, and what they tell us about the fragility and stability of life on our planet.