Category: Geomorphology

In 2006 Wallace Broeker first suggested that the sudden interruption of emergence from the last glacial maximum by a frigid climate about 12.8 ka was due to a massive release of fresh water to the North Atlantic that shut down its thermohaline ‘conveyor’ (see The Younger Dryas and the Flood in June 2006 issue of EPN). He resurrected an earlier idea that a vast lake of glacial meltwater (Lake Agassiz) to the north-west of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. His hypothesis was that the resulting freshening of surface water in the North Atlantic and decreased density stopped the formation of cold dense brines that sink and drag warm water northwards. Setting aside the notion by some enthusiastic authors that a trigger for the Younger Dryas was an exploding comet and a kind of ‘nuclear winter’ (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008) Broeker’s hypothesis is widely accepted. However there are few signs, if any, of a catastrophic glacial-lake outburst through the Great Lakes region and down the St Lawrence. An alternative is that Lake Agassiz drained northwards towards the Arctic Ocean. (Since the North American ice sheet covered Hudson’s Bay that could not have been the destination.) At the end of the last last full glaciation there was a corridor with relatively little glacial cover between the main ice over the Canadian Shield and that mantling the Rocky Mountains, roughly along the course of the modern Mackenzie River. That route would serve the hypothesis well, and there is clear evidence that an outburst flood followed it (Murton, J.B. et al. 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, v. 464, p. 740-743).

Sediments of the huge Mackenzie Delta of NW Canada contain a sharp erosion surface overlain by gravels that belie the low-energy of deposition today. Optically stimulated luminescence dating of sediment immediately below and above the erosion surface range from 13.4 (below) to 12.7 ka (above), the latter approximating the onset of frigid Younger Dryas conditions. The surface occurs all the way along the Mackenzie into its major tributary the Athabasca River. Near Fort MacMurray, 20 km north of what was the northern shore of Lake Agassiz, there is a terrace composed of massive boulders. Further evidence comes from the apex of the Mackenzie delta in the form of a 25 km long, 2 km wide spillway scoured of all loose sediment and with topographic features reminiscent of the famous Channelled Scablands of Washington State in the NW USA. Numerous beach lines record the drainage of Lake Agassiz, the highest being dated at the start of the Younger Dryas and giving a clue to the volume involved in the initial outburst flood: around 9500 km³. Dating of other features suggest that a second flooding into the Arctic Ocean occurred during the Younger Dryas around 11.5 ka, during its last stages, and a third at 9.3 ka. One effect of the Younger Dryas was a regrowth of the main ice sheet that allowed Lake Agassiz to refill periodically perhaps allowing quieter flooding events down the Mississippi and through the Great Lakes. There are no signs in the climate record of any major perturbation at 9.3 ka.

Broeker received the news graciously, commenting that a freshening of the Arctic Ocean would have been more effective at shutting down North Atlantic thermohaline circulation than a spillway down the St Lawrence, because the sites of modern day sinking of dense cold brine lie well to the north of its outlet. The only way additional water in the Arctic Ocean could escape would have been into the northernmost North Atlantic.

See also: Schiermeier, Q. & Monastersky, R. 2010. River reveals chilling tracks of ancient flood. Nature, v. 464, p. 657.

One of the delights of Google Earth is to commit a little Thesigery in the comfort of your front room and traverse the Sahara, the Empty Quarter of Arabia, the Namib or the Gobi. Not only are there dunes on gargantuan scales, but zooming-in from 30 m Landsat to 65 cm Quickbird images on Google Earth reveals a dune hierarchy down to largish ripples. And not all dunes are classic in shape. In the same issue of Nature as a retrospective review of Ralph Bagnold’s classic The Physics of Blown Sand and Desert Dunes, French, Algerian and US workers give a clue to the fundamental controls over dunes systems, that was not available to early researchers (Andreotti, B. et al 2009. Giant aeolian dune size determined by the average depth of the atmospheric boundary layer. Nature, v. 457, p. 1120-1123). They conclude that the general dynamics are analogous to those in flowing water; i.e. like a river, the wind has a capping surface that is the thermal inversion in the atmosphere marked by the tropopause. Flow that is  physically bounded involves a series of resonances (as in a flute), which help to explain the tiered nature of dune systems and also their maximum size in a particular area of desert. Together with seasonal shifts in wind direction, fluctuations in the ‘depth’ of the wind combine together to produce the hypnotically addictive disorganised order that makes big sand deserts so attractive, despite their dangers.

