Category Archives: Interpreting ancient environments

A burp and a hiccup; the volcanic contribution of carbon dioxide to the atmosphere

Of the two certainties in life, volcanoes offer the most excitement (death and taxes are basically the same thing).  They are magnificent while asleep; a primeval ruggedness that stirs the imagination. We paint them, we eulogise them. And when they awaken, we run for cover. Whether in a state of dormancy or high agitation, they leave an impression on our inner and outer landscapes.

All active volcanoes emit gas; pre-, during and post-eruption. On average, 96% of volcanic gases are water vapour, the remaining components being CO2, SO2 (most common), plus a little helium, nitrogen, carbon monoxide, hydrogen sulphide, and a few halides. Volcano-derived carbon dioxide is frequently cited as a culprit for increasing atmospheric CO2 concentrations in climate change debates.  However, it is sulphur dioxide, not carbon dioxide that does most of the short-term damage to climate. Continue reading


The life of a Tuscan wall

Montefioralle, Chianti country, Tuscany, Italy, and from where I’m sitting (happily sampling a Chianti Classico) I see rolling, wooded hills, next season’s vintage, olive groves a scattering of farm dwellings, and rock walls. Quintessential Tuscany. Except for a few ratty road cuts, there is little native rock exposed in this part of Tuscany from which a keen geologist might ascertain something of ancient pre-Tuscan history (farther south this changes).  But in fact, there are rocks aplenty. Most walls (houses, defensive, retaining, decorative) are made of limestone and sandstone, some quarried and deliberately shaped as in churches and castello (that date back to the 10th -11th century), and others that made use of whatever was handy at the time. Most of these materials were collected locally; stones that littered the hillsides, and stones brought to the surface during ploughing.  Even today, ploughs bring stones to the surface; the local clay-loam soils are incredibly stony (an important part of Chianti terroir).

So, despite the paucity of hard-rock exposure, one can make a reasonable guess at the geology beneath the hills and vineyards, based on the stone composition in local buildings. About 50-60% of the stones are cream-coloured marls; marl is an old name (medieval Latin) given to very fine grained, usually muddy limestone that breaks along curved, sharp-edged surfaces (referred to as conchoidal).  The Tuscan marls are very hard – ideal for building stone.  Variations on this theme include sandy limestones, some of which contain intricate contorted layering, and small crossbeds that indicate flowing water many millions of years ago.

Grey sandstone is also common; in fact it is found as paving stone throughout most of Tuscany.  All kinds of structures are visible in these stones, especially cut stones in larger buildings and roads; fossil ripples that indicate flowing water, trails and burrows of critters that moved across or below the ancient seafloor in search of food or finding a place to live. Some of the sandstones are not as hard as the marls, and in places show quite advanced damage where bits of rock fritter away with the vagaries of weather.

An assortment of red bricks, some rumoured to be of Roman or Etruscan derivation, has been used in most walls. It looks like odd-shaped bricks are filling equally odd-shaped gaps, but they have also been used to replace stone arches over doors, or fill holes in walls left by marauding armies (of which there were many) or neglect.

The rocks were originally deposited as sediment in an ancient and vast ocean called Tethys, that separated two supercontinents – Gondwana, and Laurasia (most of Europe and Asia).  The Tethys was closed when the Africa plate (part of Gondwana) drifted north and crunched into Laurasia, beginning about 65 million years ago. The resulting uplift produced the Apennine Ranges that now course the length of Italy.

Montefioralle is a picturesque hill-top village, typical of many in Tuscany.  Its medieval origins are still visible, but frequent battles between neighbouring villages, as well as larger fracas between Florence and Siena, put a few dents in the outer wall and houses. The hill top is crowned by a small church and tower; the last refuge in the event of siege.  There is clear evidence of repairs made over the last 800 plus years, including, I suspect, some from a more recent European conflict.

Stones in these Tuscan walls weave their tales in different threads. The limestones and sandstones have a geological story that spans 10s of millions of years, the disappearance of an ocean and the collision of continents.  Each stone and brick can also relate centuries of local history; each was carefully placed by someone, a stone-mason or perhaps a Renaissance DIY. Nameless, we can admire their handy-work, wonder what they talked about with their fellow workers, what they ate, who they loved.  There are centuries of these former lives everywhere in Tuscany. Chianti Classico loosens all their tongues.


