All of the planets were formed within a few hundred million years, and it is estimated that the Earth formed in about 70 million years.

The accretion of the Earth took place about 4.55 billion years ago. This age is based on the dating of lead isotopes in meteorites and inferences from the lead/Uranium history of materials on the Earth. About 1/2 of the Earth accreted in the first million years, and the rate of bombardment was immense. Relative to today's rate of meteorite impacts, there was about a billion times more collisions. Each collision injects heat energy into the Earth, as the kinetic energy of the meteorite is converted to another form of energy: heat. Calculations of the amount of energy available from these impacts indicate that it is easily sufficient to melt the entire surface of the planet, and perhaps the whole interior. Thus, from 4.55-4.5 billion years ago the planet had a huge ocean of magma, or molten rock: a magma ocean.

During this initial phase the Earth's core formed. There are two basic models for accretion of the Earth, but either must result in layered planet that existed by 4.5 billion years ago. We will see why this is necessary in a moment! The models are:

Heterogeneous Accretion: As the nebula cooled, the refractory materials accreted into a single core, and later, less refractory materials added on, in layers, to give an iron rich core overlain by silicates.

Homogeneous Accretion: The above process may have occurred on the scale of planetisimals, but these then accreted into a larger body, such that the initial distribution of materials was fairly uniform through the planet.

The Earth's core developed by either minor segregation of iron into the core under the heterogeneous accretion model, or by draining all iron out of the uniform planet formed by homogeneous accretion. In either case the planet was very hot from impacts, and the very motion of iron to the core releases heat sufficient to melt the rocky material of the planet.

In any case, the interior has to get hot enough to cause the iron to melt. There are two means of increasing the temperature sufficiently to allow the iron to drain toward a growing core. Radioactive heating was long favored by scientists. This hypothesis holds that the radioactive elements, which have a range of half lives many of which are quite long, decay, generating heat which warms up the interior. This would take hundreds of millions of years to accomplish. The other means is by impact heating, which we have already seen effieicntly produces a molten exterior in the form of a magma ocean. In this model, the earth acts as a continuous iron smelter, draining iron to a growing core as the earth grows, while leaving the floating silicate slag behind as what will eventually be called the mantle. The timing of core formation is therefore the most telling discriminator between these two models. Again, it is difficult to find the answer to this internally, and we have looked outward to find clues. They come in the form of meteorites and The Moon. It turns out that meteorites are all about the same age. They come in several flavors, some looking like metallic pieces of cores of broken worlds, others like the mantles of similarly segregated bodies, and yet others as loose aggregations of primitive grains that have never melted. Those coming from segregated worlds are in fact less than 100 million years younger than the oldest primitive bodies, meaning that in these other (long-ago fragmented) worlds segregation into metallic cores had occurred very rapidly. This supports the impact bombardment heated - smelter model.

Having segregated the iron into the core, the Earth was merrily on its way. Ahh, but wait. There were a few extra large planetismals out there still. One of these rare, large planetisimals about the size of Mars impacted the Earth. This sprayed much of the initial Earth's rocky mantle into space with a huge collision, converting the rocky material into liquid and even gas. While some material fell back to Earth and some escaped to space, a significant amount re-condensed and cooled, finally accreting to form the Moon. This hypothesis is the only one for the origin of the Moon which can be reconciled with all of our observations of the Moon. This includes its bulk composition (especially the low iron content), the tilt of the Earth's axis with respect to the plane of the eccliptic (which yields our seasons), and the angular momentum of the Earth-Moon system, and the age of the Moon (How do we know the age of the Moon? We went there and grabbed some rocks, brought them back and dated them!). The simulations of this collision indicate that the Moon was initially much closer to the Earth, and is slowly receding. Measurements using lasers have shown that this recession rate is now 3 cm per year. This is changing the length of the day; it is thought to have been only 6 hours at first. A similar large, late impactor may have hit Venus, which has a strange rotation, but it did not produce a moon like ours. There are other planets in fact whose spin axes are also inclined. These also are likely the result of very late, very large impacts. While they were very very rare events, they played a huge role in setting the final look of the solar system.