At first glance this section’s title seems absurd, for glaciation has the highest potential for erosion that there is on Earth. Yet it seems that at the eastern edge of the Tibetan Plateau the long-term potential for river erosion has been impeded by glacial action (Korup, O. & Montgomery, D.R. 2008. Tibetan plateau river incision inhibited by glacial stabilisation of the Tsangpo gorge. Nature, v. 455, p. 786-789). The accepted wisdom is that in the course of powerful rivers, such as the Tsangpo, steep stretches or ‘knick points’ focus erosion that proceeds headwards to drive a wave of dissection towards the sources of the main river and of all its tributaries. The Tsangpo has had the better part of 40-50 Ma since the India-Asia collision to eat away the vast Tibetan Plateau, but it has failed, as have other, lesser river systems. Repeatedly emplaced moraine dams, seem to have locked the knick points associated with the Tsangpo catchment at around 260 separate locations.

See also: Owen, L.A. 2008. How Tibet might keep its edge. Nature, v. 455, p. 748-749.

The classic notion of a floodplain is that the streams responsible for it meander to create point bars, overbank muds and all the other paraphernalia of the fluvial sedimentologist. River authorities seeking to restore floodplains see the meandering stream as the ideal to aim for, and increasingly as a means of natural flood amelioration. All this may turn out to be illusory following publication of a study on long-vanished human activities (Walter, R.C. & Merritts, D.J. 2008. Natural streams and the legacy of water-powered mills. Science, v. 319, p. 299-304). By mapping and dating alluvial deposits along 1^st to 3^rd order streams in the north-eastern USA, in relation to milldams recorded on 19^th century maps, Walter and Merritts of Franklin and Marshall College, Pennsylvania found that up to 5 metres of sediment had accumulated behind the dams since the 17^th century up to the abandonment of watermills.

The conclusion is that mill dams together with increased sediment load following deforestation for agriculture created valley flats on a vast scale – three counties in Pennsylvania had over a thousand mill dams. In places along the north-eastern Piedmont the density of water mills reaches as many as one per square kilometre, and the median density of around 1 per 10 km² involved more than 22 000 mills out of a total in 1840 of >65 000. Once the mills were abandoned, either because their dams had silted up or milling turned to larger facilities powered other energy sources, streams developed meanders that gradually incised the artificial flood plains. The situation now is that the small floodplains rarely flood, spates being unable to spill over the current bank height. Consequently, many of the low-order streams in major river catchments discharge floods quickly to the larger streams and rivers, which themselves burst their banks to cause floods with major social and economic consequences.

Walter and Merritts’ findings are also based on their analysis of the kinds of sediment that accumulated before European colonisation. In most small valleys these indicate extensive forested wetlands with multiple small channels and drier islands. A major influence over this earlier state was the formation of logjams, and perhaps beaver lodges, that spread normal and spate flows. Slow steam flow carried less sediment than nowadays, and the older Holocene alluvial deposits are organic rich. In addition, stream flow, once directly connected to groundwater, has become disconnected thereby reducing both recharge and the flood balancing achieved by truly natural streams.

