"No vestige of a beginning, no prospect of an end."

-- James Hutton, 1788

In this section, we'll put a time scale of geological history, first invoking the notion of Relative Time, and then Absolute Time. The key to probing the past is to recognize that the rocks of the continents preserve aspects of their experience (rock formation, incorporation of relics of life existence--fossils, and chemical tracers that give the age of the rock or of its constituents). Geologists have managed to extract remarkable stories by systematic application of scientific method and deductive reasoning to this rock record.

But all of this is a new idea, relatively speaking, and most of human understanding of the rock history of the planet is less than 200 years old. Throughout the preceding two thousand years theological dogma was taken to give the history of the Earth, and there was even a learned study of the biblical record to assess the age of the planet and the history of life (this continues today in the fold of the Creation Research Institute). In the mid-1600s Archbishop Ussher produced a biblical estimate of the origin of the Earth of October 22, 4004 B.C. John Lightfoot pursued this approach in 1654, obtaining the estimate of 9 am (in Mesopotamia) on October 26, 4004 B.C.! Religious doctrine was so firmly established that it was deemed heretical to invoke any arguments contesting this basic timescale, but scientific method was emerging as valid in its own right, so it was still viable to consider the relative time implied by geological structures.

One of the key notions of geological time is the simple idea that processes that produce rock formations are such that usually younger rocks are formed on top of older rocks. This was argued by Steno, who lived from 1638-1686, and is the essence of the Law of Superposition, the notion that rocks are laid down on top of preexisting rocks. Closely related is the law of Original Horizontality, in which most rock formation processes at the Earth's surface are recognized to produce nearly horizontal layers. These ideas are simple, but require an acceptance that rocks are in fact still being created today, by processes of sediment deposition and compaction, or lava flows building layer upon layer. This was observable to a limited extent, but there was a major philosophical difficulty with embracing the notion of Earth as still changing around us (versus the mountains always having been there and always to be there, etc.). The major obstacle to human appreciation of the slow changes in rock structures is the time factor; erosion works very slowly to transport sediments to make new rocks, and the slow vertical motions that take those sediments down to higher pressures that pack them into rocks are not detectable by eye. Only in volcanic areas are the rates of rock production truly rapid enough to reveal the ongoing changes.

The breakthrough in understanding of the implications of the rock record came with the work of James Hutton in the 1780s. Key to his enlightenment was the observation that many rock formations are not now horizontal, but presumably were when they were created. The superposition of flat, younger layers on dipping older layers that were truncated had a vast implication: A huge deformation had occurred to the older rocks, taking them from their original positions, uplifting, deforming and eroding them, then lowering them so that new layers could be superimposed. The contact between the older rocks and the younger rocks spanned a great albeit uncertain gulf of time, and Hutton called the contact an Unconformity. This, combined with the observation that current rock eroding and forming processes act very gradually gave Hutton the conviction that this did not all take place in the last 6000 years, but great amounts of time had transpired. In 1788 Hutton published Theory of the Earth, which was the first true geology book. It was revised in 1795 and in 1802 Playfair published supporting figures of rock formations that illustrated Hutton's logic process (Illustrations of the Huttonian Theory of the Earth). The work of Hutton profoundly influenced Charles Lyell, his disciple, who published in 1833 the Principles of Geology, which made Hutton's approach much more accessible. Lyell's writing in turn strongly influenced Darwin.

In addition to laying out simple concepts of geology, the early 1800's saw the systematic tracking of layers and continuity of structures. This is the essence of Stratigraphy, the subarea of geology that deals with strata or layers that had a uniform origin, such as in a single lava flow or a single depositional unit involving an accumulation of sediments that was later lithified (turned to stone). But, while any layer may have obeyed the basic ideas of Horizontality (at least approximately) and Superposition, all geological formations are laterally finite, and there was a need for a key to recognizing the coincident formation of two separate strata at different locations. The solution to this puzzle came in the form of recognizing that fossils of lifeforms follow the same sequence in different environments. The idea emerged that life forms in the past have had finite durations of existence, and thus, when two different rock formations hold a common fossil type, the rocks are at least as similar in age as the total duration of a particular species existence. Many fossils appear to have a very restricted distribution in the rock record, and thus provide a relative time scale for the age of rocks.

