Thursday, September 29, 2011
A Renewed Interest in Gold Panning...
is sweeping across America's "gold fields", i.e., places that have historically produced placer gold.
Here is a video from "Rock Raiders" - linked on The Blaze website - of all places. Part of the renewed interest is due to the increased cost of gold, another would probably be the higher unemployment rate leaves people with more time on their hands for such endeavors.
I have been out three times this summer with the Allatoona Gold Panners, visiting 2 different creeks in Cherokee County (or extreme North Cobb County), Ga.. I have found something in virtually ever pan, but it is mostly "flour gold", very small grains. The main idea is to have some fun - which I have. I also enjoy accumulating heavy minerals for use in lab classes or just to look through, under a microscope.
Here are a couple more videos:
From Arizona:
Here is an ad for a sluice system - sounds good, but one would need to check for local restrictions regarding the use of sluices. This Grizzly Sluice is cool, but it is a bit pricey - for the hobby miner. The MSRP is $547.00, at this website.
Here is a homemade sluice box:
Another accessory - a homemade "high banker", designed to quickly wash out the larger gravels:
American ingenuity, ya gotta love it! As a reminder, if there are any diamonds, they will be retained in the heavy mineral concentrate, so check it carefully.
Here is a video from "Rock Raiders" - linked on The Blaze website - of all places. Part of the renewed interest is due to the increased cost of gold, another would probably be the higher unemployment rate leaves people with more time on their hands for such endeavors.
I have been out three times this summer with the Allatoona Gold Panners, visiting 2 different creeks in Cherokee County (or extreme North Cobb County), Ga.. I have found something in virtually ever pan, but it is mostly "flour gold", very small grains. The main idea is to have some fun - which I have. I also enjoy accumulating heavy minerals for use in lab classes or just to look through, under a microscope.
Here are a couple more videos:
From Arizona:
Here is an ad for a sluice system - sounds good, but one would need to check for local restrictions regarding the use of sluices. This Grizzly Sluice is cool, but it is a bit pricey - for the hobby miner. The MSRP is $547.00, at this website.
Here is a homemade sluice box:
Another accessory - a homemade "high banker", designed to quickly wash out the larger gravels:
American ingenuity, ya gotta love it! As a reminder, if there are any diamonds, they will be retained in the heavy mineral concentrate, so check it carefully.
Friday, September 9, 2011
The Okie Dokie Diamond [Original Post Date: 3/13/06]
[Call it blog-post recycling or posting of "greatest hits", I am gradually transferring older posts from another blog, as time permits.]
Many people may not be aware of it, but Murfreesboro, Arkansas is the only place in the entire world where common people can pay a small fee and search for diamonds and keep everything that they find. All other diamond producing areas are owned/controlled by large corporations or governments.
From MSNBC News, an Oklahoma State Trooper, visiting the Crater of Diamonds State Park, near Murfreesboro, Arkansas, with his family - for the first time - found a 4.21 carat, canary yellow diamond, that is said to be flawless. Talk about beginner's luck!
The park is approximately 37 acres of ground that is periodically plowed. Visitors are allowed to crawl about on their hands and knees, that is how I found my small white diamond (.37 carats) on my first visit in 1973. There are other areas where people can dig and sieve sand and gravel in water to look for diamonds.
When I was there in the spring of 1978, a couple from Dallas was looking in the same area as I was looking. I left in the mid-afternoon to do some other things in the area. At dusk, I was parked along the road into town, looking for old beer cans in the woods when the Dallas couple recognized my truck and pulled over. They asked me to take a look at what they found and to tell them if it was a diamond. It was a 4 carat, brown diamond, not of gem quality, but with the classic octahedral diamond shape. I got to hold it and I was the first one to confirm that it was a diamond (the park office had closed for the day). I later saw a short newspaper article in a Dallas paper, wherein that diamond was valued at $4,000 because of its size, classic crystal shape, and it being an American diamond. And that was 1978.