This great impact would have once again melted most of the Earth, and any atmosphere or ocean would have burned away. So, we have accreted the Earth and formed the Moon, but we have to do a lot of home improvements before the planet could accommodate life.

"The Earth is a dynamic thermal engine driven by internal heat sources and solar radiation. Humans will exist on the surface of this system for only the blink of a cosmic eye." -- T. Lay

Once planetary formation was complete, the Earth embarked on a 4.5 billion year journey to the present. Along the way there have been remarkable changes from the early state of a thick molten layer of surface rocks with no oceans or atmosphere to today's blue planet. The long journey will continue well into the future, with changes both natural to the system and prompted by Human activity. Ultimately, the planet will evolve to a static planet, devoid of internal motions driven by thermal convection, and with a slow decay of the surface by erosion, unreplenished by mountain building. That state will resemble more the planet Mars, or perhaps the Moon, which have seen their internal engines run down. Of course, by then the Sun itself will be exhausting its fusion materials, and begin the core collapse that bring on the Red Giant phase. The gaseous nebula cast off from the Sun is likely to consume the Earth is one final fiery catastrophe.

The early Earth was inhospitable, with intense meteorite and cometary impacts persisting up until at least 3.9 billion years ago. The evidence for this is provided by the Moon, on which the scars of this period of intense bombardment persist, with no healing processes such as occur on the Earth. The primary structure of the Earth was established early on, during the core formation process, but the steady circulation of the rocky silicate mantle has resulted in melting of mantle rocks, from which the crust has been extracted along with the bulk of the oceans and atmosphere. This melting became more and more localized, with no longer enough heat to produce a magma ocean, and melting only occurring under special circumstances. We will explore those circumstances in our consideration of volcanic catastrophes. The cooled rocky crust no longer would vaporize gases at the surface so the ocean and atmosphere stopped boiling off into space, and it was the accumulation of liquid water at the surface of the Earth that was essential for life to evolve on the planet. This transition of the surface conditions raises a few remarkable coincidences about the particular Earth system that we enjoy. In many ways, it is remarkable that the Earth has the narrow range of conditions for some essential consequences.

1. Size: The Earth accreted enough mass to be the size that it is; not too profound of a point, seemingly, but in fact the mass of the Earth is important. For instance, the mass is sufficiently large to have enough gravity to retain large gaseous molecules, such as N, O, H2O, CO2, but to allow H and He to escape into space. This provides a chemical screening selectivity for the particular chemistry of the atmosphere retained by the Earth. In contrast, a massive planet such as Jupiter is burdened by huge amounts of H and He, held to the planet by its massive gravity.

2. Temperature: One of the most remarkable conditions of the Earth is that its surface temperatures span the relatively narrow range of the liquidus and solidus of water. This is the 100 degree C range from boiling of water to freezing of water. The Earth conditions allow water in all three phases to exist on the planet (solid ice, liquid water, and gaseous steam). If the Earth were hotter all of the water would be in vapor state in the atmosphere, whereas if it were colder it would all be frozen in the ice caps. This would only take a shift of 100 degrees either way, out of the vast range of temperatures that exist in the solar system. A secondary remarkable coincidence is that water is one of the few materials that expands upon freezing (ice floats), which has played a key role in allowing organisms to survive under frozen streams. This peculiar physics exists throughout the Universe, but may have been key to evolution of life on Earth.

The surface of Venus is hot enough to melt lead, so there is never any ocean on the planet, and hence any life would have to evolve in the thick atmosphere. The hot surface is unable to cool and sink into the interior of Venus, unlike on the Earth, so plate tectonics does not happen there, or at least not in the same way as on Earth. Mars is cold enough that its water is permanently frozen into its ice caps. In the past, Mars may have been warmer, but was it ever enough so to allow free water? This is debated.

3. Distance from the Sun: Earth is at a favorable distance from the Sun in that the amount of solar radiation which heats the atmosphere daily does not drive huge damaging storms continually. Also, the radiation received by the Earth is relatively mild, yet enough to provide good visible light and modest UV that produces mild rates of chromosomal mutation to facilitate evolution.

These facts sometimes add up to such a Cosmic Coincidence that one may question whether there has been a grand design of some sort.

