Stories tagged impact crater


3D perspective of Decorah Impact Structure
3D perspective of Decorah Impact StructureCourtesy Public domain via USGS
By studying magnetic and electrical data, geologists have found further evidence that the town of Decorah, Iowa is built upon an ancient impact crater created around 460 million years ago.

Site map of Decorah crater
Site map of Decorah craterCourtesy Vkil via Wikipedia Creative Commons
Decorah is located in northeast Iowa near the Minnesota border about 150 miles south of Minneapolis. Scientists think the Decorah Impact Structure resulted from the same meteorite barrage - known as the Ordovician meteor event - that produced similarly-aged craters found in Oklahoma, Wisconsin, and the Slate Islands in Lake Superior. The Decorah crater lines up nicely with the others.

The first evidence that an ancient crater might exist under Decorah came in 2008 when well-drilling cores from the area collected and examined by Iowa's Department of Natural Resources and Geological and Water Survey indicated that a wide-ranging layer of an unusual type of shale set beneath the surface and encircled the town. Recent aerial geophysical measurements (both gravity and electrical magnetic) by the US Geological Survey and other agencies, including the Minnesota Geological Survey, affirmed the crater's existence.

The unusual shale layer is situated 50 feet under the bed of the Upper Iowa river and was probably deposited after the crater's creation when an ancient seaway invaded the area and filled in the basin with mud and sediment. Shocked quartz found in the rock layer directly beneath the shale adds further evidence that some sort of major impact took place. Shocked quartz is a highly stressed and shattered quartz produced only one of two ways: either by a bolide impact or from a nuclear blast. The impact that created the Decorah structure is estimated to have released the energy equal to the blast of 100 megatons of TNT. To put things in perspective, the nuclear bomb dropped on Hiroshima during World War II was equal to 15 kilotons of TNT. One megaton equals 1000 kilotons so the bolide blast in Decorah would have released the energy of more than 6500 Hiroshima bombs!

Paleobiologist Bevan French, an adjunct professor at the Smithsonian Institution's National Museum of Natural History studied quartz samples from the underlying layer of breccia and concluded they held characteristics indicative of an extra-terrestrial impact event.
When it slammed into Earth, the Decorah impactor created a 3.5 mile-in-diameter crater in the planet's surface and shattered existing layers of Early Ordovician and Cambrian rocks pushing them deeper underground. Several other meteor craters discovered on earth date back to around the same time period 450 to 470 million years ago, causing French to wonder if the Decorah crater should be included in that spike in impact frequency.

According to French the shale above the breccia layer is very well preserved and contains "a very fascinating biological assemblage,"which could also be of interest to paleontologists.

"Finding structures like these and being able to study them in the geological context," French said, "is going to yield a lot of very fascinating information about the relations between the terrestrial system and the extraterrestrial influences."

Sign of demise and new beginnings: A distinct layer of white clay in southern Colorado rock exposure (and found elsewhere around the world) marks the end of the dinosaurs (non-avian, anyway) and the beginnings of the reign of mammals.
Sign of demise and new beginnings: A distinct layer of white clay in southern Colorado rock exposure (and found elsewhere around the world) marks the end of the dinosaurs (non-avian, anyway) and the beginnings of the reign of mammals.Courtesy Mark Ryan
Has past life on Earth been influenced by these impacts? If you consider the Chicxulub impact crater in the Yucatán Peninsula of Mexico and its alleged effect on non-avian dinosaurs at the end of the Cretaceous Period, you have to wonder if similar impact events weren't responsible for other extinctions and biological radiations during Earth's long history.


