Category Archives: Planetary geology

Volcanism does not cause glaciations; let’s turn this statement on its head

It is almost a truism that volcanic eruptions influence climate. Cold winters and even failed crops, particularly in the northern hemisphere, followed on the heels the Tambora, Krakatoa, and Pinatubo eruptions.  But these climate aberrations were relatively short-lived, counted in years; the stratospheric aerosols and fine volcanic ash that reflect solar radiation back into space, eventually succumb to gravity and fall to earth.  Eruptions of this kind do not result in long-lived, or permanent changes; they are temporary blips on an evolving earth and an evolving climate.

There have been stupendous volcanic outbursts in the more distant geological past, that have wreaked havoc on the global climate and threatened life itself. One such event was the eruption of the Siberian Traps, 250 million years ago, an outpouring of lava, volcanic ash, and gas that lasted almost one million years.  Individual lava flows, in a single outpouring, produced upwards of 1500-2000 cubic kilometres of basalt.  The cumulative volume of lava and ash would have been sufficient to cover all western Europe or USA in a layer more than a kilometre thick. The Siberian event is strongly implicated as the cause of mass extinctions at the end of the Permian Period, commonly referred to as The Great Dying; it was an event of truly global proportions. Events like this are orders of magnitude greater than the kinds of volcanic activity recorded over the last few thousand years.

So, my cliched introduction is true – to an extent. But how about turning this statement on its head; the excesses of a changing climate can influence volcanism. The possible links between global climate change, volcanism, and the carbon cycle have been argued for decades. First, we need to establish some time scales. Events like a Krakatoa-induced cooling persist for only a few years – usually less than 10. Contrast this with the time frame of your average glaciation, and we are now dealing with (roughly) 100,000 years. How do signals of volcanism compare with these longer-term cooling and warming episodes (glaciations and interglacials)? It turns out that there is a relationship between volcanic activity and glaciations. But it is also apparent that volcanism lags the onset of climate change that herald glaciations or interglacials, in some cases by several thousand years. Therefore, volcanism cannot be a primary cause of such climate changes, at least on a time scale of 1000s to 10,000s of years (a suggestion made almost 4 decades ago by Rampino, Self, and Fairbridge in their paper Can Rapid Climate Change Cause Volcanic Eruptions? published in Science, volume 206, 1979).  These authors hypothesis that the temporal association between volcanism and major climate change is caused not by the eruptions themselves, but changes in water-ice budgets and stress associated with the subsequent loading and unloading of the earth’s crust. The hypothesis is appealing, but until recently it has lacked any reasonable kind of testing and verification.

As happens so often in science, research in one direction can lead to unexpected results that point investigations to new and exciting directions. One such project along the subducting coast of South America, undertaken by S. Kutterolf and colleagues at GEOMAR (Helmholtz Centre for Ocean Research Kiel) provides good evidence for links between glacial cycles and volcanism.  The Initial data base consists of volcanic ash layers retrieved from cores into the sea floor off South and Central America.  The more than 80 ash layers identified provide a history of major, landward eruptions up to one million years ago.  This data set was augmented by volcanic ash – tephra records from various ocean drilling projects at several sites around the Pacific Ring of Fire (more than 400 ash layers were eventually identified). Once the age of all the ash layers had been established, the frequency of eruptions was analysed and plotted. What surprised the researchers, was the prominence of volcanic activity close to, but consistently lagging by about 1-5 thousand years, an important astronomical cycle – the Milankovitch Obliquity Cycle.  Obliquity records the gradual shift in the earth’s axis from a tilt of 21.5o to 24.5o, a change that takes 41,000 years.  Shifts in the tilt axis ‘force’ an increase or decrease in solar radiation, particularly at the poles.

The Kutterolf et al. analysis makes an explicit connection between peak volcanism and the Milankovitch obliquity cycle.  Note however, that it is not changing obliquity itself that causes the increase in volcanism, but the changing climate. For example, during deglaciation ice-sheet mass decreases as melting progresses; this means that the load on the earth’s crust will also decrease.  The opposite affect takes place in the oceans where the water load increases.  Load distributions are reversed during the subsequent glaciation. The crust needs to maintain equilibrium by balancing the changing masses, such that the land surface beneath the ice-sheet rises, and the ocean basins deepen.  This balancing act is called Isostasy.

However, the balancing of loads on the crust also results in changing patterns of stress. If stress levels decrease in regions of active volcanism, like the Ring of Fire, it may be easier for magmas to ascend through the crust, promoting increased volcanic activity.  As a final piece to their argument, the authors create a model of changing crustal stress at a site along the west coast of central America; the model calculates stress for the last 120,000 years, a period that includes the last glaciation and the interglacial that we now find ourselves in.  They also plot actual eruptions along the same time-line. There is a striking increase in actual volcanic activity and modelled stress beginning about 20,000 to 22,000 years ago – a time that corresponds to the maximum extent of glaciation. Melting began soon after (significantly in the Laurentide Ice-sheet that covered a large swath of North America), with subsequent redistribution of ice-water loads and patterns of stress.