The article mentioned that 84 diamonds have been found so far this year. When I was first there in 1973, they said about 250 diamonds per year were found by visitors. Most of them are not gem quality, but once in a while, someone finds a "blockbuster" of a diamond, worthy of faceting and mounting in jewelry. The three main colors at the Crater of Diamonds State Park are white (60%), brown (21%), and yellow (17%) - according to the linked site below. 383 diamonds were found in 2004 and 536 in 2005. The higher numbers than the 1970s may be partially a function of higher numbers of visitors and perhaps more serious methods of searching, perhaps more digging and less crawling.
As mentioned above, it is an Arkansas State Park, open to the public. The state of Arkansas has toyed with the idea of selling the property to mining company, but public pressure has so far preserved the status quo. I know that Libertarian/Conservative purists disapprove of government ownership of land, but this place is so unique, I think it should stay as a state park.
The first diamonds were found 100 years ago, when the area was a farm. The farmer, when dressing out chickens to eat, found shiny stones in their craws (not having teeth, some birds swallow small stones to aid in the digestion process and the shiny nature of the diamonds caught the eye of the chickens). The stones were identified as diamonds, but there were never enough to support a mining operation, so it became a tourist attraction. In 1972, it became a state park.
If you click on the Park link above, the middle-aged black man in the center "works" at the park. Every day the park is open, he is there to pay his fee and that is what he does all day, dig for diamonds. He was there the last time I visited the park in 1983 or 1984 and I talked to him briefly. He doesn't find a diamond every day, but he finds enough to scratch out a living. Some of his diamonds may be among the Arkansas diamonds for sale on this website.
If you ever go there, don't expect to find a diamond, but there is always a chance. There are other minerals of interest to kids, quartz crystals, amethyst, calcite, peridot, agate, conglomerate (a type of sandstone composed of rounded river pebbles) and other minerals. Just keep everything that might even look like a diamond<, and the rangers at the park are more than glad to look over what you have found and tell you "what's what".
Only in America
Many people may not be aware of it, but Murfreesboro, Arkansas is the only place in the entire world where common people can pay a small fee and search for diamonds and keep everything that they find. All other diamond producing areas are owned/controlled by large corporations or governments.
From MSNBC News, an Oklahoma State Trooper, visiting the Crater of Diamonds State Park, near Murfreesboro, Arkansas, with his family - for the first time - found a 4.21 carat, canary yellow diamond, that is said to be flawless. Talk about beginner's luck!
The park is approximately 37 acres of ground that is periodically plowed. Visitors are allowed to crawl about on their hands and knees, that is how I found my small white diamond (.37 carats) on my first visit in 1973. There are other areas where people can dig and sieve sand and gravel in water to look for diamonds.
When I was there in the spring of 1978, a couple from Dallas was looking in the same area as I was looking. I left in the mid-afternoon to do some other things in the area. At dusk, I was parked along the road into town, looking for old beer cans in the woods when the Dallas couple recognized my truck and pulled over. They asked me to take a look at what they found and to tell them if it was a diamond. It was a 4 carat, brown diamond, not of gem quality, but with the classic octahedral diamond shape. I got to hold it and I was the first one to confirm that it was a diamond (the park office had closed for the day). I later saw a short newspaper article in a Dallas paper, wherein that diamond was valued at $4,000 because of its size, classic crystal shape, and it being an American diamond. And that was 1978.
The article mentioned that 84 diamonds have been found so far this year. When I was first there in 1973, they said about 250 diamonds per year were found by visitors. Most of them are not gem quality, but once in a while, someone finds a "blockbuster" of a diamond, worthy of faceting and mounting in jewelry. The three main colors at the Crater of Diamonds State Park are white (60%), brown (21%), and yellow (17%) - according to the linked site below. 383 diamonds were found in 2004 and 536 in 2005. The higher numbers than the 1970s may be partially a function of higher numbers of visitors and perhaps more serious methods of searching, perhaps more digging and less crawling.
As mentioned above, it is an Arkansas State Park, open to the public. The state of Arkansas has toyed with the idea of selling the property to mining company, but public pressure has so far preserved the status quo. I know that Libertarian/Conservative purists disapprove of government ownership of land, but this place is so unique, I think it should stay as a state park.
