Scientists Study Earthquake Triggers in Pacific Ocean
Samples of rock, sediment from beneath the sea-floor help explain quakes like Japan's
 The scientific drilling vessel JOIDES Resolution in Puntarenas, Costa Rica. |
June 29, 2011
New samples of rock and sediment from the depths of the eastern Pacific Ocean may help explain the cause of large, destructive earthquakes similar to the Tohoku Earthquake that struck Japan in mid-March.
Nearly 1,500 meters (almost one mile) of sediment cores collected from the ocean floor off the coast of Costa Rica reveal detailed records of some two million years of tectonic activity along a seismic plate boundary.
The scientific drilling vessel JOIDES Resolution retrieved the samples during a recent month-long Integrated Ocean Drilling Program (IODP) Costa Rica Seismogenesis Project (CRISP) Expedition.
The expedition and IODP are funded in part by the National Science Foundation (NSF).
"It's critical to understand how subduction zone earthquakes and tsunamis originate--especially in light of recent events in Japan," says Rodey Batiza of NSF's Division of Ocean Sciences. "The results of this expedition will also help us learn more about our own such zone off the Pacific Northwest."
Participating scientists aim to use the samples to better understand the processes that control the triggering of large earthquakes at subduction zones, where one plate slides beneath another.
"We know that there are different factors that contribute to seismic activity--these include rock type and composition, temperature differences and how water moves within the Earth's crust," explained co-chief scientist Paola Vannucchi of the University of Florence in Italy, who led the expedition with co-chief scientist Kohtaro Ujiie of the University of Tsukuba in Japan.
Vannucchi added, "but what we don't fully understand is how these factors interact with one another and if one may be more important than another in leading up to different magnitudes of earthquakes.
"This expedition provided us with crucial samples for answering these fundamental questions."
More than 80 percent of global earthquakes above magnitude 8.0 occur along subduction zones.
The Pacific Ocean is famous for these boundaries, known as convergent margins, which are found along the coasts of the East Pacific from Alaska to Patagonia, New Zealand, Tonga and Marianas--all the way to Japan and the Aleutians.
The margins of the world's largest ocean basin are therefore a primary target for research into the triggering mechanisms of large quakes.
During four weeks at sea, the scientists and crew successfully drilled four sites, recovering core samples of sand and clay-like sediment and basalt rock.
In a preliminary report published this month, CRISP scientists say that they have found evidence for a strong subsidence, or sinking, of the Costa Rica margin combined with a large volume of sediment discharged from the continent and accumulated in the last two million years.
"The sediment samples provide novel information on different parameters which may regulate the mechanical state of the plate interface at depth," said Ujiie.
"Knowing how the plates interact at the fault marking their boundary is critical to interpreting the behavior and frequency of earthquakes in the region."
Vannucchi adds, "for example, we now know that fluids from deeper parts of the subduction zone system have percolated up through the layers of sediment.
"Studying the composition and volume of these fluids, as well as how they have moved through the sediment, helps us better understand the relationship between the chemical, thermal and mass transfer activity in the seafloor and the earthquake-generating, or seismogenic, region of the plate boundary.
"They may be correlated."
Members of the research party are currently analyzing cores from the CRISP Expedition at their home institutions.
The expedition is unique because it focuses on the properties of erosional convergent margins, where the overriding plate gets "consumed" by subduction processes
These plate boundaries are characterized by trenches with thin sediment covering less than 400 meters (1,312 feet), fast convergence between the plates at rates greater than eight centimeters per year, and increased seismicity.
The seismically active CRISP research area is the only one of its kind that is accessible to research drilling.
However, this subduction zone is representative of 50 percent of global subduction zones, making scientific insights gleaned here relevant to Costa Ricans and others living in earthquake-prone regions all around the Pacific Ocean.
The recent Tohoku Earthquake in Japan was generated in an erosive portion of the plate interface.
Other geoscience research drilling programs, such as IODP's Nankai Trough Seismogenic Zone Experiment (NantroSEIZE), near the southeast coast of Japan, focus on accretionary margins, where the front part of the overriding tectonic plate is built up by the subduction processes, sometimes forming mountains, and the plate boundary input is trench material.
In these environments, the trench sediments are greater than 1,000 meters, or more than half a mile.
Accretionary margins are known for their large earthquakes, such as the 1964 Alaska and the 2004 Sumatra quakes.
Japan's Nankai Trough was the center of two magnitude 8 earthquakes in 1944 and 1946.
The CRISP team hopes to return to the same drill site in the future to directly sample the plate boundary and fault zone before and after seismic activity in the region.
Changes observed may provide new insights into how earthquakes are generated.
The Integrated Ocean Drilling Program (IODP) is an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring and monitoring the subseafloor.
The JOIDES Resolution is a scientific research vessel managed by the U.S. Implementing Organization of IODP (USIO). Together, Texas A&M University, Lamont-Doherty Earth Observatory of Columbia University and the Consortium for Ocean Leadership comprise the USIO.
IODP is supported by two lead agencies: the U.S. National Science Foundation and Japan's Ministry of Education, Culture, Sports, Science and Technology.
Additional program support comes from the European Consortium for Ocean Research Drilling, the Australian-New Zealand IODP Consortium, India's Ministry of Earth Sciences, the Ministry of Science and Technology in the People's Republic of China and the Korea Institute of Geoscience and Mineral Resources.