USGS story
Wunderground news story
Live Science report
Ordovician Meteor Event
Tiny Traces of Big Asteroid Breakup


SEM scan no. 2 of K-Pg boundary material
SEM scan no. 2 of K-Pg boundary materialCourtesy ASPEX Corporation
Chemical analysis of sample number 2: The sample shows high concentrations of aluminum, silicon, oxygen, and sulfur, traces of iron, but no iridium.
Chemical analysis of sample number 2: The sample shows high concentrations of aluminum, silicon, oxygen, and sulfur, traces of iron, but no iridium.Courtesy ASPEX Corporation
Last December Joe made a post about a company that offered to provide free scanning electron microscope images (in this case a backscatter scanning electron microscope or BSEM) of whatever people sent in (within reason I suppose). I took the ASPEX Corporation up on its offer and mailed in some of the clay I had collected from the K-Pg boundary I visited last spring in southern Colorado. The K-Pg boundary, as most of you should know by now, marks the end of the Cretaceous period and the beginning of the Paleogene period. It used to be called the K-T boundary for Cretaceous-Tertiary but the term “Tertiary” has fallen into disfavor. Anyway, some 65 million years ago something wiped out all the non-avian dinosaurs, and left a tidy 1-inch layer of iridium-rich clay in several locations worldwide for geologists to puzzle over. Iridium is a rare-earth element (atomic number 77) but is fairly common in asteroids, meteorites, and other such extra-terrestrial space objects, so eventually, scientists came to the conclusion that such high amounts of iridium had to be from an extraterrestrial impact, and sure enough, an impact crater just the right size and age was eventually uncovered off the Yucatan Peninsula near Chicxulub, Mexico. (There’s been many years of discussion about the cause of the dinosaurs’ demise but just last month a committee of highly scientific mucky-mucks officially declared that the Chicxulub asteroid was the guilty culprit.)

SEM scan no. 7 of K-Pg boundary material
SEM scan no. 7 of K-Pg boundary materialCourtesy ASPEX Corporation
Chemical analysis of sample number 7: also contains traces of titanium and potassium.
Chemical analysis of sample number 7: also contains traces of titanium and potassium.Courtesy ASPEX Corporation
In Colorado, I found evidence of the Chicxulub impact about 10 miles west of Trinidad, Colorado. The outcrop at the Madrid East site on the way to Long Canyon is probably one of the better-defined exposures of the K-Pg boundary and is easily accessible off the main highway. A distinctive 1-inch layer of ashen gray clay can be seen sandwiched between two layers of coal – a 2-inch layer above and a 16 inch layer below - and the sequence is capped by massive Paleogene sandstone. While there, I scraped out a sample of the whitish clay to bring home.

Anyway, when Joe mentioned the free SEM scan I sent ASPEX some of the clay I’d collected hoping the scan might reveal some evidence of high iridium content, and of shocked quartz and glass spherules, telltale signs of an impact. Once sent, I promptly forgot about it until recently when a reply showed up in my mailbox.

The K-Pg boundary layer in the Raton Basin of Colorado
The K-Pg boundary layer in the Raton Basin of ColoradoCourtesy Mark Ryan
The results weren’t quite what I was hoping to see, but I did learn something. The images sent back don’t show much – at least not in the way of iridium, shocked quartz, etc. But the chemical analysis shows the clay layer (yellow arrow in photo) is indeed mostly just that - clay, or more specifically aluminum silicate hydroxide better known as kaolinite (it also contained some titanium, potassium, and iron). Kaolinitic clay is thought to result from the altering of volcanic ash beds in acidic coal swamps, but in this case it’s the result of a doomsday shroud of impact material interacting with a coal swamp. The kaolinite is the one-inch white stripe in the photo and is the layer I sampled. Unfortunately, the analysis from ASPEX shows no signs of an iridium anomaly, and here’s why:

The iridium I’ve since learned isn’t actually concentrated in the clay layer itself but in the 2 layers directly above it (red arrows in photo): that is the impact layer (smectite - blue arrow in photo), and the 2-inch coal layer directly above that. I was under the incorrect assumption the iridium was in the clay layer itself since that layer is what seems to mark the K-Pg boundary at least in the Raton Basin in southern Colorado.