The first diamonds were found 100 years ago, when the area was a farm. The farmer, when dressing out chickens to eat, found shiny stones in their craws (not having teeth, some birds swallow small stones to aid in the digestion process and the shiny nature of the diamonds caught the eye of the chickens). The stones were identified as diamonds, but there were never enough to support a mining operation, so it became a tourist attraction. In 1972, it became a state park.
If you click on the Park link above, the middle-aged black man in the center "works" at the park. Every day the park is open, he is there to pay his fee and that is what he does all day, dig for diamonds. He was there the last time I visited the park in 1983 or 1984 and I talked to him briefly. He doesn't find a diamond every day, but he finds enough to scratch out a living. Some of his diamonds may be among the Arkansas diamonds for sale on this website.
If you ever go there, don't expect to find a diamond, but there is always a chance. There are other minerals of interest to kids, quartz crystals, amethyst, calcite, peridot, agate, conglomerate (a type of sandstone composed of rounded river pebbles) and other minerals. Just keep everything that might even look like a diamond<, and the rangers at the park are more than glad to look over what you have found and tell you "what's what".
Only in America
The Pleistocene Ice Ages [Original Post Date: 3/26/06]
Currently, as defined by geologists, we live in the Cenozoic Era, Quaternary Period, and Holocene Epoch (since the last 10,000 years or so).
The previous epoch was the Pleistocene, which lasted from approximately 2 million years ago to 10,000 years ago. Before the Pleistocene Epoch was the Pliocene Epoch, which lasted from approximately 5 million to 2 million years ago.
Most people have a vision of the Pleistocene climate as being a single large Ice Age, when in reality there were several periods of glacial advance, separated by shorter interglacial periods, some of which were as warm or warmer than today's climate. The primary, attributed reasons for the fluctuations were variations in Earth orbit, Earth axial tilt, and variations in solar output. Some of these fluctuations exaggerate each other, while others moderate each other. Random events such as large volcanic eruptions may play a role, also. Evidence suggests that the global cooling began in the latter part of the Pliocene.
Some authors suggest that the emergence of the Isthmus of Panama (above Sea Level) during the Late Pliocene played a role in the cooling of the climate. Prior to the emergence of the isthmus, there was a narrow seaway that allowed the movement of warm waters from the Caribbean into the east Pacific. The closure of this seaway altered oceanic currents, while opening a land bridge that allowed the migrations of mammals between North and South America.
This series of slides is from a lecture on glaciers. Proceed forward to cover the Pleistocene. There have been previous periods of global glaciation, but those were before humans. Oxygen-18 isotopic data is one type of proxy data used to reconstruct paleoclimate history, including the Pleistocene data listed here in Wikipedia.
From youngest to oldest, as identified in North America, the Pleistocene glacial stages were:
Wisconsinan Glacial Stage
Sangamonian Interglacial
Illinoian Glacial Stage
Yarmouthian Interglacial
Kansan Glacial Stage
Aftonian Interglacial
Nebraskan Glacial Stage
Pre-Nebraskan
Slide 37 shows reconstructed temperature curves for the last billion years and the last 2 million in more detail. Slide 38 shows the maximum extent of the Pleistocene continental ice sheets. Slide 39 shows estimated sea level for the last 20,000 years. These particular slides are from the University of Portland.
After each glacial stage, the interglacial represented a period of natural global warming, a period of rebound. Since the end of the Wisconsinan Glacial Stage approximately 10,000 years ago, there have been several alternating shorter periods of warming and cooling.
Previous ice ages include:
A poorly-documented, possible ice age from 2.7 to 2.3 billion years ago, during the early Proterozoic Era.
The earliest, well-documented ice age was during the late Proterozoic Era, from approximately 800 million to 600 million years ago.
Late Ordovician Period 460 million to 430 million years ago.
Late Carboniferous Period to Early Permian Period 350 million years to 260 million years ago.
From this above-linked Wikipedia article:
..."The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around 3 million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago."
This Wikipedia link shows temperature trends during the last 5 million years.
The Pleistocene Epoch is the recent geologic past. If there were repeated periods of natural global cooling and global warming then, why are people so convinced that any and all unusual changes and variations are triggered by humans?
