The Solar System. Formation of planets. Meteorites, composition and age. Primitive earth. The origin and cosmic abundance of the elements.
It is believed that the earth accreted from the solar nebula by collecting gas, dust and solid particles orbiting the recently formed sun. The recent image from the Hubble Space Telescope below offers a fascinating view of a "star nursery" seen at the top of the dark pillar-like structures, which actually are columns of cool interstellar hydrogen gas and dust, incubators for new stars. The pillars are part of the "Eagle Nebula", a nearby star-forming region 7,000 light-years away in the constellation Serpens.
( Click on the figure for a higher resolution version).
At a mean distance of 150 million km the earth formed out of a -mostly solid- agglomeration of chondritic material. It is debated whether the accretion process was such that differentiation into the layered structure we see today occurred during or after accretion. The two competing models are referred to as inhomogeneous and homogeneous accretion, respectively. After the earth reached its present size, the number and size of accreting masses (asteroids, planetesimals) decreased substantially although for a few million years many still bombarded the surface and kept the earth's uppermost layers in a semi-molten state.
Additional material on the planets, asteroids, planetary systems external to the solar system etc, can be found at Planets of the Solar System
Cometary and asteroidal impacts were not only of importance during the earth's formation; they have probably been important factors in the subsequent and even recent history of the planet. The 1994 impact of comet Shoemaker-Levy on Jupiter is a reminder that many asteroids, comets and other objects are still around, some on orbits that may cross that of earth's...
Below is an image of planet Jupiter showing the many impact sites of comet Shoemaker-Levy ( UV band, Hubble Space Telescope ). 
The image shows the sites of the impacts on Jupiters southern hemisphere. The two larger atmospheric "holes" left in the atmosphere of Jupiter by the L and D/G fragments are approximately the size of the earth! In both cases it is suspected that each comet fragment was about 1 km in diameter. The energy released on earth by such an impact would have been close to 100,000 Megatons of TNT equivalent yield, assuming an impact speed of 20 km/sec and density of 3 gr/cm3. At the end of the Cretaceous, the Chicxulub asteroidal impact was much more energetic indeed, about 1,000 times more (Why?).
A complete description and many pictures, animations, movies and the most detailed information about the impact of comet Shoemaker-Levy on Jupiter is in the Jet Propulsion Laboratory pages. Click on the above link to get there.
An excellent description of Terrestrial Impact Craters can be found at this Web site, from Los Alamos National Lab. The page has pictures of terrestrial impact craters from all over the world and descriptions of them, as well as the most up-to-date listing of all impact craters known; with information on the age, diameter and probable composition of the impactors.
Asteroid/comet impacts
The impact of comet Shoemaker-Levy on Jupiter in the summer of 1994 made everyone aware that asteroidal or cometary impacts are not fictional occurrences, and the many blockbuster summer movies recently dedicated to the subject proves that the idea of cosmic destruction by such rocks of doom caught on with the general public. Asteroidal or cometary impacts can cause enormous disruption of the Earth's climate, especially because of the enormous release of energy that accompanies the impact of even a mid-size (~3 km diameter) comet. The energy of impact is almost purely kinetic (proportional to the product of the mass times the velocity square) and thus a doubling of the impact speed implies a four-fold increase in energy. This also implies that the impact of a relatively small comet can produce an explosion equivalent to that af a much larger asteroid. It is estimated that the explosion produced by the impactor believed to have caused the mass extinction at the Cretaceous/Tertiary boundary (K/T boundary) released energy equal to about 1000,000,000 Megatons of TNT equivalent, in an explosion about five million times greater than the largest thermonuclear explosion ever detonated, equivalent roughly to a magnitude 13 earthquake, and 10,000 times the total explosive energy in all the world's nuclear weapons arsenals.
It is a well known fact that temperature in the Earth increases with depth below the surface. Mine workers have long been aware that as they drill deep into the Earth's crust, temperature of the rock increases at rates of the order of 20 deg K/km or more. Such is the global average of the geothermal gradient This is a direct observational fact that, if extrapolated, could allow us to determine the temperature all the way to the Earth's center. Unfortunately, we soon encounter a difficulty. If the 20 deg K/km persisted to a depth of say, just 60 km, the temperature would reach the melting points of normal rocks, about 1,500 deg K. We know however that seismic wave evidence strongly suggests that at such depth the crust or the mantle are both solid. This means that the geothermal gradient must somehow decrease into the deep interior, and hence that the amount of heat generated internally from radioactivity must also decrease with depth in the mantle. Only in this way can we expect the temperature-depth profile from reaching melting point levels at depths where there is no evidence of mantle liquidity.
