Responses to the Questions Posed by Judge Alsup

Question 1. What caused the various ice ages (including the “little ice age” and prolonged cool periods) and what caused the ice to melt? When they melted, by how much did sea level rise?

Ice ages occur on our planet on two entirely different scales.

In the large scale, three “ice ages” of different durations, with warm intervals between lasting hundreds of millions of years, have taken place in the Phanerozoic, the “period of abundant life” of the last 700 million years. No known orbital parameters can explain these glacial episodes.

The small scale ice ages, to which you likely refer, are oscillations within the third, and most recent of the large scale ice ages above. We call this third large scale ice age the Pleistocene. It began about 2.5 million years ago, and we are still very much in it. Orbital variations, particularly obliquity, a ~41k year variation in the earth’s wobble, are clearly an influence–but not a controlling influence–on these oscillations. We simply do not know what caused the large scale ice ages, the glacial/interglacial oscillations within the Pleistocene, the little ice age, or the Roman and Medieval  warm periods. CO2 is not implicated in any of these changes.

The Oxygen isotope content of marine sediments cored in the Ocean Drilling Project show us the descent into the current ice age. We live at the warm peak at the extreme right side of the graphic. It is somewhat less warm than the three preceding “interglacial”  peaks. The preceding low points are times when there was a mile of ice on the Great Lakes.

To the left of the Mid Pleistocene Transition, the dominant periodicity is ~40k years. To the right of it, the period changes to ~100k years, and the amplitude of variation continues to increase.

Sea level has varied enormously through these glacial/interglacial oscillations.

It can be seen that sea level has risen some 130 meters since the most recent glacial maximum. It can also be seen that the most rapid increase ended about 8k years ago, and we are on the long tail. Current estimates of sea level rise vary between 1.5 and 3mm per year.

It can be seen that Greenland ice mass was lower in the 1930’s than it is today. According to NASA, Antarctic ice mass is currently increasing.

2. What is the molecular difference by which CO2 absorbs infrared radiation but oxygen and nitrogen do not? 

 

When the earth’s surface absorbs shortwave radiation from the sun, it warms, and creates its own longer wave (IR) radiation according to the earth’s much lower temperature. CO2 is a very efficient absorber in the earth’s longwave emission spectrum.

CO2 is a linear molecule. Its modes of bending and stretching and the applicable wavelengths are shown proportionally above. CO2 is unusually dependent on its fundamental bend at wave number 667.4 or about 15 microns. It absorbs so strongly in this band that all the earth’s radiation is completely absorbed, even at preindustrial CO2 concentrations.

The MODTRAN model above shows the difference in radiation to space between 280 ppm (preindustrial) and a “doubled” 560 ppm concentration. We are currently about 410 ppm. The differences between the red and blue are very subtle.

CO2 definitely absorbs outgoing longwave radiation the earth emits. The amount of warming this causes is limited by the saturation of the fundamental bend. The differences of absorption between CO2, Nitrogen, and Oxygen are just brute facts of the material properties of the gasses.

3. What is the mechanism by which infrared radiation trapped by CO2 in the atmosphere is turned into heat and finds its way back to sea level?

“Heat” is the net kinetic energy of all the molecules of the atmosphere. When a molecule absorbs radiation, it bumps into its neighbors more, raising the temperature. Warmed air rises, so this “conduction” by kinetic interaction is not an efficient way to get absorbed energy from any significant altitude back to sea level.

Warmed molecules also radiate in all directions, including down. This radiation can reach the surface, but it is subject to reabsorption along the way.

Above is the MODTRAN program at one meter elevation looking up in blue and down in red at 400ppm with CO2 set as the only greenhouse gas. The red looking down shows no deviation from the Planck curve. This is the case up to about 100 meters, so up to this altitude the atmosphere radiates upward like a brick. Looking up in blue, the atmosphere is radiating downward at nearly surface temperature in the CO2 bands, but falls off quickly to very low temperatures and intensities outside the CO2 bands. This radiation warms the surface.