My problem is I’m only an amateur geologist so I tend to operate on limited knowledge. Once the results of the scan showed up, I did do some further research and discovered much more information than I had when I visited the site last year. I was at the right place and was able to identify the boundary layer, I just didn’t have all the facts. But at least I’ve learned something from my mistake, so it turns out not to be such a bad thing. And now you’ve learned something, too.


ASPEX Send Us Your Sample page (I’m not sure they’re still doing it)
More about the K-Pg extinction impact
USGS K-Pg in the Raton Basin info site
Universe Today K-Pg boundary info
Buzz post on dinosaur extinction


A dazzling fireball comes blazing in: Photo by Hiroyuki Iida of Toyama, Japan, courtesy NASA.
A dazzling fireball comes blazing in: Photo by Hiroyuki Iida of Toyama, Japan, courtesy NASA.
Hundreds of Peruvian villagers have reportedly fallen ill from what they say are noxious gases coming from an impact crater left by something from space that slammed into the region.

A fiery object was seen falling to Earth last weekend over Carancas, a small town located in the Andes near in the Bolivian border, about 800 miles south of Lima.

People who visited the reported impact site say gases emitting from a large crater found there have caused them to suffer nausea, vomiting, eye irritations, and severe headaches. Livestock in the area have also become sick.

But not everyone believes the located “impact crater” has anything to do with the fiery object seen in the sky. Dr. Caroline Smith, a British museum meteorite expert, says it may just be mistaken for a crater.

"Increasingly we think that people witnessed a fireball, which are not uncommon, went off to investigate and found a lake of sedimentary deposit, which may be full of smelly, methane rich organic matter," she said.

An engineer from the Peruvian Nuclear Energy Institute reported no radiation has been detected at the site, and a team of scientists is on its way to the crater to investigate and gather further evidence. In the meantime, local authorities have been asked to warn people to stay away from the site.

Video from the site shows what appears to be a large crater 100-foot-wide by 20-foot-deep (another source states the crater is half this size). Marco Limache, a local official, reported that "boiling water started coming out of the crater, and particles of rock and cinders were found nearby."

If it proves to be a meteor crater, then it’s possible that sulfur or other elements in the extraterrestrial rock that caused the impact could have reacted with the ground water to produce the noxious gases.

Whatever it was - a fireball or a meteorite or possible space junk returning to Earth – it’s made a lot of local people nervous, and worried that the water is no longer safe to drink.

"This is the water we use for the animals, and for us, for everyone, and it looks like it is contaminated,” said one local villager.

"We don't know what is going on at the moment, that is what we are worried about,” he added.


BBC web site story
Cosmos magazine story


Ejecta from Sudbury Impact found in Minnesota: Photo by Mark Jirsa, Minnesota Geological Survey
Ejecta from Sudbury Impact found in Minnesota: Photo by Mark Jirsa, Minnesota Geological Survey
New evidence of an ancient meteorite impact in Ontario, Canada, has been found nearly 500 miles away in Minnesota.

Mark Jirsa of the Minnesota Geological Survey said the recent Ham Lake forest fire that raged over 118 square miles near the Gunflint Trail in northern Minnesota had caused the annual meeting of the Institute of Lake Superior Geology (ILSG) Institute of Lake Superior Geology (ILSG) to cancel one of its planned field trip into the region last May.

But because he’s a geologist, Jirsa wasn’t going to let a little conflagration ruin his chances to examine outcrop, so he sojourned into the area on his own to look for possible alternate sites.

Map showing Sudbury Impact and Gunflint Trail locations: Diagram by Mark Ryan
Map showing Sudbury Impact and Gunflint Trail locations: Diagram by Mark Ryan
While examining some of the Gunflint Iron Formation in an area he hadn’t visited before, he discovered an odd jumble of rounded black rock overlying the formation, which he now thinks may be partially composed of ejected material from the Sudbury Impact which took place 1,875 million years ago in Ontario.