The previous epoch was the Pleistocene, which lasted from approximately 2 million years ago to 10,000 years ago. Before the Pleistocene Epoch was the Pliocene Epoch, which lasted from approximately 5 million to 2 million years ago.
Most people have a vision of the Pleistocene climate as being a single large Ice Age, when in reality there were several periods of glacial advance, separated by shorter interglacial periods, some of which were as warm or warmer than today's climate. The primary, attributed reasons for the fluctuations were variations in Earth orbit, Earth axial tilt, and variations in solar output. Some of these fluctuations exaggerate each other, while others moderate each other. Random events such as large volcanic eruptions may play a role, also. Evidence suggests that the global cooling began in the latter part of the Pliocene.
Some authors suggest that the emergence of the Isthmus of Panama (above Sea Level) during the Late Pliocene played a role in the cooling of the climate. Prior to the emergence of the isthmus, there was a narrow seaway that allowed the movement of warm waters from the Caribbean into the east Pacific. The closure of this seaway altered oceanic currents, while opening a land bridge that allowed the migrations of mammals between North and South America.
This series of slides is from a lecture on glaciers. Proceed forward to cover the Pleistocene. There have been previous periods of global glaciation, but those were before humans. Oxygen-18 isotopic data is one type of proxy data used to reconstruct paleoclimate history, including the Pleistocene data listed here in Wikipedia.
From youngest to oldest, as identified in North America, the Pleistocene glacial stages were:
Wisconsinan Glacial Stage
Sangamonian Interglacial
Illinoian Glacial Stage
Yarmouthian Interglacial
Kansan Glacial Stage
Aftonian Interglacial
Nebraskan Glacial Stage
Pre-Nebraskan
Slide 37 shows reconstructed temperature curves for the last billion years and the last 2 million in more detail. Slide 38 shows the maximum extent of the Pleistocene continental ice sheets. Slide 39 shows estimated sea level for the last 20,000 years. These particular slides are from the University of Portland.
After each glacial stage, the interglacial represented a period of natural global warming, a period of rebound. Since the end of the Wisconsinan Glacial Stage approximately 10,000 years ago, there have been several alternating shorter periods of warming and cooling.
Previous ice ages include:
A poorly-documented, possible ice age from 2.7 to 2.3 billion years ago, during the early Proterozoic Era.
The earliest, well-documented ice age was during the late Proterozoic Era, from approximately 800 million to 600 million years ago.
Late Ordovician Period 460 million to 430 million years ago.
Late Carboniferous Period to Early Permian Period 350 million years to 260 million years ago.
From this above-linked Wikipedia article:
..."The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around 3 million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago."
This Wikipedia link shows temperature trends during the last 5 million years.
The Pleistocene Epoch is the recent geologic past. If there were repeated periods of natural global cooling and global warming then, why are people so convinced that any and all unusual changes and variations are triggered by humans?
10.5 Apocalypse...Probably Not [Original Post Date: 5/22/06]
As we proceed (lurch) towards 2012, it is possible that TV networks may rerun some of these recent-past disaster movies, especially during Sweeps Months. It is cheaper than creating new ones.
[Originally posted on 5/22/06 on another of my blogs. Modified slightly for today's "consumption".]
I don't know if any of you have checked in on the NBC 3-part disaster flick "10.5 Apocalypse" (Saturday, Sunday, and this upcoming Tuesday), where the Western U.S. goes to hell in a geologic handbasket. It has just enough science to be interesting and I am waiting to see how much of the Western U.S. is laid waste.
As for likelyhood of the envisioned chain-reaction of earthquakes, volcanic eruptions, sinkholes, etc..., it answer is...
Probably Not!
[Without my Physical Geology textbook here for refresher sake, I am going from memory.]
Plate Tectonics is driven by vertical convection currents in the Asthenosphere, a semi-molten layer beneath the rocky crust (the Lithosphere). So imagine conveyor belt systems upon which the continental plates ride "piggyback". Where there are Asthenospheric "upwhellings" of molten material, if these are upwhellings are linear, they split the overlying crust and push the plates apart. This is what happens beneath the Atlantic Ocean in regards to the Mid-Atlantic Ridge and also in the Eastern Pacific Ocean with the East Pacific Rise. So there are upwhellings of intense heat in the Mid-Atlantic Ocean and the East Pacific Ocean.