4. Does CO2 in the atmosphere reflect any sunlight back into space such that the reflected sunlight never penetrates the atmosphere in the first place?

No.

CO2 is a very minor absorber of incoming short wave radiation from the sun. It will re emit radiation ~2000 nm it has absorbed, but it is colorless, and does not reflect at all.

5.  Apart from CO2, what happens to the collective heat from tailpipe exhausts, engine radiators, and all other heat from combustion of fossil fuels? How, if at all, does this collective heat contribute to warming of the atmosphere?

This energy, including the heat radiating from our bodies, is insignificant at planetary scale. It becomes important to the tune of several degrees C in big cities.

6. In grade school, many of us were taught that humans exhale CO2 but plants absorb CO2 and return oxygen to the air (keeping the carbon for fiber). Is this still valid? If so, why hasn’t plant life turned the higher levels of CO2 back into oxygen? Given the increase in human population on Earth (four billion), is human respiration a contributing factor to the buildup of CO2?

Humans exhale about 3 billion tons (Gt) of CO2 a year , or nearly 1 Gt Carbon. Human combustion is about 10 Gt Carbon. Soil respiration is about 60 Gt Carbon per year. The annual global Carbon cycle is hundreds of Gt per year. Atmospheric CO2 is increasing, but at only half the rate of human production. The planet is greening as a result of the increased atmospheric CO2.

7. What are the main sources of CO2 that account for the incremental buildup of CO2 in the atmosphere?

Human CO2, Soil respiration, land use reduction of chlorophyll, ocean outgassing; in that order. The exact proportions are unknown.

8. What are the main sources of heat that account for the incremental rise in temperature on Earth?

The answer to this question is unknown. Humans are certainly responsible for some increase in temperature, but the increase in temperature we have seen is not exceptional. Enormous variations in temperature took place long before humans, and these changes were not driven by CO2. Careful measurements in the ice cores extending back 800k years show that without exception, changes of temperature preceded and controlled changes in CO2.  

We come full circle to the first question. Until we understand why the planet will go from paradise to ice, and within these ice ages to oscillate between warmer periods like we live in and colder periods when the ice piles up; we cannot say with certainty what the main sources of heat are that account for the recent incremental rise in the temperature on earth.

 

 

Advertisements
Posted in Climate, Judge Alsup Questions | Leave a comment

CERES and MODTRAN III

In the first post in this series we began with an assertion that according to greenhouse theory, it is impossible to warm the planet when longwave radiation to space is increasing. In the second post we discovered that it is possible for the planet to warm while longwave radiation to space increases if shortwave absorption increases. The first post has been corrected.

If shortwave absorption is increasing, the questions become, “How?” and, “Where?”.

NOAA ESRL has a long time series of atmospheric precipitable water (TPW) extending back to 1948. This time series shows a very small increase to the present and can be characterized as moving from higher values after WWII to a nadir in the late 70’s/mid 80’s, and increasing again to the present; much like surface temperature.

In the graphic above this TPW series is compared with RSS satellite lower troposphere (TLT) data. The correlation is not impressive, but there is reasonable agreement.

RSS has its own shorter TPW time series that excludes polar regions, and it is compared with ESRL TPW above. The correlation is poor, as will result from the trends being inverse from 1989 to 1997; but once again there is reasonable agreement. The burning question is what the satellites would have seen between 1948 and 1988. A reasonable supposition is that they would have generally tracked ESRL, with a poor correlation.

When the RSS TPW is compared with RSS TLT above, the correlation is good, but the trends appear divergent.

The point of all this is that if our increased shortwave absorption from CERES takes place in the atmosphere, it is probably going to be from increased atmospheric water.

 

We colored this Wikipedia image blue in the water shortwave absorption bands a while back. Oxygen, particularly in transition to ozone is a significant absorber in the SW solar spectrum. We will see when we get to MODTRAN that there is significant ozone formation near the surface as well as the stratosphere. CO2 absorption in the SW spectrum is limited to small areas near 2000 nm, shown green.