Fourier's law of heat flow applied to the earth gives
where q is the heat flux at a point on the Earth's surface, M the thermal conductivity of the rocks there, and g the measured geothermal gradient. A representative value for the thermal conductivity of granitic rocks is M = 3.0 W /m degK. Hence, using the global average geothermal gradient of 0.02 deg K/m we get that q = 0.06 W/m2 . This estimate, which is corroborated by integration of many thousands of observations of heat flow in boreholes all over the world, gives a global average of 63 mW/m2
The map above is the global heat flow, from a compilation by H.N. Pollack and his collaborators at the University of Michigan, Ann Arbor. Note that most of the flow is along the mid-oceanic spreading ridges and that most of the heat is evacuated in the southern hemisphere (Can you think of e reason why, or is this just coincidence?)
Thermal convection is the most efficient way to transport heat down a thermal gradient. 
In the earth's mantle, it has been recognized as the physical mechanism responsible for continental drift. It supplies the energy dissipated by earthquakes, and the thermal energy dissipated by volcanoes.
One important approach to understanding the workings of the earth's internal heat is through computer modeling of convection systems that help explain the known geometry, structure and evolution of the lithosphere. One of the major difficulties in the modeling is to be able to simulate the great difference in rheologies between the plates and the mantle.
Usually, results are shown in the form of movies or animations. One of them, called
"Supercontinent Aggregation and Dispersal" (1988) by M. Gurnis, B. H. Hager, and A. Raefsky (Size: 1.9 Mbytes) is available here. Just click on the picture (above, right) and if you have a mpeg viewer ( mpeg viewers are available free through the Internet ) you will be able to see an animated sequence of the two-dimensional model in which the plate on the right hand side of the picture is split in two by the ascending plume. ( The movie has over 1000 frames so it will take a few minutes to load ). Other remarkable examples of mantle convection modeling are found in the home pages of
Caltech's Seismological Laboratory.
Heat from the earth's interior is the ultimate source of the motions experienced by the plates, with the convection currents in the mantle being the consequence of the general cooling of the planet. The plates move very slowly (typical speed of the plates is close to the growth rate of human fingernails) but constantly, unrelentingly pushing and rubbing against each other, building up mountains and high plateaus, or drifting away from each other, opening up oceanic basins and rifts. An online text on plate tectonics is This Dynamic Earth from the USGS library.
Age of the Crust. One important consequence of the convective regimes of the mantle is that the crust under the oceans is in average much younger than the crust under the continents. This is because oceanic crust is relatively denser and sinks into the mantle, whereas continental crust being lighter tends to stay afloat, forming the core of continental masses. 
A glance to a geologic map of the earth will convince you also that the geologic structure of the ocean floor is extraordinarily simple, as compared to that of the land. From a structural point of view the ocean floors are just flat slabs of almost undeformed basalt...
This recently released map (left) showing the age of the ocean floors (click on the map for a higher resolution version. The globe on the right shows the same information without projection distortion) tells a very convincing story, one that is thoroughly consistent with the idea that the oceanic floor is "recycled" by convection. Just click anywhere on the map surface to see the high resolution version where the time scale is clearly visible. Notice the location of the oldest oceanic crust and that of the youngest. In the map, the oldest rocks of the ocean floor are only 180 million years old.
The Geoid The geoid is that equipotential surface of the Earth gravity field that most closely approximates the mean sea surface. This surface is also called the figure of the earth. At every point the geoid surface is perpendicular to the local plumb line. It is therefore a natural reference for heights - measured along the plumb line. The geoid surface is described by geoid heights (anomalies) referred to a suitable Earth reference ellipsoid. Geoid anomalies are small and amount up to 100 meters. At the same time, the geoid is the most graphical representation of the Earth gravity field. The earth's largest positive geoid heights are associated with subduction zones and hotspots and have no simple relationship to other elevated regions such as continents and ridges. .
The figure below is the geoid of a section of North America that includes the entire United States and parts of Mexico and Canada. Geoidal highs are red and lows are purple.
. ( Click on the map for a higher resolution version).
The geoid reflects the dynamic structure of the earth's interior. The most obvious feature of the geoid in this map is the Yellowstone Hot Spot, believed to be a plume of hot material rising through the mantle perhaps from as deep as the D" layer. The role of this hot spot on the global pattern of mantle thermal convection is not yet clear. Further to the west, the San Joaquin valley in California appears as a clear remanent of the subduction process in western North America.