There are two sides to every fault

In 1940-41, Harold Wellman, a creative but somewhat irreverent New Zealand geologist, along with his colleague Dick Willett, discovered a remarkably long, linear fault striking slightly oblique to, and a few kilometres landward of the South Island west coast; almost the entire length of the island. They called this massive structure the Alpine Fault. The Fault can be traced overland some 600 km, about 450km of this is a more-or-less single fault strand; at its northern extent the fault splits into several strands, all of which are active.

Most New Zealand geologists in the 1940s had little problem with a structure like this – admittedly it was very long, but most were familiar with faults, especially active ones.  By 1948 it had generally been accepted by the scientific community. The community did however have an issue with Wellman’s next discovery.  He realised that a certain group of rocks in the southern part of South Island (Otago region), were almost identical to a group at the north end of the Island (Nelson region).  He postulated in 1949 that these two geological domains were once a contiguous unit but had been separated some 500km by the Alpine Fault.  To many geologists at the time, this was going a bit too far, and it took several years to dispel the initial disbelief, and perhaps the odd conniption fit; one of the main criticisms was the absence of any reasonable mechanism to accomplish this geological feat. This was pre-Plate Tectonics, a time when many earth scientists still considered vertical movements of the earth’s crust to be the most important (although Alfred Wegener’s ideas on Continental Drift were discussed – it seems that Wellman was quite keen on this hypothesis). Fast forward to 1965 and a paper by J. Tuzo Wilson published in Nature, described a “New Class of Faults…”; Transform Faults.  Wellman’s discovery was about to acquire a mechanism, and become an iconic part of the new Plate Tectonics.

All plates identified by Plate Tectonic theory have boundaries, of which there are three basic types:

  • Spreading ridges and rifts, where upwelling magma creates new crust that moves away from the ridge,
  • Deep ocean trenches where two plates converge, forming a subduction zone that recycles old crust and mantle, and
  • Transform faults where two plates slide past one another. Most of this sliding is horizontal. If the movement between two massive slabs of crust and mantle were continuous then there would be few problems, other than a gradual (mm/year) change in one’s property boundary lines. But most movement along these fundamental structures is not continuous or uniform; it takes place in fits and starts – during earthquakes that commonly are very high magnitude, destructive events.

The Alpine Fault, and its close relative San Andreas Fault on the other side of the Pacific Ocean, are transform faults.  They each mark a boundary between two plates – if you walk across the San Andreas fault you pass from the Pacific Plate to the American Plate; over the Alpine Fault, from the Pacific to the Australian Plate.  There aren’t many places on earth where one can easily straddle two tectonic plates; these two transform faults provide great opportunities to become one with plate tectonics.

The Alpine Fault is geologically young.  The 500-km fault separation of the two geological domains began about 25 million years ago; from a geological perspective, this is really fast – for tectonic plates.  The west side of the fault moves northwards relative to the east side; it is referred to as a dextral (right-moving) strike-slip fault. At the same time, stresses acting against the fault have uplifted the landmass; over the last 12 million years, rocks formerly 20-30km deep, were pushed to the surface, forming the Southern Alps.  Coincidentally, erosion and glaciation have carved the landmass into the rugged mountain range that extends almost the full length of South Island. Averaged over the last 2 million years, the central part of the Alpine Fault has moved horizontally at a phenomenal 27mm/year, and vertically at 10mm/year.  It is thought that this extreme displacement of the earth’s crust is the result of large, M (magnitude)7.5 to M8 earthquakes occurring every 200-400 years, the most recent in 1717AD.

At its northern extent, the Alpine Fault splits into several large, active faults, some heading offshore, others into the southern North Island (the North Island Fault System) and these have been the focus of many destructive earthquakes in the M6 to M8 range.  More than 6m of horizontal displacement registered the M7.8 event along Kekerengu Fault in November 2016 (Kaikoura earthquake).  On January 23, 1855, up to 18m of horizontal displacement occurred during the Wairarapa Earthquake, estimated to have been M8.2 – M8.3.  The epicentre was only a few kilometres south of Wellington city, which suffered significant damage although few fatalities; there was also a tsunami that in places had a 10-11m run-up.

