Major Volcanic Eruptions and Climate Change

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Some of the larger, more explosive volcanic eruptions that the planet has witnessed in the past few hundred years have ejected large amounts of ash and finer particles called aerosols into the atmosphere and stratosphere, and it may take years for these particles to settle back down to Earth. They get distributed about the planet by high-level winds, and they have the effect of blocking out some of the Sun's rays, which lowers global temperatures. This happens because particles and aerosol gases in the upper atmosphere tend to scatter sunlight back to space, lowering the amount of incoming solar energy. In contrast, particles that get injected into only the lower atmosphere absorb sunlight and contribute to greenhouse warming. A side effect is that the extra particles in the atmosphere also produce more spectacular sunsets and sunrises, as does extra pollution in the atmosphere. These effects were readily observed after the 1991 eruption of Mount Pinatubo, which spewed more than 172 billion cubic feet (5 billion m3) of ash and aerosols into the atmosphere, causing global cooling for two years after the eruption. Even more spectacularly, the 1815 eruption of Tambora in Indonesia caused three days of total darkness for approximately 300 miles (500 km) from the volcano, and it initiated the famous "year without a summer" in Europe, because the ash from this eruption lowered global temperatures by more than a degree.

The amounts of gases and small airborne particles released by large volcanic eruptions such as Pinatubo and Tambora are dwarfed by the amount of material placed into the atmosphere during some of Earth's most massive eruptions, known as flood basalt events. No flood basalts have been formed on Earth for several tens of millions of years, which is a good thing, since their eruption may be associated with severe changes in climate.

Scattered around the globe are numerous anomalously thick accumulations of dark lava, variously known as flood basalts, traps, or large igneous provinces. These vast outpourings of lava have different ages and represent the largest known volcanic episodes on the planet in the past several hundred million years. These deposits include continental flood basalt provinces, anomalously thick and topographically high seafloor known as oceanic plateaus, and some volcanic rifted passive margins.

understanding the greenhouse effect

The term greenhouse effect refers to a phenomenon where Earth's climate is sensitive to the concentrations of certain gases in the atmosphere. The concept was first coined in 1681 by Edme Mariotte, who noted that light and heat from the Sun easily passes through a sheet of glass but that heat from candles and other sources does not. This concept was then extended by Joseph Fourier in 1824 to the atmosphere by noting that heat and light from the Sun can pass from space through the atmosphere, but heat radiated back to the atmosphere from Earth may get trapped by some of the atmospheric gases, just like the heat from a candle is partly blocked by the glass pane. Then in 1861 John Tyndall identified that the complex molecules of water (H2O) and carbon dioxide (CO2) were mainly responsible for the absorption of heat radiated back from Earth and that other atmospheric gases such as nitrogen and oxygen did not play a role in this effect. Tyndall noted that simple changes in the concentrations of CO2 and H2O could alternately cool and heat the atmosphere, producing "all the mutations of climate which the researches of geologists reveal." The next step in understanding the greenhouse effect came in 1896 from the work of Svante Arrhenius, who calculated that a 40 percent increase or decrease in the atmospheric concentration of CO2 could trigger the advance and or retreat of continental glaciers, triggering the glacial and interglacial ages. Much later, a change in the atmospheric CO2 of this magnitude was documented in cores of the Greenland ice sheet, as predicted by Arrhenius. Carbon dioxide can vary naturally in the atmosphere through a variety of driving mechanisms, including changes in volcanism, erosion, plate tectonics, and ocean-atmosphere interactions. The modern concept of linking greenhouse gases with the burning of fossil fuels by humans was formulated by Guy Stewart Callendar, who in 1938 calculated that a doubling of atmospheric CO2 by burning fossil fuels would result in an average global temperature increase of about 3°F (2°C), with more heating at the poles. Callendar made some prescient predictions that humans are changing the composition of the atmosphere at a rate that is "exceptional" on geological time scales and sought to understand what effects these changes might have on climate. His prediction was that the "principal result of increasing carbon dioxide will be a gradual increase in the mean temperature of the colder regions of Earth." These predictions were first confirmed in 1947 when Ahlmann reported a 1-2°F (1.3°C) increase in the average temperature of the North Atlantic sector of the Arctic. However, at this time the nature of the complex interactions of the carbon cycle and exchange of CO2 in the atmosphere-ocean system was not well understood, and many scientists attributed the entire temperature rise to human production of greenhouse gases. Later studies of ocean-atmosphere relationships and biogeochemistry showed more complex relationships. Later, in the 1970s, effects of aerosols in the atmosphere, principally to reflect solar radiation to space and cooling Earth, began to be appreciated as another component of the greenhouse effect. The current state of knowledge of the complex physical, chemical, biological and other processes associated with the greenhouse effect are described in the "Climate Change 2007" report issued by the Intergovernmental Panel on Climate Change.

