 |
   |
||||
|
|
|
 |
||
|
 |
 |
 |
||
|
in Publications: Font size:    
|
|
Gigaton Deep Hot Earth Gases Emitted from Flood Basalts: Cause of Mass Extinctions?
The two leading types of ancient catastrophic events posited by scientists to explain mass extinctions of life on Earth are 1) major bolide impacts, first demonstrated by Luis Alvarez in 1980, and 2) episodes of continental flood basalt volcanism, first suggested by Vincent Courtillot in 1994, and subsequently developed in his book Evolutionary Catastrophes (1999). (1-2) Courtillot theorizes that magma eruptions liberated gases such as sulphur dioxide and carbon dioxide at a rate of around one gigaton (a billion tons) per year over periods as short as one decade, suggesting a possible “life killing mechanism” to explain mass extinctions. Indeed, during the past 300 million years, scientists have identified a near-perfect association between extinction events and eruption of flood basalt lava flows (see graph below). For example, the Permian mass extinction around 250 million years ago, which was the worst ever to affect Earth’s living organisms, occurred at roughly the same time as the gargantuan outpouring of basaltic magma in what is now Siberia and a smaller outpouring in Emeishan, Panjal, in southwest China. (4) Wignall cautions that all mass extinction events on Earth coincide with giant volcanic eruptions, but the converse, that all such eruptions coincide with extinction, is not true. (5)
The rocky remains today of these ancient outpourings are called “large igneous provinces,” a phrase first coined by Mike Coffin and Olaf Eldholm (1991, 1994). Large igneous provinces, they say, are “a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalt provinces and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces [they say] do not originate at ‘normal’ seafloor spreading centers.” Placing the term “normal” in quotation marks in the previous sentence suggests that the mechanisms leading to formation of large igneous provinces may be the new “normal.” The large igneous provinces studied by Coffin and Eldholm were between 20 to 40 kilometers (12 to 24 miles) thick. These basalt outpourings represent significant volumetric contributions to Earth’s crust in the course of her long geologic history. (6-7)
Saunders adds, “The key aspect of large igneous provinces is that they represent anomalously high magmatic fluxes. The magma is usually basaltic, but it may be rhyolitic [defined below]…They are large in area, covering many thousands if not millions of square kilometers (miles), and they testify to unusual geological processes, involving large amounts of thermal energy.” (4)
Continental flood basalt volcanism is “the repeated effusion of huge batches of basaltic magma (around 100 to 1000 square kilometers, 38.6 to 386 square miles, of basalt extruded per eruption) over relatively short periods of time (less than around a million years). (8) These repeated effusions occur on the surface of the earth, not below it; thus, any gases (volatiles) mixed in with the extruded magma pass directly into the earth’s atmosphere. Examples of major continental flood basalts, also called “traps,” are the Deccan Traps in west-central India, the New Jersey Palisades, the Columbia River Basin basalts, and the Pacific Northwest scablands and coulees. (9-11)
Basalt is an igneous (from Latin, “born of fire”) rock, usually dark grey to black in color and fine-grained due to its rapid cooling after extrusion from inside of a planet (e.g., Earth, Mars, Venus). Basalts are “mafic,” meaning they are silicates rich in two heavy elements--magnesium (“ma”) and iron (“ferric,” “ic”). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Basaltic magmas contain a less amount of volatiles than “felsic” magmas (see below). Mafic magmas are 45-55% silicate by weight, which is lower than “andesite magmas” (55-65%) and “rhyolitic magmas” (65-75%). (12-13) “Felsic,” by contrast, refers to silicate magmas and rocks with a lower percentage of the heavier elements and a correspondingly higher percentage of the relatively lighter elements silica, oxygen, aluminum, and potassium (13). The term derives from “fel” for feldspar (in this case the potassium-rich variety) and “sic,” which indicates the higher percentage of silica. Felsic minerals are usually light in color (think Yosemite Valley granite). Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite. (13)