“It’s fairly dark rock,” Jirsa explained. “They look like concrete, but in this concrete you would throw pieces of rock of all sizes and shapes and in all possible orientations.”

It’s thought that the Sudbury Impact site was created when an extraterrestrial object such as a comet or asteroid between 6 and 12 miles in diameter slammed into the Earth near the present-day town of Sudbury, Ontario, just north of Lake Huron. The effects of the impact were widespread, releasing as much energy as several billion atomic bombs, blasting out more than 6500 cubic miles of debris that blanketed nearly one million square miles with impact ejecta. The crater it left was over 160 miles in diameter, the second largest on Earth.

Some of this ejecta fell into the shallow ocean that covered northern Minnesota at the time. But soon after, shock wave-induced tsunamis would have tore across the area with such force the seabed would have been severely disrupted.

“When the meteorite hit, it’s very likely that the seas went out and then the seas came back in with a vengeance,” Jirsa said.

Consequently, the ejecta and the seabed material would become mixed into a jumbled mess of sediment that would later harden into rock. This is what Jirsa thinks he’s come across along the Gunflint Trail.

Studies of lunar impact craters show that an ejecta blanket five times larger than the crater is usually created from a typical impact. Products of impact range from angular fragments of preexisting rocks and partially melted, recrystallized, or glassy fragments. But unlike the airless Moon, Earth’s atmosphere would have additional effects on the ejecta, producing such things as spherules that condense from vapor in the ejecta cloud (like hail stones forming in rain clouds).

Jirsa showed his discovery to an ILSG colleague named William Addison, who with Gregory Brumpton had first identified Sudbury Impact ejecta 15 years ago near Thunder Bay, Ontario. Addison immediately recognized that the samples exhibited typical textures of material formed in an impact. Interestingly, Addison and Brumpton are both high school earth science teachers, and not professional geologists.

William Cannon of the United States Geological Survey joined the search and has located and documented exposures of ejecta blanket in or near five iron ranges in the Lake Superior region. And Jirsa’s Gunflint Trail discovery, 500 miles from the impact’s “ground zero”, adds another iron formation to the list.

Stromatolite fossils along Gunflint Trail: Photo by Mark Jirsa, Minnesota Geological Survey
Stromatolite fossils along Gunflint Trail: Photo by Mark Jirsa, Minnesota Geological Survey
Iron deposits the same age as the Gunflint Iron Formation (about 2 billion years old) occur world-wide, and are thought to be result of high levels of oxygen present in the atmosphere, due to photosynthesis by cyanobacteria, Earth’s earliest preserved life form. Fossils of these ancient creatures called stromatolites are exposed in the upper layers of the Gunflint Iron Formation in the same area where Jirsa found the ejecta blanket evidence.

Interestingly, the deposition of iron-forming sediments declined right around the same time as the Sudbury Impact (1.875 billion years ago), and Cannon wonders if there may be a connection between the two events. Could the impact have caused the cyanobacteria to go extinct, thereby ending the favorable conditions for iron deposit formation?

“There’s a lot of work that needs to be done in the field to see what this deposit tells us that other sites don’t,” Jirsa said. “That’s the critical thing. This is a different geological setting; it’s a little farther away from the impact, the rocks are altered differently. It may reveal some secrets about the impact that other discoveries haven’t yet. That’s what we’re hoping.”


Story in Minneapolis Star Tribune
Another story source
Yet another story source
More on impact craters
More on cyanobacteria
Sudbury Impact layer in Michigan


Fallen Trees at Tunguska site: 1927 Kulik expedition
Fallen Trees at Tunguska site: 1927 Kulik expedition
A team of scientists may have finally found a possible impact crater from the Tunguska event that blasted above Siberia nearly a century ago.