San Andreas Fault is another iconic example of earth’s major fractures, and probably the most intensely studied. It is about 1200km long, and like its New Zealand counterpart, consists of a master fault with many divergent, active and inactive fault strands. It began to move things around about 28 million years ago and has continued to do so ever since, coming to public prominence on April 18,1906 with the San Francisco M7.7 to M7.9 earthquake (and subsequent conflagration); one of the largest events along this fault.  Earthquake recurrence intervals vary along the San Andreas fault system; in the southern part it averages about 150 years, but in some fault segments like Big Bend, it may be as low as 100 years.

A commonly used method for estimating earthquake recurrence interval is to date young sediments that have accumulated close to faults. Silts and muds that accumulate in river or lake beds will frequently contain peats or fossil soils, layers of woody material, and sometimes volcanic ash; along coasts, beach deposits may be raised by successive earthquakes, and these too may contain shells, wood or bone.  These materials can be dated using carbon-14 and other dating techniques. The trick is to find layers that show some disturbance (for example from ground shaking, or displacement by actual faults) and then determine their age. There is always a fair degree of slop in recurrence numbers, a bit like predicting 1-in-500-year flood events (you might end up with 2 events in the space of 12 months!).

Serious earthquakes are a fact of life on transform faults; after all, what do you expect when 10-20 kilometre-thick slabs of rock slide past one another. Recurrence numbers for major events (greater than M6 or M7) may have annoying statistical variation, but they are based on sound science. The sensible lessons learned when someone else’s backyard is reduced to rubble, like – be prepared, or, let’s do more science – are all too quickly forgotten.  I guess it’s easier to point fingers after the fact, than to be on constant alert.

J Tuzo Wilson’s 1965 paper A new Class of Faults and their Bearing on Continental Drift was published in Nature, v.207, p.343-347.


It only takes a moment; the ups and downs of earthquakes

Seismic metaphors, or seismic as metaphor? Seismic, a word that geologists and geophysicists traditionally thought was reserved for their use, has been purloined by politicians and social scientists to describe momentous shifts in things like public attitudes and voter propensities.  Seismic is the anglicized derivative of the Greek verb seien, (shake) and the word seismos (earthquake).  Apparently, it first appeared in English language writing about the mid-19th century. Personally, I find it satisfying that the social milieu sees fit to apply the scientific word in such a useful, metaphorical sense.

The word seismic is also a nice descriptor of our restless, physical world, especially the bits we live on.  Most of the earth’s crust is under stress, some parts more than others.  Most stresses are generated by the movement and jostling of tectonic plates, particularly at boundaries where plates converge, collide, or slide past one another.  Different parts of the crust respond to stress by bending; this includes seemingly hard, immovable rock. If the stress is removed then the deformed rocks return to their original states; this is referred to as an elastic response. However, if the earth materials are bent too far or too fast, they will break. An interesting analogue for this process, and a historical one, is the collapse of a Tacoma suspension bridge in 1940.

Here, steel girders and tarmac bent and twisted under stress until the deformation reached a limit (called the elastic limit), at which point the bridge failed. When the earth’s crust fails, the seismic event, or earthquake, can be devastating. Earthquakes are caused by the sudden brittle failure of rock under stress; the failure takes place along a fault, across which land (or sea floor) is moved up, down, or sideways.  The rapid displacement of rock masses produces pulses of energy, or seismic waves.  It is these waves that do the damage.

There are two main kinds of seismic wave; body waves that propagate through the earth’s interior, and surface waves that move along the earth’s surface. Body waves include a primary, or P-wave, and a secondary or S-wave.  P-waves travel fastest; they are also called compressional waves because they tend to push and pull materials as they propagate. Slower S-waves, or shear waves, produce a side-ways motion. Shear waves are not transmitted through fluids such as sea water, or the molten interior of the earth. The cartoon below illustrates how the earth reacts to these two wave types.  Surface waves are the slowest to propagate but they are also the seismic pulses that do most of the damage during an earthquake. The animation shows the earth motion for one kind of surface wave; Rayleigh Waves. Rayleigh Waves produce a circular, or orbital motion of earth materials at the surface, a bit like particle motion beneath sea waves.