The whole of Europe had a history of milling around five times as long as that in the eastern USA, as well as higher population densities. In addition, urban mill dams for metal forging and textile manufacture were on a larger scale. The UK’s National River Authority, Environment Agency and Phil Woolas, the Minister of State (Environment) need to read this study with care, as another flood season is almost certain in the summer of 2008 or the winter of 2008-9. As far as I can judge, it demands a reassessment of flood prevention ‘best practice’ in any populated humid-temperate landscape. Whatever, Walter and Merritts’ study forces a new look at the European lowland and upland geomorphology used for teaching at all levels.

The name ‘weathering’ has always been taken to indicate a direct relationship between the atmosphere and the breakdown of rocks, i.e. at or very close to the surface. This is so easy to test that it comes as a surprise to find that nobody has really tried until recently (Yokoyama, T. & Matsukura, Y. 2006. Field and laboratory experiments on weather rate of granodiorite: Separation of chemical and physical processes. Geology, v. 34, p. 809-812). Tadashi Yokoyama and Yukinori Matsukura of universities of Osaka and Tsukuba, Japan, placed small cut tablets of identical fresh granodiorite in three position: at the surface, buried above the water table and buried beneath the water table in one small catchment. These samples stayed there for 10 years. The only sample to show much sign of chemical breakdown of minerals was that buried below the water table. Does anyone claim that there is weather in groundwater? Just exposing fresh granodiorite in the laboratory to a constant flow of water chemically similar to the groundwater doesn’t accomplish the weathering (it is 50 times slower than when samples are buried). Chemical weathering needs to involve soaking, when grain boundaries break down so that individual grains can become detached and allow yet more penetration.

Most geoscientists who work on topics that involve chemical weathering, such as the changing release of tracer isotopes of strontium to estimate rates of weathering in the past, assume that it is all done by atmospheric carbon dioxide dissolved in rainwater or released by organisms in soil. It is accomplished by hydrogen ions that can be released by a great deal more processes than the formation of vary weak carbonic acid (e.g. organic acids and breakdown of sulfides). It now seems very clear that chemical weathering is a product of groundwater and burial, so should we call it weathering at all?

Several times in Earth Pages News the topic of how erosion contributes to uplift has cropped up. That is more than just the iceberg-like bobbing up of the crust as the load on the underlying asthenosphere is eased by surface rock removal. One oddity is that as large valleys are carved the ridges and peaks that they separate can rise higher than the original lands surface from which they developed (see Erosion and plate tectonics in May 2005 issue of EPN). Now it is becoming clear that sideways movement of the crust beneath mountain ranges can also be a response to erosion; thrusts and nappes can respond to erosion as well as to plate tectonic forces. The most likely place where this might be happening is in the Himalaya, which produce a huge contrast in climate and erosion rate between their southern and northern sides by creating the world’s largest rain shadow. The evidence for this possibility is nicely reviewed by Kip Hodges of Arizona Sate University (Hodges, K. 2006. Climate and the evolution of mountains. Scientific American, v. 295 (August 2006 issue), p. 54-61).

The highest erosion rates take place where rainfall during the Indian monsoon is greatest, on the SSW face of the Himalaya, especially in the foothills between about 1000 and 3500 m. The Tibetan Plateau lies in the rain shadow of the Himalaya, and erosion is far less intense. Yet the Tibetan plateau is buoyed up by crust that is double the normal thickness, to an average elevation of around 5 km. In a crude way Tibet can be regarded as having a pressure head ‘dammed’ to the north of the Himalaya. Intense erosion at the foot of the mountain ‘dam’ is likewise akin to one cause of landslides: erosion of the toe of a slope. The gravitational potential of Tibet, combined with continual undermining of the Himalayan front must create a lateral force. Where the crust is able to behave in a plastic fashion, i.e. at depth, and if there are surfaces on which movement is possible — the north-dipping frontal thrusts of the Himalaya — then deep crust should be extruded sideways. In fact there are faults systems just to the north of the Himalaya that have the same dip as the thrusts, but an opposite sense of movement, directed northwards to create extensional detachments. The crustal zone in-between is the most likely to undergo extrusion. GPS measurements there and cosmogenic dating of the surface reveal that indeed this zone is experiencing  anomalously high rates of uplift. It is producing extremely high gradients on both hillsides and valley floors.