In the last 30 million years or so, the North American Plate has pushed over and distorted a portion of the East Pacific Rise. A portion of the East Pacific Rise is present North of the Mendocino Fracture Zone, off the coast of N. Calif., Oregon, Washington, and British Columbia. Here the Juan de Fuca Plate is sinking beneath the North American Plate. The sinking Juan de Fuca Plate is remelting and that is what causes the occasional NW U.S. earthquakes and the Cascade Volcano eruptions (including Mt. St. Helens).
South of the Mendocino Fracture Zone along the Pacific Coast to south of the southern tip of Baja California, the coastal area is dominated by the San Andreas Fault Zone, which is where a small portion of the Pacific Crustal Plate is sliding past the North American Plate. Inland from this area, the mantle upwhelling (mantle plume) is under the continent and may be responsible for the hot spot vulcanism (San Francisco volcanic field (Flagstaff area), the Long Valley Caldera in Calif., Yellowstone in Wyoming, and perhaps the Rio Grande Rift).
The Rio Grande Rift represents a thinning of the continental crust from the El Paso area northward into central Colorado. If the continent were to be rent asunder, per the movie, this would be a natural "weak spot", as the crust appears to be thinned, based on heat-flow data, seismic surveys, and the presence of young volcanic rocks from the Potrillo Volcanic Field in Southern New Mexico, northward along the river.
The "geophysics" of the areas under the continent are different enough that stresses probably are not going to be quickly transferred from one area to another.
Another issue brought forth in the movie was the "Accelerated Plate Movement" theory as proposed by the discredited geologist father of Kim Delaney's character, Dr. Samantha Hill. If the plate motions were to reverse themselves, I would be looking at what was going on along the Mid-Atlantic Ridge. Had one of the plates started to sink in relation to the other? That would signify the reversal from an upwhelling to a subduction zone, where one plate was sinking beneath another.
Mantle upwhellings have stopped before, but we presume it takes millions of years to transpire. And I don't recall any mention of a mantle plume beneath the Western U.S., in the movie. Dr. Samantha Hill's character makes mention of "Sub-Asthenosphere" earthquakes, but to my knowledge, the "plastic" nature of the Asthenosphere makes seismic wave propogation unlikely. The most damaging earthquakes, by conventional wisdom (and supported by data) originate in the upper 100 km (60 miles) of the crust.
The deepest earthquakes are associated with the deep Pacific Ocean subduction zones and some of them originate from as much as 700 kilometers below the surface. With these deepest of earthquakes, the seismic waves are associated with the sinking oceanic plate, which though partially-melted, still retains enough rigidity to transfer stresses.
So, in summary, there are an endless number of "what ifs" and things that geologists and geophysicists dream of seeing (for the sake of learning), in this movie, and while anything is possible, it ain't likely. Remember, it is Sweeps Month.
[Originally posted on 5/22/06 on another of my blogs. Modified slightly for today's "consumption".]
I don't know if any of you have checked in on the NBC 3-part disaster flick "10.5 Apocalypse" (Saturday, Sunday, and this upcoming Tuesday), where the Western U.S. goes to hell in a geologic handbasket. It has just enough science to be interesting and I am waiting to see how much of the Western U.S. is laid waste.
As for likelyhood of the envisioned chain-reaction of earthquakes, volcanic eruptions, sinkholes, etc..., it answer is...
Probably Not!
[Without my Physical Geology textbook here for refresher sake, I am going from memory.]
Plate Tectonics is driven by vertical convection currents in the Asthenosphere, a semi-molten layer beneath the rocky crust (the Lithosphere). So imagine conveyor belt systems upon which the continental plates ride "piggyback". Where there are Asthenospheric "upwhellings" of molten material, if these are upwhellings are linear, they split the overlying crust and push the plates apart. This is what happens beneath the Atlantic Ocean in regards to the Mid-Atlantic Ridge and also in the Eastern Pacific Ocean with the East Pacific Rise. So there are upwhellings of intense heat in the Mid-Atlantic Ocean and the East Pacific Ocean.