In the bands where the surface irradiance is decreased by atmospheric absorption, but some light still makes it through; adding more gas will increase the warming of the atmosphere and further decrease the warming of the surface. In the water bands where surface irradiance is already zero, adding more water will have no effect on the surface. These bands are about 19% of the total absorption by water.

The amount of light can’t change much. Like the sun, the earth radiates at an intensity and wavelength governed by temperature. In this sense one can think of the earth as the sun for outgoing longwave, and the atmosphere is caught in a crossfire between the two.

The crossfire becomes particularly wicked for reflected shortwave from the sun. In this case, the graphic above must be turned upside down. The atmosphere gets a second chance to absorb the photons that made it through the first time, and the longwave from the earth and the shortwave from the sun are both heading for space. The longwave that makes it out reduces the temperature of the planet, the shortwave that makes it out does not.

Even after all the atmospheric absorption and reflection from clouds, over half of the incoming solar SW makes it to the surface. Something like 7% is reflected from the surface, leaving just less than half absorbed by the surface. The surface is warmed, and radiates longwave out. Seventy percent of the surface is water, but as the graphic above shows, the shortwave absorption by water is at its lowest at the peak intensity of solar SW radiation.

The peak of solar intensity is in the visible wavelengths, and any diver knows that visible light penetrates to considerable depth, depending on turbidity. The ocean is still absorbing SW, albeit less efficiently than it might.

We really don’t have the tools yet to determine whether the atmosphere or the surface is where the CERES increase in SW absorption is predominantly taking place. Sea surface temperature, atmospheric temperature, and atmospheric precipitable water are all rising; together, as the laws of physics decree they must.

According to NASA (and Mr. Trenberth), the surface is radiating more energy up as longwave than the TOA receives as shortwave. We can address this, and longwave radiation to space, with MODTRAN in the next post.

 

Posted in Climate | 2 Comments

CERES and MODTRAN II

The earth does not burn at 5778 K like the sun, and therefore produces no shortwave radiation of its own. The earth radiates at about 288 K, a temperature that produces longwave radiation. When molecules absorb radiation according to their individual properties, higher energy shortwave radiation tends to produce “electronic” transitions where electrons jump to higher energy levels. Lower energy longwave radiation tends to produce rotational and vibrational transitions, where the atoms within the molecules move in relation to each other. Shortwave radiation is more likely to scatter than longwave.

In the last post we showed that longwave radiation from the top of the atmosphere has increased over the period of the CERES data. The earth is believed to receive a steady 340 W/M2 in shortwave radiation from the sun. About 30% of this is reflected back to space from light colored surfaces. The possibility remains that if this reflectivity or albedo were to decrease, increasing the shortwave energy influx, the increase in longwave radiation might not cool the planet.

Fortunately, CERES gives us a net TOA flux that includes shortwave in, shortwave out, possibly some longwave in from cosmic microwave radiation, and longwave out.

 

The net flux is defined as incoming minus outgoing, so positive values mean the planet is absorbing energy and negative values mean we are losing energy to space. This is opposite the LW flux, in which higher values mean greater energy loss to space. We see that net energy absorption, like LW loss to space, is higher in the northern hemisphere summer months, and that the planet loses energy in the southern hemisphere summer. The Min/Max values show a total range of some 27 W/M2. Photosynthesis consumes 5-10 W/M2 annually, depending on assumptions of conversion efficiency, and the far larger extent of photosynthesis in the northern hemisphere higher latitudes can explain part of this swing as increased SW absorption.

 

 

When we compare LW flux with net flux above, we see that net flux is always positive, in spite of increasing LW flux’s efforts to cool the planet. There appears to be a signal in both LW and net fluxes to the moderate el Niños of 2003 and 2010; and a surprisingly strong (and delayed) response to the weak 2006 el Niño. The very strong 2016 el Nino shows a strong LW response, but no net response. Unfortunately, the record does not extend back to include the comparable “very strong” Ninos of 1983 and 1998.

 

 

When SW flux is compared to net flux it becomes apparent that a very considerable decrease in SW loss to space (=increased absorption) has taken place in the CERES record. The increase in LW flux to space is on the order of 1 W/M2, and the decrease of SW flux to space is on the order of 1.5 W/M2 over the period of record.