When Cenozoic mountain belts and high plateaux began to rise and be eroded has become a disputed topic about most of them, such as the Himalaya and Tibetan Plateau, the North American Western Cordillera, the Andes and the Ethiopian Plateau. A host of techniques have been used, including plant-leaf stomatal indices, the bubbles in lavas, cosmogenic isotopes and various stratigraphic approaches. For the Tibetan Plateau (see When did Tibet rise? and Tibetan uplift: looking a gift horse in the mouth in the March and May 2006 issues of EPN) opinion is polarised: as soon as India collided with Asia, around 40-50 Ma ago, due to crustal thickening; as a result of a slab of lithosphere delaminating from the region as recently as the Late Miocene (8-10 Ma), when the crust rose gravitationally. There is support for both hypotheses. Much the same divergence of opinion applies to the Sierra Nevada of the Western USA, which may have been a major feature throughout the Cenozoic, or subject to delamination in the Pliocene (3 to 5 Ma ago). In its case, a new way of estimating topographic elevation in the past may resolve the disputation (Mulch, A. et al. 2006. Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada. Science, v. 313, p. 87-89).

The Stanford University group led by Andreas Mulch have focused on the way in which the proportion of deuterium (²H) in rainwater changes as clouds rise over high areas. This is known quite precisely from modern hydrological studies. Of course, you cannot find ancient rainwater ponded in the place where it once fell, but that rain does find its way into clay minerals during weathering. The western flank of the Sierra Nevada is mantled by extensive Eocene graves deposited by rivers that drained the area now occupied by the mountains. Handily, those gravels were the main target for the 1849 California Gold Rush and subsequent alluvial gold mining, so they have been well exposed by the prospectors throughout the Sierra Nevada. At high elevations they are preserved in river terraces, so it is possible to trace roughly the ancient drainage courses and sample a range of modern elevations. The hydrogen isotope data from kaolinite in cobbles of weathered granitic rocks are extremely interesting. They show that the Eocene rainwater that entered kaolinite molecules at different modern heights had fallen at a very similar range some 50 Ma ago. There seems little doubt that the Sierra Nevada had risen more than 2 km before the Eocene

The land’s present topography is not just the frontier between the lithosphere and the atmosphere and hydrosphere, but where plants of many different kinds grow. Whether in the form of cyanobacteria, lichens or luxuriant tropical rain forest, vegetation affects weathering, erosion and the deposition of sediments. Animals – leaving out humans – also have some influence, whether they be subterranean rabbits, moles and worms, or heavy-footed beasts that force soils to move downslope. Inevitably life-land interactions affect landforms, although rock-type and active geological processes tend to dominate. Nonetheless, a planet with life ought to show different styles of surface shapes from one that is organically dead. The central issues for geomorphologists is whether or not it is possible to define absolutely the differences, and then to use them as a means of detecting the likely former influence of life on other worlds.

Central to such a venture (Dietrich, W.E. & Perron, J.t. 2006. The search for a topographic signature of life. Nature, v. 439, p. 411-418) is the ability to map in detail the variation of topographic elevation. Digital topographic elevation data is now available for most of the Earth’s land surface at a resolution of between 90 and 30 m, the second only publicly available for the USA, from the groundbreaking Shuttle Radar Topography Mission of 2000. Aerial photography and high-resolution stereoscopic images from satellite such as Quickbird and Ikonos, allow resolution as sharp as a few metres.  Laser scanning from aircraft potentially can even improve that to the scale of a few tens of centimetres, but such high-resolution data are far from global. The planet Mars is now better endowed with elevation data than is our own planet, thanks to photogrammetric instruments carried by ESA’s Mars Express mission, and the shyness of various intelligence agencies to share publicly what they have gleaned from high-altitude aircraft and spy satellites. Nonetheless, it is now possible to analyse elevation data from the entire range of terrestrial biomes to see what signal vegetation has imposed on surface shape. An easy way to visualise that is simple – just use Google Earth (see The Digital Earth revolution above).