In the last 30 million years or so, the North American Plate has pushed over and distorted a portion of the East Pacific Rise. A portion of the East Pacific Rise is present North of the Mendocino Fracture Zone, off the coast of N. Calif., Oregon, Washington, and British Columbia. Here the Juan de Fuca Plate is sinking beneath the North American Plate. The sinking Juan de Fuca Plate is remelting and that is what causes the occasional NW U.S. earthquakes and the Cascade Volcano eruptions (including Mt. St. Helens).
South of the Mendocino Fracture Zone along the Pacific Coast to south of the southern tip of Baja California, the coastal area is dominated by the San Andreas Fault Zone, which is where a small portion of the Pacific Crustal Plate is sliding past the North American Plate. Inland from this area, the mantle upwhelling (mantle plume) is under the continent and may be responsible for the hot spot vulcanism (San Francisco volcanic field (Flagstaff area), the Long Valley Caldera in Calif., Yellowstone in Wyoming, and perhaps the Rio Grande Rift).
The Rio Grande Rift represents a thinning of the continental crust from the El Paso area northward into central Colorado. If the continent were to be rent asunder, per the movie, this would be a natural "weak spot", as the crust appears to be thinned, based on heat-flow data, seismic surveys, and the presence of young volcanic rocks from the Potrillo Volcanic Field in Southern New Mexico, northward along the river.
The "geophysics" of the areas under the continent are different enough that stresses probably are not going to be quickly transferred from one area to another.
Another issue brought forth in the movie was the "Accelerated Plate Movement" theory as proposed by the discredited geologist father of Kim Delaney's character, Dr. Samantha Hill. If the plate motions were to reverse themselves, I would be looking at what was going on along the Mid-Atlantic Ridge. Had one of the plates started to sink in relation to the other? That would signify the reversal from an upwhelling to a subduction zone, where one plate was sinking beneath another.
Mantle upwhellings have stopped before, but we presume it takes millions of years to transpire. And I don't recall any mention of a mantle plume beneath the Western U.S., in the movie. Dr. Samantha Hill's character makes mention of "Sub-Asthenosphere" earthquakes, but to my knowledge, the "plastic" nature of the Asthenosphere makes seismic wave propogation unlikely. The most damaging earthquakes, by conventional wisdom (and supported by data) originate in the upper 100 km (60 miles) of the crust.
The deepest earthquakes are associated with the deep Pacific Ocean subduction zones and some of them originate from as much as 700 kilometers below the surface. With these deepest of earthquakes, the seismic waves are associated with the sinking oceanic plate, which though partially-melted, still retains enough rigidity to transfer stresses.
So, in summary, there are an endless number of "what ifs" and things that geologists and geophysicists dream of seeing (for the sake of learning), in this movie, and while anything is possible, it ain't likely. Remember, it is Sweeps Month.
A Follow-up on the Previous Caldera Post [Original Post Date: 8/31/06]
the ABC special did a pretty good job of explaining the basics.
In the case of the Yellowstone Caldera, it is thought to overlie a "mantle hotspot", as does Hawaii, where a particularly strong upwhelling brings heated magma towards the surface. As the magma body rises through the continental crust, it partially melts the crust, adding significant amounts of quartz, and other silica-rich minerals, such as muscovite mica and orthoclase feldspar.
From the above-linked USGS webpage:
"...Scientists infer that rhyolite lava flows as well as the caldera-forming ash-flow tuffs were fed from shallow magma chambers filled by the melting of rocks of the lower continental crust below Yellowstone. The heat needed to facilitate the melting process was supplied by the repeated injections of basalt magma from the mantle into the shallow crust."
As mentioned earlier, these silica-rich minerals make the magma more viscous. As the magma rises, the pressurized fluids (mostly mineral rich water) "wants to boil". Even though more silica-rich eruptions are the norm, from time to time, basalts can also erupt. The basalts are derived from the mantle below and when erupted, it is because the basalt has moved through the crust quickly enough not to melt much of the crust. [Note: The higher-temperature basalts can easily melt the silica-rich minerals,which crystalize at lower temperatures, than do the minerals in the basalts.]