For a fossil to be of greatest value for dating the corresponding life form should have had a relatively short duration of existence and the organism needs to have been widespread, preferably mobile. These two factors make for Index Fossils, which are the key dating agents. The fact that the number and types of taxa change upward in the rock record, with many well preserved index fossils provides the Relative Geological Time Scale. The geologists subdivided relative time into units with different fossil assemblages. With increasing age from the present the units are:

Cenozoic Quaternary Holocene

Pleistocene

Tertiary Pliocene

Miocene

Oligocene

Eocene

Paleocene

Mesozoic Cretaceous Late

Early

Jurassic Late

Middle

Early

Triassic Late

Middle

Early

Paleozoic Permian Late

Early

Pennsylvanian Late

Middle

Early

Missippian Late

Early

Devonian Late

Middle

Early

Silurian Late

Middle

Early

Ordovician Late

Middle

Early

Cambrian Late

Middle

Early

Precambrian

The flourishing of life that occurred about 600 million years ago leads to the great diversity that allows such a fine relative time scale from the Paleozoic to the Cenozoic. The Precambrian spans a much vaster time duration, but there is little in the way of fossil record to subdivide the time. So, how does this work. Basically, if you pick up a rock that has a particular index fossil in it, say a type of trilobite (a hard-shelled creature like a horse-shoe crab, but different), that will identify immediately that the rock is Cambrian in age. No younger rocks have trilobites, since all of these went extinct at the end of the Cambrian period. The particular species can sometimes place the rock in a specific epoch or yet finer subdivisions of time. Most good index fossils are ocean lifeforms, which are widespread and in rapidly evolving systems. Some creatures, such as crocodiles have been around for great spans of time and so do not provide good time resolution when their fossils are found.

Preservation of a life structure as a fossil is highly selective, and only some forms of life have been so preserved. Probably the vast majority, particularly of soft-bodied creatures like insects left no hard parts that could be silicified or calcified and preserved as rock fossils. Early paleontologists viewed the increasing diversity of fossils found in younger rocks as evidence for a cone of increasing diversity with time, like a upward branching tree with many dendritic extensions from some common root. More recent thought, and the discovery of a great diversity of different life-forms in late Precambrian rocks suggests that actually there was a vast variety of different life complexities, and many whole families failed entirely by extinction, with only one or two successful families persisting to today. This puts the particular flora and fauna found today as much more of a fortuitous selection than had originally been thought. Indeed, the whole pattern of evolution appears to have involved decimation to the same or a greater extent than diversification, at least after the first broad flourishing of complex oxygen utilizing organisms 600 million years ago.

But the Geological Relative Time Scale, as remarkable of a discovery as it is, does not provide the absolute ages that are so profound to an understanding of how evolution has occurred, and how the geological structures around us have developed. This requires absolute time. Prior to 100 years ago, the only real progress on this problem was following the Huttonian approach of appealing to present day processes to infer the age of structures. For example, if we measure the present day rates at which sediment is transported down rivers and deposited in a floodplane or delta, we can estimate how long it would take for steady accumulation of sediments (Sedimentation Rate) to pile up enough material to be turned into stone giving a layered sequence such as that, for example, exposed in the Grand Canyon. This led Hutton to suggest that at least 100's of millions of years were needed, if not an eternity. Especially given the huge gulfs of time not recorded by the rocks when they were uplifted and eroded to be visible today.

Even more direct measures include annual deposits, such as the fluctuating layers in lakes, called Varves or the growth rings of trees. Seasonal changes in sediment supply, growth, organic deposition, etc., allow one to directly count back in time, dating at least 10,000 years of Earth history, but this is but a tiny instant of the history of the planet. How to put it on a more reliable long-term absolute scale?