Each type of seismic wave is identified by the speed at which it moves, and the kind of movement that sediment and rock are subjected to.  These differences are expressed on seismograph recorders.  On a typical seismogram (below),  P-waves arrive first, followed by S-waves.   S-waves tend to have lower frequencies than P-waves (more spread out on the graph), but higher amplitude.  Surface waves usually have the highest amplitude.

Earthquake magnitude (M) reflects the severity of ground roll and shaking, and on seismograms corresponds to the amplitude of the signal (usually of surface waves).  M is expressed as a number (M1.8, M4.6, M7.8) up to a maximum of 10 (10 might be caused by a large meteorite impact – when there isn’t much left).  The numbers, and therefore the magnitude scale, are logarithmic, such that a magnitude of 4 (104) is 10 times smaller than M5 (105), and 100 times smaller than M6 (106).  We can also think of the changes in magnitude in terms of the energy released during a quake. It has been determined, (as an approximate empirical relationship) that for every unit increase in magnitude, there is a 27.5-fold increase in energy. The difference in energy is also logarithmic, such that an M8 event releases 571,914 times more energy than an M4 event (the magnitude is 10,000 times greater).  These kinds of numbers demand a degree of respect for the twists and turns our earth can throw at us.

Seismograms also help seismologists determine the Epicenter of an earthquake; the epicenter is the map location rather than the actual location, or focus, at depth. The calculation makes use of the fact that P-waves are faster than S-waves, so that the distance to an epicentre is based on the difference in arrival times for each type of wave.  Distance in this case is the radius of a circle centered on the seismograph location.  To triangulate the epicentre, at least two more seismographs in different locations are needed – in reality there can be hundreds of seismographs that allow the calculation.  If circles are drawn about each seismograph, each with a different radius, they will intersect at the epicentre.  Variations in the structure of the earth mean that seismic wave velocities can vary, so that the circles may not all intersect at a single point.  However, the large number of seismographs around the world means that location of the epicentre is usually accurate.

The November 2016 earthquake in Kaikoura, New Zealand was a M7.8 event.  We live about 500km north of the epicentre.  At 11pm we felt definite shaking (P-waves).  A few seconds later the shaking increased significantly.  At this point trees were swaying and water was slopping over the edge of the pool; this was due to ground roll from the slower surface waves.  Other than an adrenaline rush, there was no damage for us; farther south it was a very different story.  One spectacular outcome was the abrupt, lateral 6m shift along part of the New Zealand coastline; the same coast was also uplifted 1-2m.

I’ve always thought that New Zealand is a great place to witness geology in action.  But sometimes it can go a bit too far.

An excellent technical paper on the Kaikoura event by Ian Hamling (GNS Science) and a host of co-authors, has been published in Science, March 26, 2017.



Class 5; The falls and cataracts of Li Phi, southern Laos

I am of a generation that, at mention of Laos, Vietnam and Cambodia, I recall images of intense conflict, thankfully long past.  The images now are of jungle, peaceful villages nestled among ancient civilizations, and rivers; kayaks instead of gunboats. The coincidence between geology and river in Southern Laos (LDR) has created an area known as 4000 Islands.  Here, Mekong River changes from a single channel to multiple braids that thunder across a spectacular array of waterfalls and rapids; a white-water kayaker’s idea of fun. Sam Ricketts, his friend Lachie Carracher, and a film crew (Luke McKinney and Lissa Hufford), converged, in December 2016, on Don Det, an island-town in the middle of 4000 Islands; their focus – Li Phi Falls. Continue reading


Ediacara; Welcome to the revolutionary world of animals

Mistaken Point on the Atlantic coast of Newfoundland (Canada) acquired its unfortunate reputation by fooling mariners.  In a celebration of a different kind, UNESCO, in July 2016 designated the Mistaken Point coast a World Heritage Site; it is the graveyard of exquisitely preserved animals known as the Ediacaran Fauna, and at 575 million years they are the oldest known, structurally complex, multicellular creatures.

From an evolutionary context, life forms during the previous 3 billion years were dominated by much simpler algal-bacteria like organisms that constructed mats, mounds and columns (stromatolites) and even reef-like structures, all made by single-cell prokaryotes.  The Ediacaran fauna thus represents a kind of evolutionary paradigm shift – to real animals.  As Guy Narbonne (Queens University, Ontario) has suggested, this unique fauna probably formed the “root stock” of the more recent and familiar animal kingdom, but also includes some fossils that represent failed evolutionary experiments – creatures having unique form, phylum, and genetic codes that simply didn’t go anywhere.