There is an interesting corollary to the Milankovitch-volcanism hypothesis. Milankovitch cyclicity predicts that we are now in a cooling phase, which means that stress levels will increase on land as ice accumulates; there should in this case be a decrease in volcanic activity. But if surface temperatures continue to increase…?  The Milankovitch time-frame is in 1000s of years, which gives us a bit of time to work on it.

 

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A measure of the universe; Renaissance slide-rules and Heavenly spheres

Measurement is a cornerstone of science, in fact of pretty well everything we do: How far? How fast? How long?  We take most measurement for granted, with little thought to how the process originated.  We demand accuracy and precision, forgetting that these are relatively modern luxuries.  Before the universal clock chimed GMT in 1884, there were more than 200 time zones in the US.  A league in France was shorter than a league in Spain, a discrepancy for which the 16th C French scribe François Rabelais had an imaginative, if rollicking explanation.  In his tale, The Life of Gargantua and Pantegruel (1532-1564), a king required a standard distance to be determined (after all, if he was going to send his armies to battle it would be best if his advisors new how far they had to go).  He sent a trusted Knight, instructing him to ride to Spain, stopping every league to “roger and swive”; hence the discrepancy.  The leagues gradually became longer. The amusing satire of this explanation had its roots in real Medieval measures; the width of a hand, the distance one could walk in an hour. Continue reading

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Galileo’s finger

“Their final resting place…” a sepulchral phrase, redolent of a fate that awaits us all.  There is no doubt as to its finality, but resting…?  A nice metaphor that may convey a sense of comfort to the living, rather than the deceased.  Wander through any church or cathedral in Europe and Britain, and you will inevitably walk over cold marble slabs, engraved with the details of those who lie beneath, polished by the feet of a myriad worshipers and tourists.  The Basilica di Santa Croce in Florence is, in many respects, like any other magnificent church; it is old, construction beginning in 1295, with alterations and additions during the 14th -15th century overlapping the earlier Gothic forms.  The Basilica is stunning, but differs from many of its contemporaries in that it became THE place in Italy to be buried. Continue reading

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A Watery Mars; Canals, a duped radio audience, and geological excursions

Deceptive news is the art of pulling wool over the eyes of the populace, a tool (recently resurrected by certain politicians) for persuasion or dissuasion.  Orson Welles got more than he bargained for when, on October 30, 1938, he orchestrated a radio adaptation of H.G. Wells The War of The Worlds, a 1898 sci-fi that pits intelligent Martians against Victorian Britain.  Welles broadcast created a mix of amusement in some commentators, and in others panic and anger; panic in the unwitting, anger in the duped (especially other broadcasters), and amusement in all the above.

Well’s novel, apart from being the product of an agile mind, was influenced by some of the popular astronomical ideas of his time.  Italian astronomer Giovanni Schiaparelli produced, in 1888 a wonderfully detailed map of Mars showing (above image), among features such as seas, islands, and other landmasses, a network of ‘canali’, or channels.  Canali was misinterpreted in English as canals, and along with all its connotations of intelligent life, the idea of Martian canals entered popular belief.

Were the anthropomorphic connotations of the word canali deliberate?  Percival Lowell, a self-made American astronomer certainly thought so.  In 1894 Lowell announced his own findings, that there were indeed canals, 100s of them, many of them straight, intricately networked, and all artificial, which of course meant intelligent beings.  H.G. Wells simply expanded Lowell’s ideas to the point of delightful absurdity.  That O. Welles would later foist his version of events on an unsuspecting public seems quite reasonable.

Since the 1970s we have been projecting our own intelligence and sense of puzzlement on Mars, using satellites and landed vehicles.  There are no artificial canals, but there are canyons, channels and gullies, landforms that bear an uncanny resemblance to terrestrial analogues.  There is now a significant body of evidence to indicate that Mars was once watery.

On earth, sediment is distributed far and wide by flowing water. Very fine sediment from rivers or wind-blown dust is commonly suspended in water; the sediment gradually settles on the sea or lake floor.  Coarser sediment, like sand and gravel tends to be ‘entrained’ close to the sea floor or river bed by fast flowing water.  Sediment that is moved in this way forms a variety of structures such as ripples and larger dune-like structures.