This issue, born of the debates between the catastrophists and the uniformitarians (as well as the parallel debate sparked by Darwin's application of similar reasoning to invoke long times required for organic evolution), brought in the interest of physicists. One of the most prominent of the day was William Thompson (Lord Kelvin), who provided a 'definitive' estimate of the age of the Earth in 1862. He gave a number of 20-400 million years, later revised to 20-40 million years. His calculation was simple, and seemingly irrefutable (especially by a bunch of rock-loving geologists who don't like math all that much; this is still true today to a large extent). Lord Kelvin simply assumed that the Earth is cooling with time (which it must be), from some initial temperature that was close to or at the melting temperature of rock. The atmosphere sets the surface temperature of the body, and then Kelvin used Fourier's law of Heat Conduction, to estimate the time that it would take for an initially uniform temperature molten rock Earth, to cool to have the present surface temperature and the observed geothermal gradient (the rate at which temperature increases with depth into the crust). The geothermal gradient was directly measured in mines and Kelvin used a number near 25-30 degrees/kilometer, which is accurate. He then solved for the minimum age of the Earth assuming cooling by conduction. While his estimate was still deemed heretical by those preferring a biblical origin, it was too short to sit well with geologists, but no refutation of the calculation came forth for many years.

There were two things wrong with Lord Kelvin's calculation, and this is an essential lesson to remember about science. If the assumptions that are made are wrong, the conclusions tend to be wrong. The assumption that the Earth has no way of sustaining heat as a function of time, and must be cooling from its initial hot temperature is wrong. But it took the discovery of radioactivity and the recognition that many radioactive materials exist in the rocks of the Earth to recognize that Kelvin had erred here. His other assumption, that the Earth cools just by conduction was also flawed, as in fact the primary method of cooling is by thermal convection which drives plate tectonics. But this was not to be fully accepted until 100 years after Kelvin's calculations.

The discovery of radioactivity was key, not only as an erroneous assumption in Kelvin's calculation (one which allowed for a far older Earth to still be as warm as it is today), but also because radioactivity itself is a process that allows us to date Earth materials. Radioactivity was discovered in the 1890's and theoretically outlined in 1902 by Rutherford. The principle idea is that some of the very largest atoms in the periodic chart, all built in the shockwaves of exploding stars, involve so large of a nucleus that they can go unstable spontaneously and shed off parts of the atom, decaying to a new element, which may itself decay, finally reaching a stable element that no longer undergoes spontaneous decay. This process of evolving from a Parent Atom to a Daughter Atom involves the loss of alpha, beta or gamma particles. Alpha particles are the equivalent of helium nuclei (two protons, two neutrons). Beta particles are electrons, and gamma particles are light photons. The likelihood of a particular atom spontaneously going unstable has an assignable probability, with the probability differing for different materials.

We characterize the decay process for a bunch of atoms of a radioactive parent, by determining the length of time that it takes for half of the atoms to decay. This is called the half life of the radioactive substance. After time equal to two half lives, only 1/4 = 1/2 x 1/2 of the parent material remains. After three half lives, only 1/8 = 1/2 x 1/2 x 1/2 remains, and so on. Essentially, the accretion of the Earth incorporated various abundances of radioactive materials into the planet (along with the daughter products from prior decay) and ever since then the radioactivity has reduced the number of parent atoms, while increasing the number of daughter atoms. Substances that decay very slowly provide good time estimates for the age of old rocks into which the radioactive materials got concentrated. Important decaying substances include:

U235 -> Pb207 710 Million year half life

U238 -> Pb206 4.5 Billion year

K40 -> Ar40 1.3 Billion year

Rb87 -> Sr 87 4.7 Billion year

C14 -> N14 5573 years

The decay of Uranium isotopes to lead provides important constraints on the age of the planet. Decay of carbon to nitrogen is valuable for dating organic substances such as charcoal and tree parts. A host of radioactive materials have been used, and strategies evolved to stably estimate how much parent/daughter material exists relative to the initial abundance in a given substance.