Flooding basalts refers to the characteristic of basalts to flow easily, as compared with felsic magmas, which flow in a sluggish manner. The two reasons basalts are so fluid is their relatively low silicate content and their formation at very high temperatures (1,000 to 1,200 degrees Celsius). (13) The Deccan volcanic province consists of multiple layers of solidified flood basalt about 6,600 feet thick in west-central India. The province covers an area of 500,000 square kilometers (193,000 square miles, or about the size of France or Texas) and is one of the largest volcanic eruptions in Earth’s history. The original size of the Deccan was at least twice as large as what now exists; it dwindled in size because of erosion and plate tectonics. Scientists believe the volume of lava originally extruded was about 1.20 million cubic kilometers, or 288,000 cubic miles of basaltic magma. (14-15)
The Deccan Traps formed between 88 and 60 million years ago, at the end of the Cretaceous period (145-65 million years ago). The bulk of the volcanic eruption occurred at the Western Ghats (near Mumbai) some 66 million years ago. The Western Gnats are not a true mountain range; rather they are the faulted edge of the Deccan Plateau. French and Indian geologists have identified a 2,000-foot thick portion of the lava that may have piled up in as little as “30,000 years, fast enough to have possibly caused a deadly global climate shift. This series of eruptions may have lasted fewer than 30,000 years in total. The gases released in the process may have played a role in the Cretaceous–Tertiary extinction event, which included the extinction of the non-avian dinosaurs.” (5)
Hawaii’s Kilauea volcano, perhaps the world’s most active volcano, provides a modern laboratory for studying the effects on animal and plant life from magmatic volatiles injected into the atmosphere. In recent years, Kilauea on the Big Island of Hawaii has been outgassing sulfur dioxide at two to four times its usual rate (monitored since 1979). Sulfur dioxide and other gases emitted from Kilauea react with sunlight, oxygen, atmospheric moisture and dust to produce volcanic smog, known also as “vog,” which has become a serious hazard for animal and plant life in the Hawaiian Islands. (14-15)
Kilauea’s gases by volume weight are H2O (water, 37.1%), CO2 (carbon dioxide, 48.9%), SO2 (sulphur dioxide, 11.8%), H2 (hydrogen, 0.49%), CO (carbon monoxide, 1.51%), H2S (hydrogen sulfide, 0.04%) and no HF (hydrogen fluoride). The temperature of these gases reaches 1,179 degree Celsius. (18) A recent Kilauea eruption generated about 500,000 cubic meters of basalt magma (a mere pittance compared to, say, the Deccan outflows), which released 2,000 tons of sulfur dioxide gas and about four times as much carbon dioxide gas each day during periods of sustained eruption. (16) These gases cause harm to animal and plant life. For example, the acute health effects of carbon dioxide vary with concentration in the air and the length of time a person or animal breathes the gas. The usual carbon dioxide content in fresh air is only 0.03%, and in exhaled air approximately 4.5%. The life-threatening and fatal symptoms of acute carbon dioxide toxicity in a Cameroonian event are described elsewhere. (19)
Vog is toxic to humans, animals, and many plants, such as roses, sunflowers, protea, lettuce, tomatoes, koa, naio, and uki, which may burn, turn brown, wilt, and die. (Coffee, ohia trees and Peruvian lilies, however, can withstand the vog in the atmospheric concentrations now present.) In humans, vog irritates the skin and the tissues and mucous membranes of the eyes, nose, and throat. “During even moderate physical activity, SO2 penetrates deeply into the airway and can produce respiratory distress in some individuals. In the absence of strong winds, SO2 emitted by Kilauea can accumulate in the air and reach levels that exceed federal health standards. Since 1986, this has occurred more than 85 times within Hawaii Volcanoes National Park, which includes much of Kilauea.” (15-16) Vog exposure increases with altitude: it is least at sea level and most between 300 and 6,000 feet above sea level; then it diminishes. Vog exposure is maximal at 1,000 feet above sea level. (15-16)
Large igneous provinces emit large amounts of toxic gases. In addition, says Wignall, large igneous province eruptions are associated with some or all of the following climatic and environmental effects: The Karoo-Ferrar eruptions, seen today as the flood basalts in South Africa and Antarctica, occurred about 183 million years ago; they demonstrate all of the above effects.
