Geomorphological Landscapes of the World (2)

I decided to write another post about this book (first post here). I’m almost finished with Metabolic Risk for Cardiovascular disease which I’ve been reading over the last few days, but I figure coverage of that one can wait a little (it’s not that great).

I have tried to pick out passages in the coverage below which should not be too hard for the ‘uninitiated’ to understand, and I hope that I have been successful. I’ve added links here and there to help making the post easier to read. I don’t really have a lot of new stuff to say about book, so I’ll get right to it.

Badlands occur worldwide and are especially common in the Northern Great Plains of North America. […] Badlands worldwide are formed by the forces of gravity and running water, especially by the process of slopewash erosion, which reaches its maximum potential under the combination of: (1) steep local topography; (2) weakly cemented, poorly indurated, readily eroded bedrock; and (3) a semiarid continental climate that supports a sparse vegetation cover, yet delivers precipitation in relatively high-magnitude, short-lived convective storms. This combination ensures that hillslopes are highly vulnerable to erosion, and erosive forces of running water reach their maximum expression on Earth. […]

Erosion has been the primary landscape-forming process [in the Great Plains badlands] over the past 5 million years during PlioPleistocene time (Bluemle 2000). Naturally, this erosion was not uniform. Well-indurated, freshwater limestone (lake) deposits and coarse-caliber, gravel (stream) deposits served as resistant caprocks on lowlying parts of the Miocene landscape. As the overall landscape was eroded, these former low spots (lakes and rivers) were more resistant to erosion and became isolated as topographic high spots – modern-day buttes and mesas […] This process of creating high topographic points from formerly low points is known as topographic inversion.”

“Conceptually, the formation of Grand Canyon should be very simple to explain. The Colorado River floods annually in the spring from snowmelt in the Rocky Mountains. These floods exert large tractive (erosional) forces against the bed of the river. Over many millions of years the river cut the magnificent canyon into the adjacent plateaus. In actuality, the processes are far more complicated and have not been completely explained.”

“The evolution of the quartzite landscape of the Gran Sabana has been a very long-term process. The rocks are very ancient, the region geologically stable, and the highest planation surfaces, forming the tepui summits and the plains of the surrounding Gran Sabana have been exposed for more than 70 million years, probably since the mid-Mezozoic (Jurassic?) […] It is this aspect of very, very, long periods of time for weathering, longer than most places on Earth, which is probably critical for the development of the striking landscapes of the Gran Sabana.” (Below a picture of what it looks like, from the wiki:)

(I should note that that wiki article was, as far as I know, the first article I’ve come across that had multiple featured images.)

“The Iguazu Falls are one of the most beautiful in the world because of the combination of a high and wide structural step across a fluvial system with large water discharge and the tropical environmental location that sustains an exuberant forest and high biodiversity. The geology of the area consists of three layers of basalts that give a staircase-type shape to the falls. The Iguazu River is about 1,500 m wide above the falls and forms many rapids between rock outcrops and small islands. The falls have a sinuous arch-like head 2.7 km long, and part of water volume enters a canyon 80–90 m wide and 70–80 m deep, forming the spectacular “Garganta do Diabo” (Devil’s Gorge). Part of river water enters the canyon by its left side and generates a front with 160–200 individual falls that form a unique wall of water during floods. Although no absolute ages exist on the evolution of the fluvial system, it has been suggested that the falls have been continuously wandering upstream to its present position by progressive headwater erosion at a rate of 1.4–2.1 cm/year in the last 1.5–2.0 million years.”

391px-Iguacu-004(Picture from wikipedia)

“South America is drained by huge and complex drainage systems, especially in tropical areas, and […] the largest South American rivers contribute to 28% of the total fresh water to the oceans […] The Paraná River basin started to develop concurrently with the rifting processes, related to the opening of the South Atlantic, when Gondwanaland was dismembered. River valley formation surely started after the prevailing desert conditions during part of the Cretaceous age, when a big sand sea spread along a large part of southeastern Brazil (Caiua Group). During this time the development of endorreic drainage under arid to semi-arid conditions could be the first evidence of a fluvial system being present in the former Paraná Basin after the immense basaltic extrusion event linked to the Gondwana partition. Despite its long history, the Paraná fluvial system is not well-understood […] there are strong controversies about the age of the present Paraná River system.”

“The Dry Valleys comprise an ice-free part of the Transantarctic Mountains in Antarctica, bounded by the East Antarctic Ice Sheet on the landward side and a coastal ice dome at the coast. Weathering rates are among the lowest on Earth and reflect the persistent hyper-arid, cold polar desert climate. Some buried ice has survived for over 8 million years. The main escarpments and valleys were created on a passive continental margin as Antarctica split from Australia some 55 million years ago. […] Geomorphological evidence of weathering rates suggests that the climate of the last 13.6 million years has been stable and remained dry and cold throughout. The important implication is that the ice sheet that controls the climate has also been present throughout. […] Intriguingly, the climate and processes of the higher parts of the Dry Valleys overlap with conditions on Mars.”