Moving on to the Dynamic Earth System, it should be recognized that all aspects of this system are studied in the Earth Sciences. This is a field of inquiry which brings physics, chemistry, mathematics and biology into the study of the Earth system. For example, the major fields are:

Geochemistry: Study chemical processes, melting, rock formation, erosion, cosmochemistry

Geophysics: Study physical processes of the solid Earth, liquid core, atmosphere and oceans. Convection, heat transport, magnetic field generation, gravity.

Geology: Study of rocks and surface processes using analytic tools and field observations.

Paleontology: Study of biological processes through time, evolution, extinction.

Results of Earth Sciences research will be presented throughout the rest of the course, to provide an understanding of the fundamental processes underlying catastrophic phenomena.

The Earth is, to first order, best described as a chemically differentiated, radially stratified planet. The original soup of atoms of the solar nebula from which the planet accreted have been processed through gravitational, chemical, and melting processes to produce the present Earth. In particular, once the primary mass of the planet was accreted, melting has played a key role in chemically separating (differentiating) the components of the Earth. The result is that a thin (6-70 km) thick crustal layer of very light rocky materials overlies a thick (2840 km) layer of mantle materials that are mainly Mg, Si, O rocky silicates. Below the Mantle lies the iron and nickel rich core, which has minor components of sulfur, silicon, oxygen and carbon.. The core is itself stratified, in that there is an inner core which is solid iron, and the inner core is slowly growing as the core cools. Melting during the magma ocean phase, accretion, and the core formation process produced the primary chemical layering of the Earth. Key to melting differentiation is that in a melt the light material rises and the heavy sinks very efficiently, which allowed the crustal materials to separate as well as the iron in the core. Blended all back together, the bulk composition of the Earth is thought to be very similar to that of primitive meteorites that fall to Earth.

The shallow Earth is of most concern in this class, both the upper mantle and the surface, for this is where the dynamic systems affect Humans as catastrophic processes. An important element of the shallow Earth is the upper 100 km which comprise the Lithosphere. This is a relatively cool, stiff region of the Earth that behaves as a rigid plate when the surface moves laterally. The plates of Plate Tectonics are a large mosaic of lithospheric chunks which move relative to one another. The lithosphere is a region of heat transport by thermal conduction, and the temperatures are below the melting temperature of rocks near the surface. Beneath the lithosphere, the temperature increases and the next 100-200 km are more ductile, comprising the asthenosphere. This region is partially molten in some regions, and does not move coherently, but shears readily, accommodating the motions of the lithosphere. It is the increase of temperature with depth that controls the transition in material properties (rheological properties, the mechanical response to stresses).

At greater depths in the upper mantle there are global increases in sound velocity at depths of 400 and 660 km. These are believed to be the results of phase changes, as the atoms in the crystal silicate structures collapse to denser forms which transmit sound more efficiently.

If we look at the surface of the dynamic system, the crust is the shallowest part of the lithosphere, and it differs between oceans and continents. A plot of the proportion of surface area at different elevations shows that the Earth's surface elevation is very bimodal, with most of the 30% of continental surface area being low elevation platforms only a few hundred m high, and most of the oceanic regions being about 4 km deep. There are localized regions of very high mountains on continents and localized regions of very deep trenches in oceans, each comprising only a small portion of the surface. To sustain even these small regions requires ongoing dynamic processes, as erosion quickly reduces mountains and deep trenches quickly fill with sediments. To understand the extremes of elevation we must consider the dynamic processes that are responsible.

The Earth has evolved, because it is a dynamic system, only about half way through its life cycle. In the past 35 years or so, Humans have attained a pretty good understanding of the dynamic system.

Key to the evolution of the solid mantle is that it is not really solid at all! The notions of solid and fluid are based on human time scale experience. We now know that while rocks are very solid for short-time scale loads, they do behave fluidly given steady application of forces over long periods of time. In fact, the whole mantle is convecting, with rising hot regions and sinking cold regions, turning over the whole mantle on a billion year time scale. This is solid state convection, and is the primary mechanism by which the planet is cooling off.