These are large magma bodies, termed "batholiths" that become magma chambers for the overlying eruptions when they reach shallow-enough depths. More than 100 square miles is usually bulged by the rising magma.
When the buoyancy (sp.?) and the gas pressure of the magma overcomes the strength of the overlying rock, as depicted on the program last night, eruptions will break through in several places. These multiple eruptions taking place at the same time will produce gas-driven pyroclastic flows and partially drain the magma chamber. The magma chamber will then collapse inward, opening up a circular to round "ring fracture" system, which will provide more conduits for molten rock to reach the surface, likely triggering even more eruptions. Some of the eruptions may be restricted to the collapsed crater, while others will erupt outward.
Watching the pyroclastic flows from the smaller composite volcanoes gives an idea of the speed of these types of eruptions, but the caldera eruptions may be two to three orders of magnitude larger.
In the case of the Yellowstone Caldera, it is thought to overlie a "mantle hotspot", as does Hawaii, where a particularly strong upwhelling brings heated magma towards the surface. As the magma body rises through the continental crust, it partially melts the crust, adding significant amounts of quartz, and other silica-rich minerals, such as muscovite mica and orthoclase feldspar.
From the above-linked USGS webpage:
"...Scientists infer that rhyolite lava flows as well as the caldera-forming ash-flow tuffs were fed from shallow magma chambers filled by the melting of rocks of the lower continental crust below Yellowstone. The heat needed to facilitate the melting process was supplied by the repeated injections of basalt magma from the mantle into the shallow crust."
As mentioned earlier, these silica-rich minerals make the magma more viscous. As the magma rises, the pressurized fluids (mostly mineral rich water) "wants to boil". Even though more silica-rich eruptions are the norm, from time to time, basalts can also erupt. The basalts are derived from the mantle below and when erupted, it is because the basalt has moved through the crust quickly enough not to melt much of the crust. [Note: The higher-temperature basalts can easily melt the silica-rich minerals,which crystalize at lower temperatures, than do the minerals in the basalts.]
These are large magma bodies, termed "batholiths" that become magma chambers for the overlying eruptions when they reach shallow-enough depths. More than 100 square miles is usually bulged by the rising magma.
When the buoyancy (sp.?) and the gas pressure of the magma overcomes the strength of the overlying rock, as depicted on the program last night, eruptions will break through in several places. These multiple eruptions taking place at the same time will produce gas-driven pyroclastic flows and partially drain the magma chamber. The magma chamber will then collapse inward, opening up a circular to round "ring fracture" system, which will provide more conduits for molten rock to reach the surface, likely triggering even more eruptions. Some of the eruptions may be restricted to the collapsed crater, while others will erupt outward.
Watching the pyroclastic flows from the smaller composite volcanoes gives an idea of the speed of these types of eruptions, but the caldera eruptions may be two to three orders of magnitude larger.
ABC Discovers Calderas [Original Post Date: 8/30/06]
[Posted originally on another blog of mine.]
Or by another name "supervolcanoes". There are two active calderas or cauldron-type volcanoes (very large volcanoes with huge oval shaped craters) the lower 48 states, the Yellowstone Caldera and the Long Valley Caldera (eastern California). The oval crater for the Yellowstone Caldera measures approximately 36 miles X 48 miles and has erupted three times over the last 2 million years or so. The last major eruption was approximately 630,000 years ago.
The Cascade Volcanoes (Mount St. Helens, Mt. Rainier, Mt. Baker, etc.) are a smaller type of volcanoes, called composite volcanoes. Both types of volcanoes, calderas and composites are both capable of producing pyroclastic eruptions, which are the explosive eruptions which consist of gray clouds of superheated gases, volcanic ash, rock fragments, crystals, and pumice fragments, that are capable of moving across the countryside at speeds of up to 125 mph. Ancient pyroclastic flows (also called ash flows) have been traced from composite volcanoes to a distance of 60 miles and from calderas a distance of 100 miles. Once the ash flows begin their travel, a trapped layer of air beneath them serves as a cushion, allowing for less friction and greater travel distances.