Another noticeable feature is the mid-continental high, an ancient tectonic rift or suture zone that runs from the western tip of Lake superior through Minnesota, ending in Texas. In the eastern U.S., the geoid shows a large low centered offshore the Carolinas, coinciding with the location of the oldest oceanic floor. 
To the very top of the map the broad low reaching as far south as the Great Lakes corresponds to the depression created in the crust by the ice sheet of the last ice age.
Compare the geoid on the US to its major topographic features (left). Do you see any connection? One way to discover what relationships are there between the geoid and the topography is to examine the two maps on high resolution and spend some time comparing their major features, such as the Rocky Mountains, the Appalachians, the central plains, the San Joaquin valley, etc. There are some features that appear in both maps, but there are others that while appearing prominently in the geoid map are absent or show a very subtle signature in the topo map. (Click on either map to get a higher resolution version).
Earth's interior. An entertaining description from the inner core to the uppermost crustal layers is available at the University of Nevada's Earth's Interior. Read about the fact that the Earth having a magnetic field is an independent piece of evidence for a molten, liquid core. A compass magnet aligns with the magnetic field anywhere on the Earth, but other bodies like the Moon and Mars have no or very weak magnetic fields. The earth cannot be a large permanent magnet, since magnetic minerals lose their magnetism when they are hotter than about 500 degrees C. Almost all of the earth is hotter, and the only other way to make a magnetic field is with a circulating electric current. Circulation and convection of electrically conductive molten iron in the Earth's outer core produces the magnetic field. To make the magnetic field, the convection must be relatively rapid (much faster than it is in the plastic mantle), so the core must be fluid. Does the earth's magnetic field rotate with the earth? And if it does not, why? And if it does, why? .
GPS (Global Positioning System) has become an important means to measure plate motions and displacements produced by earthquakes. The link here is to
GPS-Geodynamics a Web page with many subsidiary links that describe many GPS applications in the earth sciences. In essence, the system works by having GPS satellites transmit signals that allow one to determine, with great accuracy, the locations of GPS
receivers. The receivers can be fixed on the Earth, in moving vehicles, aircraft, or in low-Earth orbiting satellites. The time evolution of receiver locations allows researchers to study the motions of tectonic plates, displacements associated with earthquakes, earth orientation, and other geophysical interesting phenomena. Outstanding results were recently obtained for the magnitude 7.2, Jan 17th, 1995 Hyougo-ken Nambu earthquake (also known as Kobe earthquake). Inspect the data on the Geographical Institute of Japan (GSI)
One of the most interesting results to date in the application of space technology to seismology and tectonics was obtained by GSI researchers from what is called "radar interferometry", 
whereby two or more satellite microwave-radar images of a region taken before and after an earthquake are compared creating thus a pattern of interference ( optically, this would be analogous to the fringes seen in any interferometric analysis, such as Newton's rings ) tracing out contours of equal land displacement if there has been a change in the height of the spacecraft over points on the earth's surface. The figure (left) shows the radar interferometry result for the island of Awaji, southeast from the epicenter. The map image of Awaji Island shows eight or more
colored interference fringe lines approaching the fault, at
11 cm of vertical displacement per fringe contour, demonstrating almost 1 meter of uplift by the earthquake. That is, this part of the crust was uplifted by about one meter as a consequence of the fault displacement, even though the causative fault had a left-lateral strike-slip displacement. How can this be?
In the GSI home pages you can see more of these remarkable fringes along the the coast and through the city of Kobe, showing about 20 cm of displacement across the city from the buried fault.
One of the most impressive results of these new techniques is much closer to home: radar interferometry of the Landers earthquake epicentral area. This and other spectacular examples of new technologies applied to monitoring current global dynamics are available from the Active Tectonics web page, a new NSF program for interdisciplinary study of tectonism and its societal effects. Be sure to visit it.
If you would like to see just how earthquakes are located and how magnitudes are determined, visit the Virtual Earthquake Laboratory. This is an interactive link that illustrates nicely how triangulation was used in the old days to determine epicentral location and how nomograms were used to determine Richter magnitude. Although presently the work is done automatically by computers, the principles are basically the same. The exercise is excellent practice for those interested in earthquake monitoring.
To monitor the earth's earthquake activity there is an interactive educational display (produced by I.R.I.S. and the University of Washington) of global
seismicity that allows you to monitor earthquakes in near real-time,
view records of ground motion, and visit seismic stations around
the world.