Earth Scientists can measure the rates of decay using particle detectors, but the most common approach is to actually count the number of atoms in a given rock sample using a Mass Spectrometer to separate the various radioactive isotopes. Essentially, the rock is vaporized by heat or a laser, and then the rock gas is passed by a magnet, which deflects the lighter elements more than the heavier ones, allowing their relative numbers to be measured. This allows many rock samples containing fossils to be dated, and thus we now have absolute times put on the Geological record. For example, the boundaries of the major eras are now known to be:

Cenozoic/Mesozoic = 65 Million Years

Mesozoic/Paleozoic = 225 Million Years

Paleozoic/Precambrian = 570 Million Years

From Lead isotope studies, the age of the Moon, primitive meteorites, and the Earth is given as 4.55 billion years. This is quite an increase from Ussher, Kelvin, and others, and begins to approach the nearly infinite time envisioned by Hutton. Nonetheless, it is a finite time, albeit vast, and the Earth system probably has a comparable span yet to go.

"The geological record is extremely imperfect."

-- Charles Darwin

The major boundaries in the geological record are defined primarily by major transitions in the life assemblages as recorded in fossils. Some of these transitions appear to be relatively gradual, while others are abrupt and dramatic. But, is the apparent abruptness an artifact of incomplete geological preservation of the history of time and life? A strong requirement of the Huttonian theory of geological evolution is that large gaps in time exist in the rock record, from the moment at which a region stops receiving new rock deposition and is elevated and eroded before sinking again and being overlain by rock. The gap in time may be immense, and almost must be immense, for the rates of vertical motion are presently observed to be relatively slow (1 cm/thousand years is pretty fast). The nature of unconformities predicts that there will be huge gaps in the rock, and hence in the fossil records.

From the work of Charles Lyell (Principles of Geology, 1833) to Charles Darwin (Origin of Species, 1859), the notion of great gouts of time, incomplete preservation of the rock record, and Uniformitarianism became so ingrained in Earth Science thought that Gradualism dominated the field for 150 years. Invoking uniformity of Law, Process and Rate, there was a strong dogma that no special catastrophic processes need be introduced to understand either geological history of evolution. >

This was invoked repeatedly by Darwin, to explain the absence of gradual transitions in evolutionary history. The many 'missing links' between successive fossil lifeforms was attributed to an incomplete geological record. Darwin assumed that the environment changed only very gradually, and that competition drives the process of speciation and evolution. In a uniform environment, each organism will thrive by widening its niche, at the expense of other organisms sharing a common pie of life. This requires mutation and gradual adaptation which leads to the concept of survival of the fittest.

However, in the 1970's this dogma came under scrutiny by paleontologists, who were looking at much more comprehensive rock and fossil records than available to Lyell or Darwin. Steven Gould articulated the notion of Punctuated Equilibrium in a series of works from 1972-1977, arguing that speciation occurs in subpopulations, largely in response to competition with a changing environment, rather than under uniform conditions. While accepting some aspects of gradualism, this notion invokes external triggers which change the environment abruptly to perturb the system, allowing some organisms to flourish and others to abruptly go extinct, by an accelerated natural selection.

Presently, Earth Sciences involve a merging of the ideas of Gradualism and Punctuated Change, recognizing that both play a role in both geological processes and evolution of life. In particular, mass extinctions prompted by rapid environmental changes have modulated the history of life.

Let us now consider one of the most intensively studied mass extinctions. This is the Cretaceous/Tertiary (K-T) extinction event which took place about 65 million years ago. This is long ago, but only a tiny fraction of the Earth's lifetime, and the rock records are quite good for this time interval. As with any extinction event, the first issue to address is what lifeforms were involved in the extinction process that defines the transition. In this case, the K-T event involved extinction of 64% of all species on Earth, and included organisms in both Marine and Terrestrial environments:

Marine:

* mollusks: e.g., Belemnites, ammonites

* plankton: coccoliths and forams with calcium carbonate shells

* bryozoans, brachiopods and many corals

Terrestrial:

* Dinosaurs were zeroed out

* Marsupials were decimated,

Equally important are the survivors: small mammals, crocodiles

These facts suggest a global extinction that was not confined to just oceans or land, and which involved a wide range of organisms with different life needs.