Map showing Tunguska event location: Image source: Public Domain
Map showing Tunguska event location: Image source: Public Domain
On June 30, 1908, some sort of extraterrestrial object, such as a comet or an asteroid (at least according to the consensus), exploded in Earth’s atmosphere above the Tunguska River with such force that it flattened more than 2000 square miles of forest. But until now, no viable impact site had ever been found.

This whole Tunguska thing is cloaked in so much mystery and mythology, that agents Muldar and Scully could do a whole X-Files episode about it (in fact, they did). Well, the truth may be out there, but there’s a whole lot of it that remains unknown.

What’s is known is that something big exploded over the Tunguska river region in 1908. The place, unfortunately, is so out-in-the-middle-of-nowhere; it wasn’t scientifically investigated until more than two decades later when mineralogist Leonid Kulik led the first official expedition into the region in 1927. Kulik had initially come upon the site six years earlier when he doing a survey for the Soviet Academy of Sciences. Local eyewitness accounts convinced him that the explosion had been caused by an enormous meteorite impact, and he persuaded the Soviet government to fund the expedition in hopes of salvaging meteoric iron for Soviet industry. But to his disappointment, no possible impact crater was ever located (except in one bare location that later proved to be just a bog). What the expedition did find was a huge area of forest flattened out in a butterfly pattern. Oddly, the only trees still standing were located at ground zero, but those had been stripped bare of all their leaves and bark.

Kulik didn’t find any chunks of iron either, although later expeditions did find microscopic traces of nickel and iron in the soil.

But now, a University of Bologna team of scientists claims that Lake Cheko, which is located just 5 miles north-northwest of the explosion’s epicenter, shows some interesting features that could be interpreted as resulting from some sort of impact, perhaps from a small chunk of the disintegrating space rock – if that’s what it was. The team’s research appears in the online journal Terra Nova.

But other scientists aren’t jumping on the bandwagon just yet. For one thing the lake exhibits almost none of the usual telltale physical markings of an impact crater, other than being uniquely funnel-shaped unlike other neighboring lakes. And even then, its shape is more elliptical than circular. Cheko’s rim is not raised and lacks any sign of upturned ejecta. The scientists have found no shocked terrestrial rock in or around the lake, and to date no meteoric material either. And even if some is found, skeptics say it could have washed into the lake from the surrounding landscape. Also, trees older than a hundred years old are still standing near the lake. If Cheko were an impact crater, the force of the collision would have knocked them all down. It’s true that the lake doesn’t appear on any map prior to 1929, but the region is extremely remote, and there is some folklore evidence of its existence before then.

Other scientists speculate that the source of the event wasn’t from outer space at all, but rather was caused by geophysical forces, such as a cataclysmic gas blow out from deep inside the Earth. It just so happens that the Tunguska event epicenter sets at the intersection of a number of tectonic faults, and atop the ancient crater of a paleovolcano. Kimberlite pipes are also found in the area, an indication of magma reservoirs deep beneath the surface. And evidently there was a lot of earthquake activity in the Tunguska epicenter region back in 1908.
Andrei Ol'khovatov, a former Soviet scientist who is now -in his own words- “an independent researcher/expert” on everything Tunguska, has an entire website addressing this and other possibilities about the event. He has participated in a number of International Tunguska conferences, and I found his site very interesting to peruse.

So, whatever the Tunguska event was, whether it was a comet or asteroid, a UFO, an errant radio transmission, or the real cause of global warning - it exploded about 3-6 miles above the ground, knocked down a whole lot of lumber, scared the dickens out of the locals, and illuminated the sky so brightly it could be seen in London, a third of the way around the globe!

The Italian team plans to return to Lake Cheko in 2008 to perform further tests, including drilling into the core of the lake to examine an anomaly detected some 10 meters below the lake bottom. It could be a meteorite fragment or maybe just some compacted mud. I guess we’ll have to wait and see.


More on the Tunguska event
The Tunguska event in fiction
BBC website story
Lake Cheko story on Sky& Telescope website