 

 

It must be noted again that January and February data for the year 2000 and October to December data for 2017 have been infilled according to the “nearest living relative” approach; 2001 and 2016 data respectively. The 2017 data will eventually be corrected, the 2000 data probably not.

What we see above is that the SW flux added to the LW flux shows a decline generally in agreement with the increase in net absorption, as one would expect. There was some sort of aberration in 2005.

In the next post we will explore what can be discerned about where this increased SW absorption is going, before leaving SW radiation to focus on LW radiation and the disagreement between CERES and MODTRAN.

 

Posted in CERES, Climate | Leave a comment

CERES and MODTRAN

You don’t radiatively warm the third rock from the sun without reducing long wave radiation from the top of the atmosphere to space. Period. [Unless shortwave absorption increases]

For far too long, we have ignored CERES data here at the Trunkmonkey Research Institute. Partly, this is because the data is only available in netCDF format developed by the “University Corporation for Atmospheric Research”. This format is good for painting raster images with color shaded values on 1×1 degree grid cell maps, but it is difficult to extract numeric values for use in other sorts of analysis.

At https://www.unidata.ucar.edu/software/netcdf/software.html, UCAR has published a long list of free and proprietary software for reading netCDF. We tried netCDF4Excel as there was some internet chatter that it worked well and we wanted the data in EXCEL anyway. We downloaded the latest version, only to be greeted with a popup that “this app cannot be used on this computer”. We also tried a cloud based app perporting to read netCDF to EXCEL, but EXCEL refused t open the output, claiming it was corrupt.

We use ARCMAP all the time and it is straightforward to paint color shaded rasters, but the ESRI toolbox utility supposed to create a table from netCDF  does not work with CERES data.

You can go to the CERES data and look at visualizations, and when you click on them this apparently very helpful screen pops up as above. Unfortunately, the “Save as ASCII” and “Save as PNG” buttons do not work. What you see is a screenshot.

Normally we would just digitize this, but the one thing that does work is when you mouse over the above screens, the data pops up. This particular graphic is so important that we deemed it worth the trouble, and while entertaining vile thoughts about the NASA nerds, duly recorded all the monthly values in a notebook.

The CERES data above comes from the TERRA (before 2002) and then the AQUA Moderate Resolution Imaging Spectroradiometers  (MODIS) instruments. The satellites are just above 70 Km altitude in sun synchronous polar orbits that cross the equator at the same time every day. The TOA is considered to be 20 Km altitude.

The first striking feature of the above data is the large swings in TOA longwave radiation to space between the northern and southern hemisphere summers. As the min/max values in the screenshot show, the range is some 12.5 W/M2, with the northern hemisphere always radiating more to space than the southern hemisphere in their respective summers. Land must radiate to space more effectively than ocean.

The second striking feature of the above data is that it does not seem to be decreasing as MODTRAN predicts from the some 2 W/M2 additional absorption from human CO2.

It is actually increasing. Above is the raw data, but this overstates the case a bit because the data ends in September, 2017. We wish the data continued to finish 2017, and as we approach the end of the first quarter 2018, it does not seem unduly burdensome for NASA to provide the data. Anyway, there are various mathematical tricks one might use to correct this, all with drawbacks. What we have done below is arbitrarily complete 2017 with 2016 monthly data.

The result looks like this. There has been a substantial increase in the long wave radiation to space in the CERES data. This is how you cool the planet, not warm it. This data glaringly contradicts MODTRAN to the extent that one or the other is wrong. Exploring this discrepancy will be difficult because the formats are different, but it is very important. The discrepancy between CERES and MODTRAN will be the subject of this series.

Posted in Climate | 3 Comments

Differential Motions of the Continents, Mesozoic and Cenozoic Eras: The 120mya Inflection and Finale

A square kilometer of extruded ocean floor exerts force in all directions. The “squareness” is partly our own construct for convenience of measurement, but also justified by lateral offsets where the linear ruptures we call ridges accommodate the curvature of the planet surface.