The next issue is how abrupt was the extinction process? This involves the imperfection of the rock record. Precision in timing requires a well-documented, continuous record. Given the statistical vagaries of fossil preservation to begin with, sparse sampling of a gradual extinction may give the false impression of a catastrophic abrupt extinction.

For the K-T boundary, there has been extensive analysis of extinctions before the boundary, which show about 5-8 million years of reducing diversity in some groups such as the Rudists and Inoceramids. These well-preserved fossil records clearly indicate that there was a period of slow environmental change preceding the boundary which was responsible for some extinctions. The general cause is believed to have been the gradual lowering of sea level and recession of the huge interior seaways, such as had inundated much of North America. The question then arise; were the gradual changes responsible for some run-away mechanism that caused more abrupt extinction events at the K-T boundary? Or are there two independent processes operating?

The way to address the abruptness of any of the extinction events is to improve the record quality for the time interval of interest. One seeks appropriate age rock formations that were deposited in sites with continuous sedimentation (no intervals of uplift and erosion), which points toward marine environments. In addition one wants high sedimentation rates, so that a thick layer of rocks is deposited across the time interval. The faster the rate of sedimentation, the better the resulting time resolution. This points toward marine environments adjacent to continents, where there is good sediment supply.

This search to find a good record section led Walter Alvarez, a U.C. Berkeley geologist to the Gubbio rock formation in Italy. In this formation, a high rate of continuous sedimentation straddles the K-T boundary, and there were good characteristic fossils deposited throughout the section. At the 'top', or most recent part of the Cretaceous, the Gubbio formation involved pelagic marls, or carbonate rich sedimentary rocks, with stable Cretaceous age marine planktic (floating) foraminifera. This marl is disrupted by an layer of clay, dated right at 65 million years, indicating a major change in sediment type, that slowly was restored to pelagic marls, but now with Tertiary assemblages of foraminifera. The clay layer is very distinctive, and being bracketed by Cretaceous fossils and Tertiary fossils, it is taken to lie right at the boundary. What caused the change in sediment? Is it related to the change in lifeforms?

Walter asked the question, is there anything unusual about this clay? He benefited from the fact that his father, a professor of Physics at Berkeley, Luis Alvarez, had set up an analysis procedure to determine the composition of heavy metals in substances. Some of the boundary clay was tested, and revealed an unusually high concentration of the Platinum group element IRIDIUM. Iridium is a heavy metal very sparsely found on Earth, and with no known mechanism for concentrating it in a geological process. This suggested an extraterrestrial origin, as many iron-rich meteorites are much enriched in Iridium relative to the Earth's rocks (most of the Earth's Iridium is concentrated into its core). Thus, the Alvarez father and son advanced the idea that a large meteorite, enriched in Iridium by its own prior history of metal separation hit the Earth and the debris deposited a global layer.

This was immediately testable by examining the K-T transition at other regions with continuous sediment deposition across the boundary. Again and again, the materials, often involving a clay layer, but sometimes not, showed enriched Iridium relative to the background level of normal rock. In detail, the abundance of the Iridium relative to other heavy metals was very close to that for meteorite samples. Systematic mapping of this Iridium anomaly ensued. Further examination of K-T record sections revealed an equally unusual attribute in 1984. This was the discovery of quartz grains with shock lamella, or bands. Shocked quartz requires very high pressures and temperatures (it was previously only observed near underground nuclear explosions), and is very difficult to account for by anything other than a large impact. The K-T sections also revealed a large amount of microspherules, small glass beads, which appear to be drops of molten rock that cooled rapidly while flying through the air.