What happens is that hot rock rises to the surface, as crust pulls apart. This hot rock cools to form new crust, releasing the heat energy to the surface to radiate into space. The main regions where upwellings occur is under the central mountain ranges in oceans, which are in fact large volcanic edifices. The new ocean crust that is formed spreads laterally away from the mid-ocean rises in the process called Sea Floor Spreading. So, the age of the ocean crust is youngest at the rise and increases with distance laterally away from the rise. How is the creation of new sea floor balanced on a planet with constant surface area? What happens is that once oceanic crust and lithosphere age to 100-200 million years, they cool and become so dense that they sink back into the interior. This occurs near deep ocean trenches, as the surface is pulled down by the sinking oceanic plate. The margins where ocean plate sinks produce large volcanic arcs, either as islands or on continents. This is the site of most surface volcanoes (those at the rises are more extensive, but covered by water), as well as most large earthquakes. The cycling of oceanic crust/lithosphere occurs on a time scale much shorter than the age of the Earth, and therefore 70% of the planet surface is very young.

To find older rocks, we look to the continents, which are concentrations of lighter components which cannot sink back into the interior. The continents all have central nuclei of very old rocks, for example in North America the Superior Province rocks of Eastern Canada are 2-3 billion years old, and surrounding rocks are progressively younger overlapping layers. The continental rocks provide the geological record of all processes older than 200 million years; the oceanic rocks have all been recycled for early times. Thus, all old fossils, all reconstructions of past motions of the continents, and all inferences about mountain building events more than 200 million years old are based on continental rock records.

A key to looking at the past is provided by the fact that continental rocks preserve markers of where those rocks formed in space and time. We can date the age of the rocks by various means of radioactive decay, and we can tell the latitude at which they formed by the magnetic record preserved in the rocks. This allows us to fairly reliably reconstruct the history of motions of the continents over the past 600 million years. This time interval has seen great reshaping of the surface map, with the process of Plate Tectonics and the creation and destruction of oceanic plates moving the continents into great aggregations such as Gondwanaland and Pangea, or dispersing them as in the present state, with oceans spreading the continents apart. This is the surface manifestation of the mantle convection process by which the Earth is cooling.

We also know that the Earth's core is an active dynamic system by a somewhat different method, which is that the Earth has a magnetic field. In general, the present field is like that of a large magnetic dipole, with axial symmetry close to the spin axis of the Earth, giving rise to north and south magnetic poles. The magnetic field is not constant, but changes with time, and is known to undergo intermittent reversals of polarity. The origin of the magnetic pole is convection of the molten iron outer core. The movement of the liquid iron core in cylindrical columns surrounding the inner core, paralleling the spin axis, results in generation of the magnetic field. This is by dynamo processes, which involve the motion of the electrical conducting iron in the presence of a magnetic field, which in turn generates a magnetic field. The core convection regime is very turbulent and complex, but the net magnetic field at the surface has simple symmetry.

The atmosphere is a very complex dynamic system as well, and one that has changed composition dramatically through time. From an initial state in which the atmosphere was predominantly carbon dioxide, with no free oxygen, the Earth has progressively evolved to have a present oxygen rich environment (20% of the atmosphere). This transition has been critical to the presence of complex organisms, and in fact has proceeded in parallel with the evolution of life on the planet. The earliest cells did not use free oxygen, but produced it as a waste product, a mechanism that was incorporated into plant photosynthesis. Only in the last 600 million years has the oxygen level been sufficient for the evolution of complex shelled invertebrates and the higher forms of life enjoyed by Earth.

At every stage, the atmosphere has been a complex dynamic system, heated from the Sun and influenced by gravity and rotation of the planet. We will consider some of the complex systems, such as the water cycle, involving evaporation, transport, precipitation, ground water storage, etc.. Understanding the solid Earth and fluid Earth dynamic systems is essential for our understanding of consequent catastrophic phenomena accompanying these systems, as well as to our ability to project the consequences of human activities in these dynamic systems.

"The Earth's history of life dates back to near the beginning, and the impact of life on the Earth system has itself been profound."