The Siberian Traps, which erupted 250 million years ago at the Permian-Triassic boundary, show some of the above effects. Wignall says some scientists have proposed volcanic cooling as the cause for several extinction events, but he thinks the evidence is insubstantial. Instead, he believes, The preponderance of evidence favours warming [caused by carbon dioxide degassing from the basaltic magma]. This strongly suggests that CO2 emissions do all the damage, although in many extinction scenarios this effect is envisaged as a trigger. Indeed other factors are almost mandatory given the volume of CO2 likely to be released during LIP [large igneous province] eruptions. The amount of CO2 released during the eruption of the largest LIPs is unlikely to have exceeded 1013 tons of CO2, with the amount released during individual flow events likely to have been at least two orders of magnitude lower. Thus, the gas released during a major flow of 1000 cubic kilometers [240 cubic miles], which may have occurred as frequently as every few thousand years, is unlikely to have greatly exceeded the current anthropogenic CO2 release of 25 x 109 tons per year. This modern flux comes not even close to recreating the conditions during these ancient catastrophes. (5) Wignall proposes that carbon dioxide release from large igneous province eruptions promotes a “volcanic greenhouse scenario,” which may explain how degassing during flood basalt eruptions causes mass extinctions, as follows: Ideas concerning the role of volcanic gases have been developed primarily from events during the end-Permian and Early Jurassic mass extinction. It is proposed that the volcanic extinction mechanism is triggered by the release of CO2 during the eruption of the giant lava flows that form LIPs [large igneous provinces]. The resultant increase of atmospheric carbon dioxide levels would [slowly raise the atmospheric and oceanic temperatures, thus producing] several deleterious consequences for the oceans. For example, an increase in carbon dioxide concentrations in surface waters causes a pH decline [becomes more acidic] and thus problems for carbonate-secreting organisms. This is known as a calcification crisis and is manifest as a decline in carbonate content in many sections. Global warming can also cause the development of oceanic anoxic events (and there marine extinctions). These are intervals of time when large areas of the oceans and shelf seas were either oxygen poor or oxygen free (anoxic). Modern oceans typically have around 5-6 milliliters of oxygen dissolved in a liter of water, but conditions become stressful for most organisms if values decline below 1.0 milliliter/liter, and no metazoan life can survive below values of 0.3 ml/liter. Low oxygen conditions are restricted to only a few small areas of modern oceans but they became much more widespread during oceanic anoxic events due to several feedback factors associated with global warming. First, warmer waters hold less dissolved oxygen that cooler waters, contributing to an anoxic ocean event. Second, the ocean’s circulation system is primarily driven by the temperature gradient between the equator and the poles, with deep circulation driven by the generation of old and dense waters in the polar regions. This system slows down as polar waters warm up, thus decreasing the supply of oxygen to the ocean’s deeper waters. A possible third factor may relate to the supply of nutrients from land, which will increase with global warming during increased rainfall and runoff in a warmer, more humid climate. Increased nutrient flux to the seas will foster increased biological productivity, which in turn will decrease oxygen levels in sea water as the plankton biomass decays—the same phenomenon is see in many modern shelf seas oversupplied with anthropogenic “nutrients” such as fertilizers and sewage. Evidence of increased global runoff during LIP eruptions is substantial and includes several lines of geochemical evidence, such as increase in the trace metals rhenium and osmium. However, many mass extinction events coincide with a collapse, not an increase, of primary productivity, and this third factor may not be significant until after the mass extinction has run its course. Indeed, its main significance may be as a vital negative feedback loop for drawing down atmosphere carbon dioxide. (5) Life was flourishing on Earth about 250 million years ago, when, over a period of between 8,000 and 100,000 years, nearly all of it was wiped out. The end-Permian-early Jurassic extinction ranks as the Earth’s most severe, causing death of up to 96% of all marine species and 70% of terrestrial vertebrate species; it is the only known mass extinction of insects. (21)