Rivers in particular, generally move sediment to larger repositories, or basins such as seas or lakes; The kinds of landforms that represent these processes are very distinctive.  On Mars, there are several landform-indicators of flowing liquid (most likely water), most of which have direct terrestrial counterparts; deltas, straight and meandering river, point bars, alluvial fans, and gullied crater margins.  One such Martian landform, imaged by NASA’s Mars Global Surveyor, is the Eberswalde Delta which contains many of the ingredients that also make up terrestrial deltas.  In this case, sediment making up the delta was probably derived from outside the Eberswalde Crater and subsequently transported by rivers into the crater:

  • The delta consists of one or two main river channels (left side) that split into many smaller channels,
  • Bifurcating channels form distinct lobes – there are at least 6 of these, where each lobe represents a specific period of delta formation.
  • Switching of delta lobes is common in terrestrial delta. Each lobe represents a period of sediment movement and deposition, in this case into the deep crater basin.  At a certain point in time, the channel will switch direction and begin to build a new lobe.
  • Each new lobe partly overlaps older lobes, such that the younger deposits appear to lie on top of older deposits.
  • The Lena Delta in Russia provides a nice analogue for the overall shape of channels, with some active parts of the delta (especially the centre-right) juxtaposed with less active segments.

The Eberswalde Delta has another remarkable set of structures.  Meander loops (opposite image), seemingly identical to those seen in meandering rivers on Earth, contain patterns of progressive channel movement.  The Martian meander channel loop was eventually cut off, perhaps forming an oxbow lake like its terrestrial counterpart.

Martian landforms like these are mostly found in regions assigned to the Noachian Period, a geological interval that extended from 4.1 to 3.7 billion years.  All the evidence (so far) points to a time when surface water was common as rivers, lakes, and possibly seas; groundwater can be added to this mix.  If this was the case, there must also have been water vapour in the atmosphere.  The surface must have been significantly warmer than the present frigid temperatures; water vapour probably provided some degree of greenhouse protection.  Overland flow of water also produced sediment, much of which ended up in impact craters and broad lowlands.

 

However, some extremely large outflow channels, such as the Kasei Valles formed sporadically during a later, generally drier and colder time known as the Hesperian Period (3.7 to 2.9 billion years).  This massive system of channels and canyons extends about 3000km from its source in the Tharsis volcanic region, and through about 4km of topographic relief. The overall form of the channels, plus more detailed images of flow-like structures within the channels, indicates possible catastrophic outbursts of humongous volumes of water.  One popular hypothesis to account for this involves massive volumes of frozen groundwater being released either during meteorite impact or volcanic activity and heating.

The comparison between Eberswalde Delta and Kasei Valles mega-floods is quite stark; the delta represents relatively continuous river flow over a long period of time, into a crater.  The Kasei Valles outflow formed almost instantly, driven by the forces of impact and directed away from the crater.

Scientific understanding of Martian geology will continue to evolve; some hypotheses will stand the test of experimental and observational rigour; others will become history.  Modern science has developed the technology to actually do the field work, albeit remotely.  Perhaps we shouldn’t be too hasty to consign Schiaparelli’s and Lowell’s ideas to the theatrically amusing; their observations and explanations were not without context. Keep in mind the possibility that another H.G. Wells may point a satirical finger at 21st century science.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The Introductory chapter by John Grotzinger and Ralph Milliken provides an excellent technical summary of the Martian sedimentary realm.

 

 

 

 

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Sand dunes but no beach; a Martian breeze

When James Hutton, in 1785 presented to the Royal Society of Edinburgh his ideas on the uniformity of earth processes (over vast tracts of time), he did so with both feet planted firmly on good Scottish ground.  Hutton’s Principle, for which Archibald Geikie later (1905) coined the phrase “The present is the key to the past” gave to geologists a kind of warrant to interpret the geological past using observations and experiments of processes we see in action today (see an earlier post for a bit more discussion on this philosophy).  One wonders whether either of these gentlemen gave thought to the Principle being used to interpret processes elsewhere in our solar system.

There is of course, no logical reason why we cannot use terrestrial environments and physical-chemical-biological processes to unravel the geology on our solar neighbours.  We may need to extend our thinking beyond purely earth-bound processes, but the Principle remains a starting point for scientific thinking, interpretation, and discovery.  Mars provides the perfect opportunity for this scientific adventure.

I am constantly amazed with the images that various satellites and extraterrestrial vehicles provide of our celestial neighbours.  The surface of Mars is no exception.  There is clear evidence for ancient processes involving water; landforms and landscapes, strata, and minerals, all pointing to a once-watery planet – ancient rivers, lakes, and seas, beset by Noachian floods.  But all this aqueous excitement was in the distant past; more than 3.7 billion years past.  The Martian surface is now cold and bone dry (although the solid carbon dioxide ice does melt from time to time).  Active surface processes on Mars, i.e. those operating now, mainly involve loose sediment (sand, dust), and wind.  On earth, our experience tells us that if there is enough wind and enough sand, then sand dunes will form. Martian sand dunes look pretty much like our earth varieties.