-- T. Lay

The most interesting aspect of the Earth system is that life developed in it, and together the animate and inanimate elements of the system have journeyed through time. The course of life has been dramatic, with great diversity of lifeforms through time, massive catastrophic death events, and even dramatic changes of the atmospheric chemistry. As we delve into the Earth Catastrophes affecting life, it is important to first come to some working definitions and understandings of life itself.

For the scientific perspective of this course, we view life as a PROCESS, involving constant change. Yet, how to distinguish this from the process of thermal convection in the mantle, which has constantly changed the Earth's surface, or the atmospheric circulation that leads to storms and tornadoes? The biologists define some specific activities of the process which distinguish living systems from inanimate ones:

Life Activities involve:

* Reproduction/Birth

* Metabolism (Intake, use, waste elimination of materials in the environment).

* Death

While mantle convection uses up heat energy ultimately will freeze up and die, there is no perspective of it reproducing, or passing on its essence to a successor. The life activities can be defined on the macro-scale of the entire organism, or on the micro-scale of the smallest life unit, the CELL.

On Earth, all lifeforms are based on carbon compounds, such as

* CH4 (methane)

* C2H6 (ethane)

Carbon bonds into complex molecules, and allows for very complex structures. There was fairly abundant carbon in the primordial soup of materials incorporated into the early Earth, and exhumed from the rocks in volcanic emissions. This soup involved C, N, H, O, and other elements that had been produced inside stars long before. As the magma ocean phase ended, and water could reside in liquid phase on the surface of the Earth, C began to react in the soupy mixture to build up larger and larger carbon compounds. We recognize the following building blocks of even the simplest cell:

* Amino Acids (20) 5-27 atoms, involving C-H primarily

* Nucleotide Bases (5) larger stable forms of carbon compounds

* Proteins (thousands of varieties) such as insulin, hemoglobin. Complex molecules

* Nucleic Acids (DNA,RNA) may involve more than 100 million nucleotide bases

The Nucleic Acids, built up of large agglomerations of ordered carbon compounds are distinctive in having the ability to split and self reproduce. This function opened up the chemical process of renewal and reproduction.

If Amino Acids are the fundamental building blocks, are they the rate-limiting factor in the development of life? Is it hard to make amino acids? These questions were addressed in the 1950s, and in 1953 Stanley Miller conducted an experiment that showed that amino acids are readily produced. His experiment was to mix a primordial cocktail of steam, ammonia, methane, hydrogen and water in a large vat, and jolt the mixture with electrical discharges that input energy into the system. The input of energy catalyzed reactions, that lead carbon reactions to produce amino acids that precipitated out of the fluids. Having constructed his cocktail to replicate early Earth chemical conditions, and recognizing that lightning, impacts, eruptions, etc., provided ample energy for the environment, it became clear that amino acids would have come into existence quite readily. This is supported by more recent discoveries of amino acids in 4.5 billion year old meteorites, which suggest that some of the materials may have come in from outer space as well, and that the building blocks of life are not isolated to Earth.

But, it is a long way from amino acids to the Cell. Allowing for a soupy concentration of carbon compounds under stable conditions, how could the Cell come into existence? In the 1930s de Jong found that rich amino acid cocktails do in fact produce spontaneous agglomerations in the form of spherules. These may involve vast numbers of nucleotide bases and complex amino acid systems.

In the 1950's it was found that comparable complex protenoid spherules could spontaneously form. These spherules are not yet cells, but are essentially semi-organisms that may chemically work toward more complex functions.

In the 1990s the view is that cells may evolve from symbiotic populations of such semi-organisms, as energetically favorable chemical pathways are found in a particular agglomeration.

While this relatively agnostic take on the origin life still lacks a complete understanding of how the process came into being, it is agreed that scientifically life is a general chemical process prompted by the many reaction opportunities of carbon molecules.