Dietrich and Perron review the mathematical approaches to modelling life’s topographic influences, beginning with an equation that relates elevation and time to rates of uplift, erosion and entry of sediment into storage, thereby expressing conservation of mass.  All the variables are themselves governed by a variety of processes, theoretically amenable to quantification, summarised in Dietrich and Perron’s review. In each there will be some potential biotic influence. On Earth there are sufficient landscapes devoid of all but a minute veneer of organisms to assess both end-members clearly. Mars and Venus ought to be good tests.  But, should such a rigorous quantification of lifeless and lively surfaces at a spectrum of scales be achieved, where would we deploy it?

The world’s most awesome natural spectacle is probably the Brahmaputra River in full spate.  Unlike most large rivers, it is constrained for most of its course within a deep, narrow gorge that has to take the snow melt from a huge catchment on the northern flank of the High Himalaya, brought partly by the Tibetan Tsangpo River.  Each spate hurtles onto the plains of  Bangladesh, loaded with debris, at a rate of around 70 thousand cubic metres per second.  Although that is but a third of the flood discharge of the Amazon, for much of the  Brahmaputra’s course it must pass through a gorge only a few hundred metres wide in places.  This gives not inconsiderable erosive power, indeed probably the highest anywhere.  Not surprisingly, little is known about the Tsangpo-Brahmaputra valley, because of its inhospitable character.  With the recent release of ~90m resolution elevation data from the Shuttle Radar Topography Mission, it is now possible to analyse the whole catchment’s morphology in detail, without needing to follow the individual rivers.  Parts of the lower Tsangpo have remarkably high gradients, including a 100 km stretch with a fall of more than 2 km, through a gorge with almost 7 km of relief on either flank that cuts N-S across the axis of the Eastern Syntaxis of the High Himalaya. The gorge lies downstream of a west to east stretch with lower gradients, falling around 1 km in 300 km, which suggests some dramatic incision begins at the junction of the two sections.  US and Chinese geomorphologists visited the area and discovered that high on the flanks of the upper Tsangpo are terraces of lacustrine sediments, at about 3100 and 3500 m (200 and 600 m higher than the river) (Montgomery, D.R. et al. 2004.  Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet.  Quaternary Research, 9 September 2004 issue).  Charcoal in the sediments gives radiocarbon ages between 1200 to 1600 BP and 8800 to 9800 BP for the lower and higher terrace levels, so the lakes formed during the Holocene.  The terraces stop at a zone of thick glacial moraine, cut by the Tsangpo, which suggests that both formed in lakes behind two ice dams.  Using SRTM data allows the volume of water ponded in both ice-dammed lakes to be estimated.  The older and higher level indicates about 830 km³, and the lower some 80 km³.  Breaching of the dams would have caused the largest recorded erosive events in recent Earth history, and explains the gorge below.  Each flood discharge would have been between 1 and 5 million cubic metres per second, equivalent 3 to 15 times the maximum flood discharge of the Amazon.