The complete 2016 Mistaken Point UNESCO Heritage Site dossier by Richard Thomas and Guy Narbonne can be found here, but NB, it is a large file!

What kind of animals were they?

Although discovered in Namibia, the age and evolutionary significance of the fauna were first recognised in Flinders Range strata, Australia. The name Ediacara is probably Aboriginal.  Ediacaran fossils range from 575-542 million years; the period immediately prior to what is commonly called the Cambrian Explosion. Ediacaran fossils are now found on all continents except Antarctica.

The iconic Ediacaran fossils are those that appear petal-, feather-, or sea-pen-like, creatures that in some beautifully preserved examples exhibit complex growth patterns. Guy Narbonne has described these growth patterns as “quilted fractals”, an analogy that is quite apt. They were soft-bodied animals; fossils with hard parts, shells or hard skeletal frames did not appear until the very end of the Precambrian, becoming abundant in the Cambrian.  The petal-like structures had a central stem that was attached to or grew into the sea floor; in some cases only these holdfasts are preserved. Other forms that appear frond-like grew to almost 2m in length. Some were fan-, bush-, and comb-shaped; others simple domes or discs. Imagine the ancient seafloor covered in a forest of these soft, delicate forms, swaying in the wash of gentle sea currents.  It must have been quite stunning.

Trace fossils are also present, becoming abundant in rocks younger than about 555 million years.  These are not static impressions of animals, but tracks and burrows of worm-like creatures that moved on or through soft sediment.  Many traces resemble those made by animals in much younger strata, and if the same interpretation is applied to the Ediacaran types, then they too represent animal behaviours such as feeding, or burrowing a new home.


Preservation – an interesting conundrum

Paleontologists frequently consider the preservation potential of the fauna and flora they study.  Animals having hard parts are more likely to be preserved than those without.  However, even skeletal remains may not survive the vagaries of scavenging or dislocation.  Complete dinosaur skeletons, although celebrated, are rare; after death the animal is prone to being eaten, crunched by powerful jaws, or dismembered by flooding rivers. Preservation of soft-bodied animals is even more fraught – they tend to decay rapidly, are eaten by scavengers, or are dismembered by ocean currents and waves.

Most Ediacaran fossils were preserved as impressions in sediment. The uniqueness of the Ediacaran fossil record is a testimony to the absence of scavengers during this geological period.  Many, like the Mistaken Point communities (and also in Mackenzie Mountains) lived in relatively deep water where currents were subdued but strong enough to ensure a continuous supply of nutrients.  That the fossils are intact means that they were buried by sediment before decay set in.

Those animal communities that lived in shallower seas (there are examples in Australia, Namibia and Russia) were periodically subjected to stronger currents and waves and had correspondingly lower preservation potential.  The buried parts of stems and fronds, and some animal burrows could be preserved (after all they were already buried), but the more delicate structures above the sea floor were easily broken up.   In some environments, such as those now found in the Flinders Range, the dead fronds or bushes were covered by a thin microbial mat that enhanced preservation.  Elsewhere (Newfoundland and England), volcanic ash falling into the sea filtered quickly through the water column, gently smothering the live animals – a bit like Pompeii.

In the grand scheme of things It is generally understood that complex, multicellular animals like the Ediacara fauna require oxygen.  For much of the preceding 3 billion years, free oxygen was in short supply. By about 1800 million years the oxygen levels are thought to have been about 10% of the concentration in our modern atmosphere (based mainly on stable isotope chemistry).  The biomass back then was dominated by single cell, prokaryotic microbes (such as cyanobacteria).  There is good evidence that simple, multicell eukaryotes were present at least 1300 million years ago, for example in forms like red algae, but they were in the minority. Sudden appearance of the Ediacaran fauna indicates that oxygen levels may have increased abruptly 600-580 million years ago, creating the right conditions for evolutionary expansion; some estimates put oxygen concentrations at about 50% present atmospheric levels.

Continued research will probably refine these numbers. Regardless, the Ediacaran fauna provides fantastic evidence of significant evolutionary trajectories and ancient environmental conditions for one of the most crucial periods in the history of our earth.


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.