These types of eruptions occur in composite and caldera eruptions because of the presence of silica (quartz) in the magmas. Quartz tends to increase the viscosity of magmas and as the magmas rise towards the surface, the quartz makes the magmas "want to freeze", while the trapped gases and liquids "want to boil". If the viscous magma plugs the volcano neck, pressure builds until the immovable object is overcome by the irresistable force, which results in an often catastrophic explosive eruption, a la Mount St. Helens, Mt. Pinatubo, El Chichon, Krakatau (1883), Mt. Vesuvius, etc....
As explosive as composite volcanoes are, calderas are much more so, as the quartz content is usually higher, i.e., the underlying magmas are generally similar to granite and the calderas are usually larger. Another caldera, which is approximately 1 million years old is the Valles Caldera, near Los Alamos, NM.
ABC is televising a special tonight [8/30/06] (9 PM EDT) called "The Last Days on Earth", which among other disasters, focuses on the damage that might be done to humanity by a large caldera eruption. The last major caldera eruption occurred about 74,000 years ago. That particular eruption of Mt. Toba in Indonesia reportedly put a major hurt on the DNA of early humans, i.e., there were mass casualties, apparently.
[As with typical Indonesian volcanoes, Mt. Toba was probably a large composite volcano, the explosive eruption of which triggered a caldera collapse. That is somewhat different from the caldera-type "supervolcano" mentioned in the first paragraph.]
Or by another name "supervolcanoes". There are two active calderas or cauldron-type volcanoes (very large volcanoes with huge oval shaped craters) the lower 48 states, the Yellowstone Caldera and the Long Valley Caldera (eastern California). The oval crater for the Yellowstone Caldera measures approximately 36 miles X 48 miles and has erupted three times over the last 2 million years or so. The last major eruption was approximately 630,000 years ago.
The Cascade Volcanoes (Mount St. Helens, Mt. Rainier, Mt. Baker, etc.) are a smaller type of volcanoes, called composite volcanoes. Both types of volcanoes, calderas and composites are both capable of producing pyroclastic eruptions, which are the explosive eruptions which consist of gray clouds of superheated gases, volcanic ash, rock fragments, crystals, and pumice fragments, that are capable of moving across the countryside at speeds of up to 125 mph. Ancient pyroclastic flows (also called ash flows) have been traced from composite volcanoes to a distance of 60 miles and from calderas a distance of 100 miles. Once the ash flows begin their travel, a trapped layer of air beneath them serves as a cushion, allowing for less friction and greater travel distances.
These types of eruptions occur in composite and caldera eruptions because of the presence of silica (quartz) in the magmas. Quartz tends to increase the viscosity of magmas and as the magmas rise towards the surface, the quartz makes the magmas "want to freeze", while the trapped gases and liquids "want to boil". If the viscous magma plugs the volcano neck, pressure builds until the immovable object is overcome by the irresistable force, which results in an often catastrophic explosive eruption, a la Mount St. Helens, Mt. Pinatubo, El Chichon, Krakatau (1883), Mt. Vesuvius, etc....
As explosive as composite volcanoes are, calderas are much more so, as the quartz content is usually higher, i.e., the underlying magmas are generally similar to granite and the calderas are usually larger. Another caldera, which is approximately 1 million years old is the Valles Caldera, near Los Alamos, NM.
ABC is televising a special tonight [8/30/06] (9 PM EDT) called "The Last Days on Earth", which among other disasters, focuses on the damage that might be done to humanity by a large caldera eruption. The last major caldera eruption occurred about 74,000 years ago. That particular eruption of Mt. Toba in Indonesia reportedly put a major hurt on the DNA of early humans, i.e., there were mass casualties, apparently.
[As with typical Indonesian volcanoes, Mt. Toba was probably a large composite volcano, the explosive eruption of which triggered a caldera collapse. That is somewhat different from the caldera-type "supervolcano" mentioned in the first paragraph.]
Subscribe to:
Posts (Atom)