Category Archives: Geology
Another delightful posting from my friend Marli Miller.Thanks, Marli. I’ve blogged here earlier myself about the famous unconformity at Siccar Point, and the depositional contact at the Giants Causeway between a later lava flow, and the paleosol formed by weathering of the one before it.
Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!
Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.
In a cross-sectional sketch they may…
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Landscape as process, beautifully illustrated. Students with Marli Miller as instructor are fortunate indeed
Most of us love landscapes –and many of us find ourselves wondering how they came to look the way they do. In most cases, landscapes take their shape through the combined processes of weathering and erosion. While weathering and erosion constitute entire fields of study unto themselves, this primer outlines some of the basics—which pretty much underlie all the further details of how natural processes shape landscapes.
Aerial view of incised meanders of Green River, Utah.
Two definitions: weathering describes the in-place breakdown of rock material whereas erosion is the removal of that material. Basically, weathering turns solid rock into crud while erosion allows that crud to move away.
Weathering processes fall into two categories: physical and chemical. Physical weathering consists of the actual breakage of rock; any process that promotes breakage, be it enlargement of cracks, splitting, spalling, or fracturing, is a type of physical weathering. Common examples…
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Reblogging from Steve Drury at wileyearthpages. In an earlier post (How to learn from creationists), I mentioned the case of the buried Nile canyon as one where I had learnt from answering creationists’ questions. This piece gives much more detailed interesting information, and I am surprised (not for the first time) to learn how young great river systems are in their present form.
The longest river in the world, the Nile has all sorts of riveting connotations in terms of archaeology, Africa’s colonial history, the romance of early exploration and is currently the focus of disputes about rights to its waters. The last stems from its vast potential for irrigation and for hydropower. It is probably the most complex of all the major rivers of our planet because it stretches across so many climatic zones, topographic systems geological and tectonic provinces. Mohamed Abdelsalam of Oklahoma State University, who was born in the Sudan and began his career at the confluence of the White and Blue Nile in its capital Khartoum, is an ideal person to produce a modern scientific summary of how the Nile has evolved. That is because he has studied some of the key elements of the geology through which the river and its major tributaries travel, but most of all…
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I have discussed the Grand Canyon myself at https://paulbraterman.wordpress.com/2018/03/09/why-science-needs-philosophy-cont-and-why-it-matters-with-examples-from-geology/ , and strongly recommend this beautifully written and illustrated guide to its unconformities
A prominent ledge punctuates the landscape towards the bottom of the Grand Canyon. It’s the Tapeats Sandstone, deposited during the Cambrian Period about 520 million years ago, when the ocean was beginning to encroach on the North American continent, an event called the Cambrian Transgression. Above the ledge, you can see more than 3000 feet of near-horizontal sedimentary rocks, eroded into cliffs and slopes depending on their ability to withstand weathering and erosion. These rocks, deposited during the rest of the Paleozoic Era, are often used to demonstrate the vastness of geologic time–some 300 million years of it.
But the razor-thin surface between the Tapeats and the underlying Proterozoic-age rock reflects the passage of far more geologic time –about 600 million years where the Tapeats sits on top the sedimentary rocks of the Grand…
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Quite a challenge, I expect, to the local Free Presbyterians.
My friend Kim Johnson commented on the strange appearance of the footprints, Steve Drury (author of the blog of which this is just an annotated repost) referred me to Paige Depolo, senior author of the paper on which Steve’s post is based, and Paige replied as follows:
When it comes to the depositional environment, the tracks were formed in a low-energy lagoon and are generally preserved today as impressions into shaley limestone. Later, additional limestone layers were laid down at the site and in-filled the impressions. Those layers form the casts that we can still observe for some of the tracks today. In some cases at this site, the cast remains while the surrounding impression which it was originally infilling has been almost completely eroded. These rocks were deposited during the Middle Jurassic. Later, likely during the Paleogene, a sill was intruded immediately below the track bearing layer and the surrounding rocks were baked. The low-level contact metamorphism of the track-bearing layers definitely makes for some interesting looking exposures!
h/t Kim and Steve, and many thanks to Paige
The Isle of Skye off the northwest coast of Scotland is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser…
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EVOLUTION: What the Fossils Say and why it Matters, Donald R. Prothero (2nd edition)
If you are interested in evolution, get this book. And make sure that your library gets it. And your children’s highschool library. Incidentally, it’s incredible value; list price $35.00/£27.95 from Columbia University Press, with over 400 lavishly illustrated pages.