The formation of terrestrial ripples and dunes requires certain (well understood) sediment attributes and physical processes.  We define sand grains as particles of rock or mineral that range in size from about 0.1mm to 2mm diameter.  Most dunes and ripples on earth form in sands that are in the 0.1mm to 0.3mm range.

  • The sand needs to be dry.
  • Sand grains must be loose, and not stick to one another (in technical jargon they are cohesionless).
  • Wind speed must be greater than the threshold for moving the grains. Stronger winds can move larger grains.

Satellite imagery of the Martian surface has revealed that sand dunes and ripples are common. They have formed, and continue to form in crater floors, broad flat plains, and ancient river valleys; basically anywhere there is sufficient loose sand available.

The variety of dune sizes and shapes is similar to that seen on earth.  Dune shape, like the Barchan dunes shown in the above images, provides clues to prevailing wind directions.  Other dune forms suggest greater variability in wind direction and strength, like the smaller, ripple-like structures (below) imaged by Curiosity ; here, changes in wind direction and speed are indicated by the very small ripples that move in different directions across the larger structures.  These patterns of sand movement are identical to those we commonly see on earth.

The larger dune imaged by Curiosity (below) is moving directly towards the camera.  Sand blown over the top of the dune (the crest) tumbles or slides down the steep front, or lee face, a process that continues as the dune moves forwards.  The inset, taken with a special camera that acts like a small magnifying glass (or hand lens), shows that the sand grains are indeed loose and cohesionless.

All the above images show dunes and ripples that are currently active.  It is also possible to identify fossil dunes in much older strata, using structures within the rocks.  In the examples below, the Martian fossil dune structures are very similar to those we see in some familiar rocks at Zion National Park (Utah).  The kind of layering visible in both Martian and earth examples is called crossbedding (see here for an explanation of how crossbedding forms).

Despite the obvious similarities between Martian and Terrestrial dunes, one in-depth analysis has led a couple of researchers (Gary Kocurek and Ryan Ewing, see below) to suggest that there are also important differences.  The Martian atmosphere is much less dense than that on Earth; there is no oxygen or water vapour.  Mars is consistently much colder; permafrost is probably widespread but in this case the ice is solid carbon dioxide.  Even if the CO2 melts, it will sublime (to gas) rather than form a liquid.  In some cases, permafrost may even stabilise, or solidify dunes.

One important difference between Earth and Mars is in the availability of loose sand.  On earth, much sand is produced by weathering of rock, by all the elements of weather plus chemical reactions that commonly involve oxygen.  Sand is available along most ocean, lake and river shores.  On Mars, physical weathering is very slow; sand is no longer moved by water, as it is on Earth.  Chemical weathering on Mars is also extremely slow.  Thus, the only sand available will likely be from local permafrost melt, the very slow breakdown of rocks, and the occasional meteorite impact.  Some sand may be left over from the period on Mars when water was abundant, the Noachian Period 4.1 to 3.7 billion years ago, but that seems like an awfully long time for sand grains to hang around.

Fossil dunes on Mars, like those on earth, provide clues to the kinds of environments that once existed. Some of these very ancient dunes may even have been associated with Martian lakes and seas.  Modern, active dunes on Mars have little or no association with water.  But they do provide information on sediment availability, wind direction, and wind strength.  The latter may be important when deciding where to locate future sites for human habitation.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The paper by Kocurek and Irwin, cited above, is one of these papers.

Kocurek and Ewing, 2012.  Source-to-sink; an Earth/Mars comparison of boundary conditions for eolian dune systems, p. 151-168.

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Comets; portents of doom or icy bits of space jetsam?

Omens, God’s wrath, or just plain misfortune; comets were seen by our Medieval forebears as a disturbance in the natural state of the heavens, portending disaster, pestilence, or famine, and if you were really unlucky, all three.  Harold, Earl of Wessex and later King, before he did battle against William of Normandy in 1066, must have had some misgivings with Halley’s comet nicely lighting up the northern sky (we now know it was comet Halley); he probably should have kept both eyes on the battle. Portent indeed; the Norman conquest changed irrevocably the history of Britain.

It seems that the ancient Chinese were a little more rational in their deliberations on comets – they referred to them as brush stars, and as early as 613 BC were computing approximate orbits.  In fact it is ancient Chinese astronomy records that have enabled modern astronomers to confirm calculated orbit periodicities for comets like Halley. Continue reading

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