The display is updated every 30 minutes from the National Earthquake Information Center. Earthquakes that have occurred within the last 24 hours are shown and stay on the map for 15 days as circles of changing color. After 15 days, the circles are replaced by light purple dots that remain on the map for five years. The distribution of seismicity over the past 5 years demonstrates how earthquakes define the boundaries of tectonic plates, and the relationship between topography and seismicity.
Climate through geological time. The greenhouse effect and its role in the history of climate and evolution of life. Present climate. Atmospheric physics and the ozone layer. Energy balance models (EBM). Monitoring of global climate change. General circulation models (GCM).
There are a number of links to Web sites with information about Climate Change. I suggest you start from the following link to the United Nations Environmental Program, especially the Intergovermental Panel on Climate Change (IPCC) web site, with the latest scientific data and analyses on the state of the earth's climate and other documents related to Global Change.
The National Geophysical Data Center (NGDC) manages environmental data in the fields of solar-terrestrial physics, solid earth geophysics, marine geology and geophysics, paleoclimatology, and glaciology (snow and ice). In each of these fields it also operates a World Data Center (WDC A) discipline center. Data, meta-data, and information at NGDC are available via the Internet using Web browsers, Gopher, and Anonymous-FTP. While not all of our data holdings are available through NGDC's Geophysical On-Line Data (GOLD), new data and information are continually being added. Visit NGDC at the link above.
GLOBE visualizations is one of the most interesting web sites that deal with actual data on the changing earth's environment. Here you can see for the global day's maximum or minimum temperatures, evaporation, albedo, vegetation cover, in an interactive manner. For instance, the figure below was downloaded the day it refers to: July 17th, 1997.
One of the most advanced computer programs that atmospheric scientists use is called GENESIS. This is one of the latest (late 1995-early 1996) Global Circulation Models (GCM) with which climatologists can reproduce the conditions of the atmosphere and the oceans (as well as their interactions) when subjected to prescribed disturbances, such as increase in the greenhouse gases, the seasons, the ice sheets of the Pleistocene, etc. The code runs normally on a supercomputer.
The purpose of the visualization effort of the
GENESIS Earth System Modeling Project is to aid in the analysis of
global climate research and communicate its findings through the creation of broadcast quality video products.
These video products are in turn suitable for use by students and educators at many levels. An example of what the GENESIS program can do is available here. A click on the blue earth icon will download the "Dance of the Seasons" movie to your computer.
Research on global change using this and other modeling tools includes the study of climate changes of the past. For instance, an important use of GENESIS is in providing geologists with an accurate view of the climatic consequences of the huge asteroidal impact 65 million years ago, at the end of the Cretaceous period.
Climate change and the immediate future
An example of actual data of the way in which industrialization influences the climate of the earth is the CO There are other periodicities associated to the behavior of the atmosphere and the earth's climate whose origin is not as clear however. An example is shown and briefly discussed in the next section.
There are many sites on the Net that describe fractals, show amazingly intricate pictures and dazzle you with filigrees and computer fakeries. There are some fractal Web sites however that deserve a second look. This is a good one BACK TO THE TOP
The Origin of the Earth
6. THE EARTH SYSTEM.
From the inner core to the top of the atmosphere, the Earth is a complex system of interconnected parts. Superplumes. The Ice Ages of the Pleistocene. El Nino. Mass extinctions and asteroid impacts. Is there a correlation between geodynamic reversals, intense volcanism, hot spots, climate and mass extinctions?
The Earth system as seen from the Galileo spacecraft
..and closer to home, the US East Coast
There are a million lightning flashes every year, world wide
... and every four years or so, El Nino
BACK TO THE TOP
The Origin of the Earth
7. PRINCIPLES OF FRACTAL GEOMETRY AND CHAOS.
Fractal geology. Power laws and self-similarity in earth's sytems. Predictability vs randomness.
Fractal structures are fairly common in nature, and in geological environments fractals are perhaps more common than in any other natural system. The following picture is a simulation of crystal growth by a method known as DLA, or diffusion limited aggregation. Note the fine structure of the "crystal", it is self-similar indeed.
An introduction to geodata searching on the Internet is in the Earth Science Resources of the UNC-CH Geology Department.
jar@email.unc.edu
This page maintained by J.A. Rial. Please send me your comments and suggestions at this email address. Students taking geo15 can send their answers to the questions posed above to the same address.
Back to Geology Homepage