LATE DEVONIAN (365 Ma)
Severity: 3rd worst
Cause: Still unclear, changes in sea level and ocean anoxia (?)
Climate: Abrupt cooling
Aftermath: Marine filter feeders diversify
Rather similar to the End-Ordovician extinction, warm-water marine invertebrates were the hardest hit during the Late Devonian extinction. 22% of known marine families and 57% of marine genera were wiped out during a global cooling event. Gondwana glaciations were once again the likely culprit, triggering a massive overturn of ocean waters, bringing cold, non-oxygenated bottom waters to the surface. As opposed to a sudden impact event (which will be showing up later), the Late Devonian extinction was characterized by slow glacial cooling. Estimates for this extinction’s duration range from 500,000 yrs to 25 My, so although it was the third most severe, it’s timing is still rather poorly constrained.
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In 1906, the seismologist Henry Reid developed the “elastic rebound theory” to explain earthquakes. When rocks begin to press against each other, they initially bend, like a spring, to accommodate the opposing forces. Eventually, when the rocks reach a point where they cannot bend further, they break. The bent rocks snap back, or rebound, to their original shape. The break is the fault itself, and the shock waves emanating from the rebound are the earthquake. The shock waves vibrate through the Earth, making it “ring” like a bell.
A fault is a rock fracture along which movement occurs. Normal faults develop where the crust stretches apart, as in the East African Rift Valley. In thrust faults, which are found at subduction zones, the rocks on one side of the fault are pushed up and over those on the other side. A third type of fault is the strike-slip fault, where the rocks on either side of the fault slip by each other horizontally. The San Andreas Fault is a strike-slip fault.
Learn more about earthquakes in the Gottesman Hall of Planet Earth.
I thought I’d share my final project from Introductory Geology! I made this back in fall of 2011 when I had no idea I’d be going on to study geology. I was just psyched to be able to use my graphic design skills, and this is still probably my favorite thing I’ve ever made.
Shock metamorphism & shatter cones
Above: A shatter cone in dolomite from the Kentland, Indiana impact structure (Specimen from Lawrence University.)
Meteorite impacts are pretty amazing things. An impact crater forms instantaneously on a human time scale, with the shock wave from the impact propagation through the target rock in the following stages: contact and compression, crater excavation, and crater modification.
Some rocks and minerals are deformed by shock-metamorphism that is caused by the high pressures and temperatures; certain shock-metamorphic features can be used to unambiguously identify an impact crater, as there are no other natural processes capable of producing such effects. During the contact and compression stage, shock waves will travel faster than the speed of sound through the medium and pressures will reach in excess of those in the upper mantle.
Shatter cones are the only shock-metamorphism indicators visible at the outcrop scale and form at pressures from around ~1-10 GPa. There are other shock-metamorphic features such as planar fractures or planar deformation features in quartz grains that are visible on the microscopic scale, but if you find a shatter cone, you know for sure that you’ve got an impact crater on your hands.
END-ORDOVICIAN (440 Ma)
Severity: 2nd worst
Cause: Some type of C cycle disturbance, not well constrained
Climate: Abrupt ice age followed by rapid warming
Aftermath: Cambrian organisms (e.g. trilobites) decimated
During the End-Ordovician mass extinction, 25% of known marine families and 60% of marine genera were wiped out. Warm-water invertebrates were the hardest hit, as the event was likely caused by a severe cooling event in the world’s oceans triggered by Gondwanan glaciation.However, as with just about all of the mass extinctions, other causes have been offered, such as a gamma ray burst and volcanism and weathering.
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The Giants Causeway- Rock Formations - Northern Ireland.
I am honored to be following a genius on tumblr vancouver watch out you got an amazeballs expert coming your way o read your abstract and just woweowow ;)
I am honored by your comment! My amazing prof deserves a lot of that credit, I’m just psyched to let the world know about Brussels Hill!
Presenting at GSA!
Just got the official email that my GSA abstract has been accepted, so I’ll be heading out to Vancouver, BC in October!
I’m giving an oral presentation in The Holey Solar System session, so I’m partially terrified but pretty excited. It’s crazy seeing my name up there with the leading experts in the field of impact cratering!
This’ll be my second time at GSA, but last year I just did a poster presentation in the undergraduate session, so I’m jumping right into the big leagues now!
Here’s my abstract if anyone’s curious about the research I’ll be presenting!
Sklodowskite | ©Webshop for mineral collectors
Animas Mine, Francisco Portillo, West Camp, Santa Eulalia District, Mun. de Aquiles Serdán, Chihuahua, Mexico.
Gemmy crystals of Sklodowskite perched on contrasting matrix of Gypsum. Crystals are 2 mm in size with a bright neon yellow color.
Sklodowskite (Hydrated Magnesium Uranyl Silicate) is a rare, radioactive, uranium mineral that forms from the oxidation of uranium-bearing minerals.
The mineral is named for the famous chemist Marie Sklodowska Curie, who discovered the element Radium. Its green-yellow to yellow velvety tufts are attractive and make for an unusual mineral specimen.