Mapping of the Iridium anomaly, shocked quartz, and microspherules, indicated a worldwide event at the end of the Cretaceous. Additional field studies revealed very thick, complex deposits around the Caribbean, some in Haiti, enriched in microspherules and quartz, and some in Texas and Louisiana which involved jumbled piles of debris that appears to be the deposit of a large tsunami wave. These thick deposits suggest that the impact took place in the Gulf of Mexico. In 1989, subsurface imaging revealed the existence of a 180 km diameter crater underlying the northern Yucatan peninsula, buried by the last 65 million years of sediment. The rock type there is rich in carbonates and evaporites and there is a thick deposit of broken rock overlying the subsurface crater. This is the Chicxulub crater, now the most likely candidate for the primary impact of the K-T period (there is actually evidence for two impacts, perhaps one year apart). Other candidate craters for later impacts include the smaller Manson crater in Iowa and the Popigai crater in Siberia, both of which have ages of 65 million years. Perhaps the asteroid broke up and their were multiple impacts, just as happened with the Shoemaker-Levy comet that hit Jupiter recently.

If we accept this evidence for a large impact, we must ask the question, how plausible is such an event? Is it a likely or highly improbable event? Studies of the crater density on Earth and the Moon provide some estimate of the rate of impact of different size objects. The results are startling:

several meters 1/yr 20 Kilotons (Hiroshima=14Kt)

10's of meters 1/100yr several Megatons (1908 Tunguska)

>1 km 1/1000000yr 1 million Megatons

There should be a 100 m object hitting the Earth about every 10,000 years, which is the size of impactor that is associated with Meteor Crater in Arizona. A 10 km object, such as needed to account for the volume of Iridium at the K-T boundary should hit Earth about once every 50-100 million years. Thus, in a way, meteor falls are common phenomena, and are indeed to be expected with time.

Could such an impact actually kill so many forms of life, as observed at the K-T boundary? What are the kill mechanisms? Several are postulated:

Direct Hit: The explosion produced by impact of a 10 km diameter projectile is about 100 million megatons of TNT, or 5 billion times the strength of the Hiroshima bomb. It would produce a 100+ km crater, like that found in the Yucatan. Over 100 x 10exp(12) tons of pulverized rock would be ejected into the atmosphere.

Since the impact site was under water, there was a huge wave generated.

The Dust in the atmosphere would produce a darkening for at least one year, sometimes called the Impact Winter scenario. This is long enough for most plants to die, disrupting the food chain on both land and sea.

The fireball from the impact would set the world's forests burning, and indeed in the carbon isotope record for the K-T boundary there is evidence for massive fires.

The impact site was rich in carbonates, and this would vaporize to produce carbonic acid, with a short duration of acid rain.

Carbon dioxide released from the impact would add to a short-term Greenhouse effect, causing planetary warming.

These effects could have combined to eliminate many of the species that became extinct at the end of the Cretaceous.

"Your chance of dying in a global impact catastrophe is 1/10,000."

-- Thorne Lay

The impact hypothesis for the K-T boundary event can account for the global environmental change that would account for the widespread extinction that occurred. However, is it a unique explanation? It provides an independent punctuation of the relatively gradual decrease in sea level and resulting reduction in shallow water environments that had been occurring for about 5 million years. It can explain many observations, such as the global presence of a thin Iridium-enriched clay layer, shocked quartz, microspherules of glassy materials, large wave deposits, and the presence of a 180km diameter crater in the Yucatan. As a scientific hypothesis, it is viable and testable. But that does not ensure that it is a unique interpretation, nor does it necessarily establish a causal link between the impact and global extinctions.

Another clear geological record of the late Cretaceous is massive layers of basalts, in the form of Flood Basalts, found in several places. Immense deposits of basalt flows are found in India, in the Deccan Traps, and in other countries including Siberia, Brazil, and the western U.S.. At the end of the Cretaceous India had not yet collided with Eurasia, and the Deccan Traps formed rapidly by massive volcanic extrusion over the present position of the island of Reunion. It is speculated that the dust arising from these prolonged and extensive eruptions could have opaqued the atmosphere, much as an Impact Winter would, causing global disruption of the food chain and massive extinction. There is no clear explanation for why the Cretaceous finished with massive volcanism, but presumably deep mantle dynamics and turn-over were involved. The onset of basalt flows in the Deccan Traps appears to have slightly preceded the end of the Cretaceous, but it is striking to plot the position of India at the time, relative to the impact site in the Yucatan. They are almost on opposite sides of the Earth, which has prompted the speculation that the impact sent shock waves through the Earth that accentuated the volcanic activity on the far side of the planet. In this way, the catastrophic effects of the impact and volcanism could be linked.