The book is a comprehensive survey of the fossil record, supplemented at times with other evidence, and framed as one long argument against creationism. It opens with a general discussion of the ideas behind current evolutionary thinking, moves on to a survey of specific topics in (mainly animal) evolution, from the origins of life to the emergence of humanity, and concludes with a brief discussion of the threat that creationism poses to rational thinking. The argument is laid out clearly in the seemingly artless prose of an accomplished writer in love with his subject matter, with plain language explanations that presume no prior knowledge, while the detailed discussions of specific topics give enough detail to be of value, I would imagine, even to a professional in the field. The author is an experienced educator and researcher, with thirty books ranging from the highly technical to the popular, some 300 research papers, and numerous public appearances to his credit, and the work is copiously illustrated with photos, diagrams, and drawings by the author’s colleague, Carl Buell. These illustrations are an integral part of the work, graphically displaying the richness of the data at the heart of the argument. Read the rest of this entry
“The wise learn from everyone.”1 The freak success (half a million reads) of my recent piece How to slam dunk creationists, and the subsequent discussion, have again set me thinking about how to learn from creationists. It is not enough to say, as Dawkins notoriously said, “[I]f you meet somebody who claims not to believe in evolution, that person is ignorant, stupid or insane (or wicked, but I’d rather not consider that).” Conversation is a two-way street, I have certainly learnt from creationists’ attacks on evolution, and if I am learning from them it is at least possible that they are learning from me.
Types of comment
Comments I have had from creationists fall into three broad groups (and note that contrary to what Dawkins says, some of these are at least partly informed, highly intelligent, and completely rational):
1) Simple misstatements
2) Appeal to the Bible
3) Purportedly scientific arguments, some without merit, while others refer to important issues.
From simple misstatements, not very much can be learnt, except perhaps the source of the misinformation. Remember that if someone quotes wrong information, the burden of proof is not on you but on them. Leave it there, as in this actual exchange: Read the rest of this entry
If there were decompression melting of magma in the West Antarctic volcanic province as the icesheet thinned, that would not be good news.
If the geological Society link in the article doesn’t work for you, try this one; http://sp.lyellcollection.org/content/early/2017/05/26/SP461.7
Most volcanic activity stems from the rise of hot, deep rock, usually within the mantle. Pressure suppresses partial melting, so as hot rock rises the greater the chance that it will begin to melt without any rise in its temperature. That is the reason why mantle plumes are associated with many volcanic centres within plates. Extension at oceanic ridges allows upper mantle to rise in linear belts below rift systems giving rise to shallow partial melting, mid-ocean ridge basalts and sea-floor spreading. These aren’t the only processes that can reduce pressure to induce such decompression melting; any means of uplift will do, provided the rate of uplift exceeds the rate of cooling at depth. As well as tectonic uplift and erosion, melting of thick ice sheets and major falls in sea level may result in unloading of the lithosphere.
During Messinian Stage of the late Miocene up to 3 km…
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From my friend Michael Roberts. And next time anyone tells you that fracking causes earthquakes, ask them what magnitude and refer them to the diagram at the end of this post.
As we mark the 70th anniversary of India and Pakistan, we would also remember that the Assamese did not have a good day on the 15th August.
During the night a massive Mag 8.6 earthquake struck the area, with its epicentre just inside Tibet.That makes it far bigger than the Mag 7.8 in Nepal in 2015. However the damage or loss of life, though considerable, were far less as the area was less densely populated.
My own interest is that we were living in Jorhat on the tea plantations of Assam at the time and the quake became part of family history. We lived in a bungalow, which was built on stilts for protection against earthquakes and had about ten servants as was the norm.
So what happened?
The bungalow on stilts began to sway. Wardrobes fell over, but didn’t hit anyone.
The car, an american Studebaker, rolled out off…
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Ten minutes difference, and Earth would still be Planet of the Dinosaurs
We have suspected for some decades that the dinosaurs1 became extinct as the result of a massive meteorite, an asteroid, hitting the Earth. We have known where the impact site was since 1990, if not before. But it is only last year that we successfully drilled into the impact site, and only now, for the first time, do we really understand why the impact was so fatal. And if the meteorite had arrived ten minutes earlier, or ten minutes later, it would still no doubt have inflicted devastation, but the dinosaurs would still be here and you wouldn’t.
66.1 million years ago, dinosaurs covered the Earth. 66 million years ago, there were none. And not only the dinosaurs, but the pterosaurs in the skies, the long necked plesiosaurs and even the ammonites in the oceans, and 75% of all complex animal life. No terrestrial vertebrate heavier than around 25 kg seems to have survived. What happened? Read the rest of this entry