The greater the rainfall, the more effective streams become as agents of erosion.  So, “common sense” suggests that very wet mountain areas should be eroded more quickly and develop a more profound relief than those that are drier.  With the advent of detailed digital elevation models that cover the world, it is very easy to calculate slope angles and relief over huge areas, and match them with rainfall records.  Geomorphologists from the Universities of Montana and California have done this for the wettest and most rugged area in the world, the Annapurna area of the Himalaya (Gabet, E.J. et al. 2004.  Climatic controls on hillslope angle and relief in the Himalayas.  Geology, v. 32, p. 629-632).  The main agent of erosion there as streams cut downwards is by landslides.  The region also shows a profound gradient in annual rainfall from about 1000 mm in the High Himalaya to 4500 at the front of the range, where the monsoon rains hit hardest.  “Common sense” is wrong, for the slopes decrease from an average of 35º to 25º as rainfall increases.  The authors believe this is due to the influence of deeper weathering in more humid parts that reduces the strength of slope materials so that they must stabilise at lower angles than those in dry areas.  Their other finding is that relief (elevation difference in small segments of an area) and slope angle have a strong positive correlation, so that relief itself is inversely related to rainfall.  They are able to comment interestingly on various ideas about mountain evolution.  Their main conclusion is that in any particular area, a transition from dry to wet conditions lowers mountain ridges faster than valley incision can shift the debris, whereas during drying, ridges are barely lowered, while streams cut unhindered into bedrock, thereby sharpening up the landscape.

Formation of gorges in tectonically quiet areas

The flanks of the North Atlantic probably became tectonically inactive in Mesozoic times, yet rivers large and small have cut large gorges, often through highly resistant bedrock.  But they also have developed broad valleys over millions of years, and it is into them that the gorges are incised.  Slow upward flexing caused by sediment loading on the continental shelves, a general lowering of sea level since Antarctica first formed a permanent ice cap, and isostatic response to gradual denudation help explain the full extent and shape of the rivers drainage basins.  The gorges are young, and must have developed rapidly.  Old ideas focussed on W.M. Davis’ theories of landscape evolution, particularly rejuvenation associated with changing base levels of erosion, but with no quantitative backing.  The development of means of dating eroded surfaces using the decay of short-lived radioactive isotopes that cosmic-ray bombardment creates now offers an opportunity to test hypotheses rigorously and come up with others.  Quite a few published works on cosmogenic dating applied to landform development seem to add little to geomorphological knowledge, so it is a relief to find one that does (Reusser, L.J. et al. 2004.  Rapid late Pleistocene incision of Atlantic passive-margin river gorges.  Science, v. 305, p. 499-502).  The authors, from the Universities of Vermont and Maryland, the USGS and the Lawrence Livermore National Laboratory, focus on impressive gorges in the lower reaches of the Susquehanna and Potomac Rivers as they drain the eastern US into the Atlantic, and a series of higher surfaces which they cut into to leave as rocky straths.  The oldest ages occur on the highest of these straths, as expected, and age decreases on successively lower ones to the rocky flood plain of the modern rivers just above their current channels.  The highest levels are between 85 and 97 ka, the most prominent strath formed between 30 and 33 ka, succeeded by  one at 19 ka and the lowest level seems to have formed between 13 and 14 ka.  Interpreting the periods of intense erosion that cut each level must involve late Pleistocene climate change, sea-level shifts, and the bulging effect due to the North American ice sheet which reached its maximum extent in the northernmost part of the Susquehanna basin.  It seems that during the early part of the last glacial episode, incision was slow, although probably faster than during the Holocene.  But around 30 to 33 ka ago it accelerated rapidly to half a metre every thousand years, some 1 to 2 orders of magnitude greater than at present.  This was at a time when ice loading was only half that at the glacial maximum around 20 ka, so it seems likely to have been initiated more by increased storminess and torrents, and indeed correlates with an abrupt increase in sea-salt content in the Greenland ice cap brought in by winds at that time.  Lasting through the glacial maximum, increased frequency of flooding combined with more rapid sea-level fall, also beginning at around 32 ka, were probably the main driving forces for gorge incision.  This still leaves a puzzle.  Both drainage basins had been in existence since well before the cycles of glacial and interglacial periods began on the flanks of the North Atlantic around 2.5 Ma ago.  Similar periods of accelerated incision must have been repeated, at least during the last 6 or 7 glaciations which were the most extensive.  Did earlier topographic features exert any control over later ones, and do any relics of them remain?