“If there is a single landform that might typify the landscape of Africa, then this would be an isolated bare rock hill or mountain rising from the vast plains. Hills of this sort struck an early German explorer of East Africa, Walter Bornhardt, so much that he invented a special term and called them inselbergs, literally meaning “island hills” […] Inselbergs vary in terms of lithology, dimensions, height, and shape, but have a few characteristics in common. First, as the name implies, they stand in isolation and are surrounded by a flat or gently rolling topography. Second, the topographic boundary between the hillslope and the plain around is fairly abrupt. Third, inselbergs are residual landforms due to wearing down of the surrounding terrain. Hence, volcanic cones and up-faulted blocks are traditionally not regarded as members of the inselberg family. The vast majority of inselbergs is built by strong and resistant rock, and very often this rock is granite […]

Spiztkoppe is one of the tallest, if not the tallest inselberg on Earth. Its summit rises to 1,728 m a.s.l. and overlooks the adjacent plains by 600 m. […] The evolution of the granite landscape of the Spitzkoppe inselbergs has been a complex and long-lasting process. The granite belongs to the family of Early Cretaceous (137–124 Ma ago) intrusions [according to the wiki, ‘The granite is more than 700 million years old’ – having read the chapter about it in this book, I’d say that claim merits a ‘citation needed’] […] The intrusions were emplaced at depths of several km below the ground surface that existed at the time. When the granites of Spitzkoppe were exposed to daylight is not known with precision […] The mean denudation rate was high in the late Cretaceous/early Tertiary, and perhaps several kilometers of rock were lost, but then surface lowering proceeded at a lower rate, decreasing further since the Miocene. Cosmogenic isotope dating suggests that the mean denudation rate in the last 10 million years has been of the order of 5 m/1 million years. Hence, by simple extrapolation and assuming (unrealistically!) no denudation at the site of the future inselberg, 120 million years would be required to produce a 600 m high residual hill. Allowing for an increasing denudation rate prior to 10 million years ago, these rates indicate that the tops of the inselbergs may have been exposed as early as in the late Cretaceous, ~80–70 million years ago. Over time, their height increased as the surrounding terrain built of less resistant rock was worn down. Evidently […] inselbergs have a very long history”

“The Afar Triangle is a barren lowland bounded by the Red Sea and the two blocks of Ethiopian Highlands […] its terrain is a casebook of tectonic geomorphology. Plate divergence is at its most obvious where the Red Sea has opened, and is still opening, between the Arabian and African plates. The African plate is breaking apart along the well-known East African Rifts, separating the Somalian plate from the main continental block (often known as the Nubian plate in the north). These three divergent boundaries have a triple junction at the Afar. The Triangle is the one place where the coastlines and plateau margins cannot be fitted neatly back into their pre-divergent entity – because the locally excessive constructive process of basalt generation has created anew the youthful lowland that is the Afar. […] ‘Hostile environment’ is a term tailor-made for the awful, hot, barren desert of the Afar […] Just one river enters it, and none leaves it. A few salt lakes contain almost the only water not yet lost to solar evaporation. Daily temperatures are 30–40°C in the cool of winter; summer regularly sees shade temperatures of 50°C on the floor of the Danakil Depression – and there is no shade.”

“The largest single feature of the Afar is the Danakil Depression, which descends to 126 m below sea level over the line of current plate divergence, and would be larger except that half its floor is occupied by shield volcanoes. […] Of the 34 volcanoes listed within the Afar Triangle, five have recorded activity within historical time. The largest and most frequently eruptive is Erta Ale, rising from the floor of the Ethiopian sector of the Danakil Depression. It is a classic shield volcano […] Its perimeter is more than 100 m below sea level within the depression, and its summit rises to 613 m above sea level […] Erta Ale is unique in that it has contained lava lakes that, between them, have been persistently active for at least 100 years […] The currently active lake lies within the central vent, which is a spectacular pit crater, developed by collapse when magma pressure declined beneath it. Only 60 m across when first recorded in 1968, it is now 150 m across, and about 80 m deep. A lava lake normally covers all or part of its floor, and has periodically overflowed. The lava has a temperature of about 1,200°C, while the rafts of chilled crust that cover most of its lake surface are at about 500°C. […] The continuing survival of Erta Ale’s lava lake relies on a substantial heat supply to match its thermal loss into the atmosphere […] This heat supply is from rising magma that is feeding a zone of active emplacement of dykes and sills. Within the immediate vicinity of the volcano, these intrusions currently fill new fissures to keep pace with the plate divergence, so that they largely prevent further fault-related subsidence in the Erta Ale sector of the main graben.”

February 15, 2014 - Posted by | books, Geology

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