Most scientists acknowledge the role of the flood basalts of Siberia in fomenting the Permian extinction. The Siberian traps consist of 1.5 million cubic kilometers (360,000 cubic miles) of basalt (5,000,000 square kilometers or 1,900,000 square miles, the world’s largest continental flood basalt) that flowed out of a fissure in the Earth’s crust at the same time the Permian mass extinction occurred. (18) For example, Renne and Basu noted in 1991, The Siberian Traps represent one of the most voluminous flood basalt provinces on Earth…The bulk of these basalts was erupted over an extremely short time interval (900,000 plus or minus 800,000 years), beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year…Magmatism was not associated with significant lithospheric rifting; thus, mantle decompression resulting from rifting was probably not the primary cause of widespread melting. Inception of Siberian Traps volcanism coincided (within uncertainty) with a profound faunal mass extinction at the Permo-Triassic boundary 249 plus or minus 4 million years ago; these data thus leave open the question of a genetic relation between the two events. (22) The Deccan Trap eruptions 65 million years ago in what is now India affected the global environment to some degree. Researchers have detailed the chronology of volcanisms and climate change in the Maastrichtian Stage, which is the latest age or upper stage of the Late Cretaceous epoch (c. 70 to 65 million years ago). At the end of the Maastrichtian, there was a mass extinction commonly referred to as the Cretaceous-Tertiary extinction event at the so-called “K-T boundary.” Wignall writes: The mid-Maastrichtian was rather a cool interval, but a rapid phase of warming began around 400,000 years before the K-T boundary. This was reversed by a rapid cooling trend around 100,000 years before the boundary, when the 4-5 degree Celsius temperature gain was lost. The cooling coincides with a sharp sea level fall, and a lowstand was reached shortly before the K-T boundary. Thereafter, sea level began rising again across the boundary. These substantial oscillations in climate and sea level did not cause much in the way of extinctions….The possibility that the Deccan Trap eruptions were implicated in some or all of these changes has of course been known for some time. However, only recently has it been appreciated that the main eruptive phase coincided with the late Maastrichtian warm pulse. The release of volcanic carbon dioxide is the most likely driver of environmental change, with a calcification crisis in the oceans and global warming of the order of 4 degrees Celsius the most direct consequences. Thus, like the other LIP eruptions of the Cretaceous, the Deccan Trap eruptions appear to have caused significant climatic effects, but only modest biotic effects, perhaps because the oceans did not become anoxic. It has been argued that the biosphere was already rather stressed at the moment of meteorite impact (i.e., the six-mile meteorite that created the 110-mile diameter Chicxulub Crater beneath the Yucatan Peninsula in Mexico), but without that impact one suspects the end-Maastrichtian event would only have ranked alongside minor Cretaceous crises such as the Selli Event. (5,23) In other words, Wignall attributes the mass extinction at the K-T boundary to the combined effect of the Deccan Trap eruptions and the meteorite impact at Chicxulub Crater. Indeed, the idea that large meteorite impacts may trigger volcanic activity (impact-induced volcanism) has been around for several decades. (24) “Very large terrestrial impact craters [diameter greater than 310 miles; Chicxulub is more than 110 miles diameter] can create more [basaltic mantle] melt than the volume of the impact crater, i.e., sufficient for large igneous provinces (around 106 cubic kilometers [about 240,000 cubic miles, two-thirds the outflow of the Siberian traps]. For more information on impacts and melting as causes of continental basalt magma flows, see Note 23 below. Huge erupting continental flood basalts of low viscosity, possibly triggered by bolide impacts, have contributed significant basalt to the Earth’s crust during the past 300 million years and probably longer. Basalt magmas contain toxic volatile gases such as carbon dioxide and sulphur dioxide, which enter the atmosphere in huge volumes, setting in motion a cascade of events that may lead to mass extinctions. There is a remarkable correlation between mass extinctions and eruption of continental flood basalts. However, not all eruptions of continental flood basalts lead to mass extinctions of fauna and flora. The observation and conceptualization of “large igneous provinces” and examination of their natural history is a thrilling advance in geology. Notes:
|
Copyright
2007 - SEMP INC., All Rights Reserved.