The early Earth environment was highly reducing, and corrosive, and no free oxygen was around for use by lifeforms. How could life have formed in this hostile environment? Part of the answer has actually been provided by Earth Sciences. This came with the discovery in 1977 of deep sea hot springs, in regions where upwelling mantle rock comes close to the surface at mid-ocean ridges. Water circulates down into the crust and is heated by the magma, and rises out of the rock in hot springs. The water is enriched in rock materials and gases leached from the rock. This environment is actually very reducing, as there is no free oxygen coming from the rocks, thus it is an analog to the early Earth (such leaching has gone on through the ages). What was most remarkable was that many strange organisms were found near the hot springs, and these had evolved to use the materials in the reducing environment. Rather than photosynthesis, they rely on chemosynthesis, extracting energy for life from H2S (hydrogen sulfide) carried in the rising vents. These very distinctive forms of life, with completely different pathways to existence than most Earth life may have strong parallels with early lifeforms. Indeed, such deep hot vents may have been the main source of energy and environmental stability for life to evolve, given the heat from the geothermal engine and the sheltered status deep in the ocean. There is evidence in the DNA of most organisms that have evolved to survive in the current oxygenated environment that suggests we all have common links to chemosynthetic predecessors.

In the early Earth "Archaen" times, the Earth system was not only highly reducing, but there were other differences from the present:

1. Earth was hotter, both because it had formed recently and it was richer in radioactive materials that have decayed.

2. Convection of the mantle was more vigorous, and the plate tectonics process ran faster. There was more volcanism, and many more earthquakes.

3. Volcanic exhalations provided H2O and CO2 to quickly build up the atmosphere and hydrosphere. Large impacts may have repeatedly burned these off, but eventually they got established.

4. Light rocks like granite separated from molten mantle rocks and formed the continents, which persisted even while ocean lithosphere was continually recycled.

5. The period of great bombardment wound down, and impacts were infrequent after 3.9 billion years ago.

Throughout this initial 600 million years, it is unlikely that life could survive with any continuity, and it is speculated that the diminishing rain of meteorites consistently reset the diversity of life back to near zero again and again. This could have been by vaporizing the oceans or melting the surface. But, again and again the building blocks would form in the post cataclysmic environment and time would work to allow chemical reactions to enhance the complexity of the system. Early life would likely have started in the deep seas and muds: We are the stuff of ooze....

Finally, by 3.5 billion years ago, life took hold and has not yet again totally relinquished its presence on the planet. How do we know? We actually have rocks that are 3.5 billion years old that contain fossils of simple bacteria that lived then. These are single celled structures with no nucleus (prokaryotes), and are found in rocks of the oldest central shields of continents. At the time that these organisms lived, the environment was more hospitable than in the Archaen, but there were major differences still:

1. the Sun was 75% as bright as today

2. the atmosphere was a carbon dioxide CO2 greenhouse, very warm and deadly to us

3. the moon was close to Earth and there were much stronger tides, which particularly influenced the shallow water environments where many lifeforms exist.

4. the Earth rotated faster, in about 18 hours. Thus, there were 500 days/yr.

From the simple bacteria 3.5 billion years ago, there were slow increases in complexity of organisms. By 3 billion years ago algae were present, and left the record of their existence in large pods called Stromatolites. These still form today in regions with favorable tidal conditions. A major development was that photosynthesis was discovered as a means for releasing chemical energy, and many simple cells adopted it. This led to progressive conversion of CO2 to O2, and slowly the percentage of oxygen in the atmosphere increased. About 1.5 billion years ago cells began to have nuclei, and are called Eukaryotes. This allowed more complex specialization of cells, which in turn led to methods for transporting oxygen around in the organism to exploit its favorable chemical reactivity. Thus, the availability of free oxygen, itself produced by the early life, allowed life to seek new ways of tapping chemical energy. This feedback spawned great complexity, and 700 million years ago there was great diversification of more and more complex organisms, involving many cells that now differentiated to achieve separate functions, with oxygen transported through the system. Most were soft-bodied creatures, but some were preserved in special geological rock formations such as the Burgess Shale. Some were so weird, they were called Hallucigenia by the paleontologists that discovered them.

Soon after, the first exoskeletons appeared, such as trilobites, and within the last 500 million years the vertebrates and land plants appeared.

The last 600,000,000 years have been particularly interesting for life, after several billion years of boring bacteria, algae and mushy creatures. We will see in a later section that the very changes in lifeforms have defined the geological time scale, with major boundaries in the scale associated with massive extinctions.