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During eruption of these vast piles of volcanic rock Earth moved more material and energy from its interior in extremely short time periods than during the entire intervals between the massive volcanic events. Such large amounts of volcanism also released large amounts of volcanic gases into the atmosphere, with serious implications for global temperatures and climate, and may have contributed to some global mass extinctions. Many are associated with periods of global cooling where volcanic gases reduce the amount of incoming solar radiation, resulting in volcanic winters.

The largest continental flood basalt province in the United States is the Columbia River flood basalt in Washington, Oregon, and Idaho. The Columbia River flood basalt province is 6-17 million years old, and contains an estimated 1,250 cubic miles (5,210 km3) of basalt. Individual lava flows erupted through fissures or cracks in the crust; it then flowed laterally across the plain for up to 400 miles.

The 66-million-year-old Deccan flood basalts, also known as traps, cover a large part of western India and the Seychelles. They are associated with the breakup of India from the Seychelles during the opening of the Indian Ocean. Slightly older flood basalts (90-83 million years old) are associated with the breakaway of Madagascar from India. The volume of the Deccan traps is estimated at 5,000,000 cubic miles (20,840,000 km3), and the volcanics are thought to have been erupted in about 1 million years, starting slightly before the great Cretaceous-Tertiary extinction. Most workers now agree that the gases released during this period of flood basalt volcanism stressed the global biosphere to such an extent that many marine organisms were forced into extinction, and many others were stressed. Then the planet was hit by the meteorite that formed the massive Chicxulub impact crater on the Yucatán Peninsula (Mexico), causing the massive extinction including the end of the dinosaurs.

The breakup of east Africa along the East African rift system and the Red Sea is associated with large amounts of Cenozoic (less than 30 million years old) continental flood basalts. Some of the older volcanic fields are located in east Africa in the Afar region of Ethiopia, south into Kenya and Uganda, and north across the Red Sea and Gulf of Aden into Yemen and Saudi Arabia. These volcanic piles are overlain by younger (less than 15 million-year-old) flood basalts that extend both farther south into Tanzania and farther north through central Arabia, where they are known as Harrats, and into Syria, Israel, Lebanon, and Jordan.

An older volcanic province also associated with the breakup of a continent is known as the North Atlantic Igneous Province. It formed along with the breakup of the North Atlantic Ocean at 62-55 million years ago and includes both onshore and offshore volcanic flows and intrusions in Greenland, Iceland, and the northern British Isles, including most of the Rockall Plateau and Faeroe Islands. In the South Atlantic, a similar 129-134 million-year-old flood basalt was split by the opening of the ocean and now has two parts. In Brazil, the flood lavas are known as the Paraná basalts, and in Namibia and Angola of West Africa as the Etendeka basalts.

The Caribbean Sea floor represents one of the best examples of an oceanic plateau, with other major examples including the Ontong-Java Plateau, Manihiki Plateau, Hess Rise, Shatsky Rise, and Mid-Pacific Mountains. All of these oceanic plateaus contain between six- and 25-mile- (9- and 40 km-) thick piles of volcanic and subvolcanic rocks representing huge outpourings of lava. The Caribbean Sea floor preserves 5-13 mile (8-21 km) thick oceanic crust formed before about 85 million years ago in the eastern Pacific Ocean. This unusually thick ocean floor was transported eastward by plate tectonics, where pieces of the seafloor collided with South America as it passed into the Atlantic Ocean. Pieces of the Caribbean oceanic crust are now preserved in Colombia, Ecuador, Panama, Hispanolia, and Cuba, and some scientists estimate that the Caribbean oceanic plateau may have once been twice its present size. In either case, it represents a vast outpouring of lava

Photo of limestone rocks, deposited in a shallow sea in the Carboniferous (300 million years ago) on shoreline of Fayette Village State Park, Michigan, showing how climate and sea level rise have at times in Earth history raised sea levels so that much of the continental interior is covered by shallow seas. (CORBIS)

Photo of limestone rocks, deposited in a shallow sea in the Carboniferous (300 million years ago) on shoreline of Fayette Village State Park, Michigan, showing how climate and sea level rise have at times in Earth history raised sea levels so that much of the continental interior is covered by shallow seas. (CORBIS)

that would have been associated with significant outgassing with possible consequences for global climate and evolution.

The western Pacific Ocean basin contains several large oceanic plateaus, including the 20 mile (32 km) thick crust of the Alaskan-sized Ontong-Java Plateau, which is the largest outpouring of volcanic rocks on the planet. It apparently formed in two intervals, at 122 and 90 million years ago, entirely within the ocean, and represents magma that rose in a plume from deep in the mantle and erupted on the seafloor. It is estimated that the volume of magma erupted in the first event was equivalent to that of all the magma being erupted at mid-ocean ridges at the present time. Sea levels rose by more than 30 feet (9 m) in response to this volcanic outpouring. The gases released during these eruptions are estimated to have raised average global temperatures by 23°F (13°C).

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