The very linearity of ridges attests to the propagating force along the axis, even though we tend to ignore this in favor of the cross axis force that separates continents and creates ocean basins.

In the post on constraints, we concluded that the motions of all the continents except Antarctica and Australia could be reasonably explained by the formula of 1 kilometer of continental motion for each kilometer of cross axis spreading along the Atlantic ridge, for the period of zero to 10mya. During this period nearly all of the ocean floor currently exists.

The problem of subducted and therefore missing ocean floor arose as a significant problem in our attempt to explore the most recent inflection point in the continental wander paths in the last post. We concluded that a what was notable about this inflection point was both a concentration of apparent (we lack information on spreading rates in missing ocean floor) spreading in the southern hemisphere, and widespread “ridge jumping”. In many cases, 80 to 70mya ocean floor spreading jumped to both sides of the prior 90 to 80mya ocean floor.

The 80mya inflection seems the last of a series inflections beginning at 120mya. This 80mya inflection was chosen first because it is the first disturbance of coherent paths tracking to the present, and because the greatest amount of existing ocean floor is available for the analysis.

We have avoided the motion of India as it is the most outrageous, and difficult to explain. India seems worthy of a separate series, that will better be pursued after some principles can be established for the more sedate continents. The series of inflections beginning 120mya seems bound to the launch of India.

 

Above we have compared the total continental motions (NIC India) with the total seafloor spreading for the periods 70 to 80 and 0 to 10mya. According to Christopher Scotese’s polygons, only about 1/3 of the current ocean floor existed 70mya, so the low surviving spreading is no surprise. The pink represents what it might have been. The units of spreading are important. At a factor of 10k to avoid a very tall graph, the spreading (at one linear Km /Km2) might have moved the continents a lot further 0-10mya. This spreading  includes includes the Pacific, which loses its efforts to trenches on two sides.

 

We attempted to separate North-South spreading from East-West spreading and compare with continental motion bearings above. The rather surprising result was that a small amount of East-West  continental motion resulted from a lot of East-West spreading, and a larger amount of North-South motion (NIC India) resulted from a much smaller amount of North-South spreading; for the 0-10mya period.

Digging deeper, many bearing data points cluster around the definition break points (green circles), meaning that there was lots of oblique motion that fits poorly into a N-S/E-W scheme.

We simply don’t know what sorts of spreading may have taken place in the 2/3 of the ocean missing 80mya and the less so for 120mya. The exercise above at 0-10mya shows us that comparing the directions of spreading and motion is difficult, even when the ocean floor is substantially existing.

We suggested above that the 80mya inflection was the most recent of a series of disturbances beginning 120mya as India separated from Antarctica. Above we bracket this range with 120 to 130mya ocean floor in green and 110-120mya ocean floor in red. Like 80mya, there is lots of ridge jumping going on. The older floor is sandwiched the entire length of the Atlantic ridge.

Whatever that is out in the Pacific, it does not look like anything we have seen or would be inclined to call seafloor spreading. Astonishingly, only a single DSDP drill site has investigated this massive blob blow 30S. It was drilled at the southeastern margin. DPSP 35-323 drilled only to the basement through 701 meters of sediment. The first 60 meters [!] above the basement is volcanic ash, containing only fish teeth. We really need to drill this blob to find out if it is just a volcanic province deposited over older ocean floor.

As the surviving ocean floor fades away in deeper time, we are left with only the continents to work with.

 

Albert Einstein was no fan of continental drift. He could imagine no force great enough. From GPS measurements we know that continents continue to move, and furthermore that there is differential motion within continents just as there was differential motion within Pangea. Below are some GPS velocities and vectors from NASA.

 

We have seen that there is plenty of ocean spreading to explain the motions of the continents, but that it is not always easy to show how the spreading leads to the motions. We have seen ridges jump. We suspect that the only meaningful plates are the stable continental cratons. Sometimes the fickle ridges will sweep the continents into a pile, other times they will split the continents up.