From our consideration of the frequency of impacts, it appears that a large meteorite or comet may strike the Earth about every 50 million years on average. Is this the explanation for the many dramatic extinctions in the geological record? Some extinctions were in fact much more extensive than the K-T event. 250 million years ago, at the boundary between the Permian and Triassic periods, 75% of all genera and 95% of all oceanic species went extinct rather abruptly. In fact, life on Earth was almost wiped out. This again suggests a global catastrophe. Other mass extinctions took place 360 and 435 million years ago, and lesser extinctions have speckled geological history.

Following the Alvarez hypothesis of a large impact, paleontologists began to scrutinize the record of extinctions for statistical properties. If one plots the rate of genus level extinctions as a function of time over the past 250 million years, a statistically significant periodicity of 26 million years is found. The same is true if one considers extinctions of entire families of taxa. This is extraordinary, and suggests some sort of regularity in what might be assumed to be a totally random process of environmental perturbations.

If one considers terrestrial craters that have 140-200 km diameters and ages over the past 2 billion years (well preserved in some parts of the continents, particularly long stable regions such as in North American, Europe and Australia, a total of 130 are found. Many smaller craters are also found, and if those with a diameter greater than 10 km are considered, there proves to be a 32 million year peak in the time between impacts, which is quite close to the 26 million year extinction number. Variations in low sea level show peaks with periods of 21 and 33 million years, while changes in plate creation (sea-floor spreading) peak at 18 and 34 million years. Are these processes independent or linked? This is very hard to establish, but one can ask the question, What in the Solar System has a 26 million year period? The answer is that nothing does, so any explanation for an extraterrestrial cause must invoke a larger scale phenomenon.

One idea that emerged in the last decade is that there may be a 26 million year perturbation of the Oort Cloud, which is a vast halo of comets that lies at large distances from the sun. Based on the number of comets that penetrate into the inner solar system, Oort proposed in 1950 that there are around 10exp18 comets about 1 light year away from the sun, the relic of the original solar nebula which formed the solar system. These are in a spherical shell of orbits, and some process is needed to periodically perturb them.

Two ideas for how to perturb the Oort Cloud have been proposed:

1. A dark sister sun called Nemesis: Our sun formed as a binary, with a secondary star that never began to shine (too small for fusion). Interaction of the orbit of the two stars could regularly cause gravitational perturbation of the Oort Cloud. Systematic search for Nemesis has not yet revealed such a dark star.

2. Galactic Plane Oscillation: Our solar system is not static within the galaxy, but oscillates up and down through the main symmetry plane of the Galaxy with about a 60 million year periodicity. Each passage through the plane could lead to the surrounding Oort cloud being perturbed by increased interstellar mass in the galactic plane. This would release a hail of comets, some of which would strike the Earth. The timing is not quite perfect, but it is on the right scale. We are now pretty much in the middle of a cycle, so would not expect another mass extinction for millions of years.

But, there are many objects in space that could fall to Earth at any time, and they need not all originate by some perturbation of the Oort cloud. By the best estimate now, your chance of being killed in a global catastrophic impact is considered to be 1 in 10,000. This is the same as your chance of dying in an airplane crash. It is about 1/60th of the chance that you will die in an auto accident.

This investigation of extinctions has awakened humans to their vulnerability to debris from space. Ideas for a prevention system have been advanced, including Project Spacewatch, in which near-Earth orbiting comets and asteroids are detected and tracked. Some nuclear physicists (Edward Teller, for example) have advocated setting up nuclear missile defense systems to deflect any infalling objects on a collision course with Earth. We may be able to avoid the fate of the dinosaurs using technology!