Our understanding is entirely dependent on the magnetization of rocks. The earth’s magnetic field is also fickle. It is possible that the inflection points we explored were distortions of the magnetic field rather than real changes in the directions of continental motion.

Plate tectonics is far from settled science. The mysteries of the Pacific Triangle, the lack of trenches around Antarctica, large volumes of anomalously old rocks on theoretically much younger ocean floor, and the apparent continuity of Paleozoic rocks across the Bearing Strait; all need explanation.

To conclude this adventure, we leave you with an image stripped of as many assumptions as possible. We know that continents move, but like Einstein, we still have no idea how. The ridges are shallow features purported to move deep rooted behemoths. To simply contemplate is humbling.

 

 

 

 

Posted in Continental Wander Path, Paleogeography, Plate Tectonics | Leave a comment

Differential Motions of the Continents, Mesozoic and Cenozoic Eras V: The 80mya Inflection

In the last post on constraints we decided that the east Pacific and Juan de Fuca ridges were useless in constraining the motions of the Americas. We decided that the Atlantic, Greenland, and Lomonosov ridges were acceptable explanations for the motions of the Americas; and that we simply have no explanation for the motions of Antarctica and Australia, as they are out of scale with the adjacent spreading.

There was clearly a disturbance of the forces acting to move the continents about 80 million years ago.

On the 250mya base map above, Eurasia can be seen to very nearly reverse field, and set a course that rotates  continuously to its current position.

North America made about an 80 degree change of course to the west.

South America was the least affected, but changed course modestly. Africa ceased drifting eastward and headed north.

 

 

Antarctica came to a flying stop and reversed field slightly. This projection exaggerates the retrograde motion. Australia slowed and drifted north.

Above is the 80mya base map with the 80 to 70mya ocean floor extrusion. Even at 80mya one begins to appreciate the difficulty caused by missing ocean floor. There really does not seem to be anything extraordinary going on here.

The south polar 80mya view tells a different story. The map has been simplified to mere suggestions of the continental shelves to focus on the relation between the ocean spreading and the relevant  points. At this scale the blue 70mya Antarctica points are barely visible below and to the right of the red and green 80mya marker. Australia drifted somewhat more down and to the right 80 to 70mya as the separation of the two continents begins.

There is a lot of southern hemisphere and South Polar spreading. India launched about 120mya and its astonishing trajectory is aided by nearly 600 kilometers of spreading between India and Antarctica.

Ridge jumping is a murky concept in tectonics as the discipline has been fixated on the notion of plates. Ridges define plates, and it is very inconvenient when they jump, since real estate formerly on one plate gets transferred to another; or entirely new plates are created. We have zoomed in above and it is easier to see the 70mya Antarctica points behind the 80mya marker. The green is the 90 to 80mya seafloor production, and the red is the 80 to 70.

In a sense, the Pacific ridge has been “jumping” continuously since its inception about 175mya as it moved away from the Pacific Triangle. It continued this habit from 80 to 70mya, except at the very bottom of the image we can see a secondary jump to the other side of the 90 to 80mya ocean floor. We believe this “double” extrusion is the hallmark of the 80mya inflection.

Well behaved ridges extrude into and separate symmetrical bands of older ocean floor. On the 80mya globe this did not happen much. Ridges either jumped to offset or doubled to enclose older ocean floor.

In the next post, we will explore the need to get beyond plates. Ridges must be the focus, and a coherent system of naming the ridges must be developed.

 

 

 

Posted in Continental Wander Path, Paleogeography, Plate Tectonics | 1 Comment

Differential Motions of the Continents, Mesozoic and Cenozoic Eras IV: Constraints

We know the position and velocity of stable points on our six cratons. A reasonable question to ask is, “Can we account for this motion?”

Above is a map of the ocean floor created in the last 10 million years. We will start at the present and work backwards, as 100% of the ocean floor is available to aid our constraints.

The present ocean floor is 361 million square kilometers. This map shows 27 million square kilometers, or ~7.5% of the current ocean floor extruded in the last 10 million years.

Of the last 10my extrusion, nearly 12 million square kilometers, or ~44%, was extruded by the East Pacific Ridge, shown in magenta. The Nazca, shown in rose, extending toward Chile, and the Juan de Fuca shown in lighter magenta off the Pacific Northwest add another ~1.4 million square kilometers.

A careful examination of the isochrons around the prominent “arm” of the Pacific ridge pointing towards Panama, and the Nazca arm as well, shows that even these exerted spreading force according with the predominant east-west axis of the ridge.

Very nearly 50% of the ocean floor extruded in the last 10 million years has been in the Pacific Ocean basin. The Americas are poorly connected to the Pacific basin and have continued moving westward even as the spreading ridge has moved eastward. The Pacific basin has been subducted beneath the advancing Americas. Whatever braking effect this subduction may have had, half of the ocean spreading in the last 10 million years cannot be used to constrain the motion of the Americas.

 

There is currently little or no subduction going on around Antarctica. The Australian-Antarctic ridge in rose and the Pacific ridge join in a peculiar fashion. In a  2013 post we discussed the oddity that Antarctica is circumscribed with ridges, yet has no trenches. Antarctica has moved about 135 kilometers and rotated about 3 degrees as shown. This Antarctic motion is about 50% more that the spreading of the Atlantic and Indian ridges apparently pushing the right direction, and is less than half of the antagonistic spreading of the Pacific and Austral-Antarctic ridges.

In order for this motion and growth to be accomplished without expanding the earth, the entire system must have moved with Antarctica. The motion of Australia is ~28% greater than the combined spreading of the ridge and motion of Antarctica.

Africa is also caught between spreading ridges with little or no current subduction. It has moved only about 40km west to east at its 58 degree bearing. The motion of South America is in reasonable agreement with the spreading of the Atlantic ridge.

It would be truly astonishing, and call into question our most fundamental concepts of plate tectonics, if the motions of North America and South America could not be accounted for by the spreading of the Atlantic ridge. The pieces fit together like a puzzle, existing magnetic lineations cover the entire interval since separation, and the existing ridge tracks the shapes.

We can see that the motions do generally conform to the scale of Atlantic spreading over the last 10 million years. What remains to be explained is why Africa and Eurasia remained relatively fixed in longitude while the ridge and the Americas moved to the west.

There are two areas in the mantle with the chewy name of LLSVP’s where shear waves are attenuated. I call these Low Velocity Provinces doughboys. They appear to be extrusions from the core. One is under Africa, and the other out in the Pacific. Their footprints at the core-mantle boundary (the bottom of the box) are at the 1% anomaly contour and all sorts of plumes and kimberlite pipes seem to originate there as suggested in the upper panel and red dots at the surface.

Above we have plotted the 1% footprints of the doughboys (after Torsvik). These footprints are thought to have remained fixed in relation to the core. Their highest reaches flatten out at the 660km discontinuity, and the crust has moved around above them. It seems possible that Africa is somehow grounded in its doughboy, and being substantially connected to Eurasia for the last 10 million years, has restrained this entire mass while the Americas moved to the west.

We have the extrusions of the Greenland Ridge, seemingly an extension of the Atlantic ridge; and the Lomonosov ridge, nearly crossing the North Pole, to complete our 0-10mya analysis. These ridges seem to be in accordance with the very modest SE motion of Eurasia and the bearing of North America ~15 degrees south of due west. Once again, with no trenches, the Arctic spreading ridges must have moved away from Eurasia their spreading distance less the 49km opposite motion of Eurasia.

The answer to the question at the beginning of this post, “Can we account for this motion?”, is ambiguous. We can account for the motions of the Americas, Eurasia, and Africa; with the caveat that the explanation for the relative fixation of Africa and Eurasia is hypothetical. The motions of the Austral pair, Antarctica and Australia, are wildly out of scale with adjacent spreading. As the ridges must necessarily have moved with Antarctica, we must conclude that the motions of Antarctica and Australia remain unconstrained.

It seems far too tedious at this point to step back 25 times through our 10my increments. In the next posts, we will focus on the inflection points.

 

 

Posted in Geology, Paleogeography | 2 Comments