-NSF-
JOIDES Resolution: http://www.joidesresolution.org
Integrated Ocean Drilling Program: http://www.iodp.org
| Heavy Metal Meets Hard Rock: Battling through the Ocean Crust's Hardest Rocks | |
Heavy Metal Meets Hard Rock: Battling through the Ocean Crust's Hardest Rocks to Capture the Boundary Between Magma and Water
Scientists and drillers recovered a remarkable suite of heat-tempered basalts that provide a detailed picture of the rarely seen boundary between magma and seawater. These samples were collected during a return to ODP Hole 1256D, one of the deepest "hard rock" penetration sites of scientific ocean drilling. ODP Hole 1256D has been stabilized, cleared to its full depth, and primed for further deepening.Panama City, Panama – Integrated Ocean Drilling Program (IODP) Expedition 335 Superfast Spreading Rate Crust 4 recently completed operations in Ocean Drilling Program (ODP) Hole 1256D, a deep scientific borehole that extends more than 1500 meters below the seafloor into the Pacific Ocean's igneous crust – rocks that formed through the cooling and crystallization of magma, and form the basement of the ocean floor.
An international team of scientists led by co-chief scientists Damon Teagle (National Oceanographic Center Southampton, University of Southampton in the UK) and Benoît Ildefonse (CNRS, Université Montpellier 2 in France) returned to ODP Hole 1256D aboard the scientific research vessel, JOIDES Resolution, to sample a complete section of intact oceanic crust down into gabbros.
 A granoblastic basalt viewed under the microscope (picture is 2.3 mm across). Magnification shows a rock formed of small rounded mineral grains annealed together. These rocks are the hardest material ever drilled in more than 4 decades of scientific ocean drilling. The rocks are very abrasive and aggressive to the drilling and coring tools, and difficult to penetrate. Still, the samples recovered provide a treasure trove of information, recording the rocks' initial crystallization as a basaltic dike then their reheating at the top of the mid-ocean ridge magma chamber. These rocks represent the heat exchanger where thermal energy from the cooling and solidifying melt in the magma chamber below is exchanged with seawater infiltrating from the oceans. (Credit: IODP) |
This expedition was the fourth in a series and builds on the efforts of three expeditions in 2002 and 2005.
Gabbros are coarse-grained intrusive rocks formed by the slow cooling of basaltic magmas. They make up the lower two-thirds of the ocean crust. The intrusion of gabbros at the mid-ocean ridges is the largest igneous process active on our planet with more than 12 cubic kilometers of new magma from the mantle intruded into the crust each year. The minerals, chemistry, and textures of gabbroic rocks preserve records of the processes that occur deep within the Earth's mid-ocean ridges, where new ocean crust is formed.
"The formation of new crust is the first step in Earth's plate tectonic cycle," explained Teagle. "This is the principal mechanism by which heat and material rise from within the Earth to the surface of the planet. And it's the motion and interactions of Earth's tectonic plates that drive the formation of mountains and volcanoes, the initiation of earthquakes, and the exchange of elements (such as carbon) between the Earth's interior, oceans, and atmosphere."
"Understanding the mechanisms that construct new tectonic plates has been a major, long-standing goal of scientific ocean drilling," added Ildefonse, "but progress has been inhibited by a dearth of appropriate samples because deep drilling (at depths greater than 1000 meters into the crust) in the rugged lavas and intrusive rocks of the ocean crust continues to pose significant technical challenges."
ODP Hole 1256D lies in the eastern equatorial Pacific Ocean about 900 kilometers to the west of Costa Rica and 1150 kilometers east of the present day East Pacific Rise. This hole is in 15 million year old crust that formed during an episode of "superfast" spreading at the ancient East Pacific Rise, when the newly formed plates were moving apart by more than 200 millimeters per year (mm/yr).
"Although a spreading rate of 200 mm/yr is significantly faster than the fastest spreading rates on our planet today, superfast-spread crust was an attractive target," stated Teagle, "because seismic experiments at active mid-ocean ridges indicated that gabbroic rocks should occur at much shallower depths than in crust formed at slower spreading rates. In 2005, we recovered gabbroic rocks at their predicted depth of approximately 1400 meters below the seafloor, vindicating the overall 'Superfast' strategy."
 Granoblastic dikes samples were recovered in abundance by fishing tools during successive hole remediation operations. Sumiyo Miyashita and Yoshiko Adashi, from Niigata University, Japan, examine large rock samples from Hole 1256D. (Credit: IODP) |
Previous expeditions to Hole 1256D successfully drilled through the erupted lavas and thin (approximately one-meter-wide) intrusive "dikes" of the upper crust, reaching into the gabbroic rocks of the lower crust. The drilling efforts of Expedition 335 were focused just below the 1500-meter mark in the critical transition zone from dikes to gabbros, where magma at 1200°C exchanges heat with super-heated seawater circulating within cracks in the upper crust. This heat exchange occurs across a narrow thermal boundary that is perhaps only a few tens of meters thick.
In this zone, the intrusion of magma causes profound textural changes to the surrounding rocks, a process known as contact metamorphism. In the mid-ocean ridge environment this results in the formation of very fine-grained granular rocks, called granoblastic basalts, whose constituent minerals recrystallize at a microscopic scale and become welded together by magmatic heat. The resulting metamorphic rock is as hard as any formation encountered by ocean drilling and sometimes even tougher than the most resilient of hard formation drilling and coring bits.
Expedition 335 reentered Hole 1256D more than five years after the last expedition to this site. The expedition encountered and overcame a series of significant engineering challenges, each of which was unique, although difficulties were not unexpected when drilling in a deep, uncased, marine borehole into igneous rocks.
The patient, persistent efforts of the drilling crew successfully cleared a major obstruction at a depth of 920 that had initially prevented reentry into the hole to its full depth of 1507 meters. Then at the bottom of the hole the very hard granular rocks that had proved challenging during the previous Superfast expedition were once more encountered. Although there may only be a few tens of meters of these particularly tenacious granoblastic basalts, their extreme toughness once more proved challenging to sample– resulting in the grinding down of one of the hardest formation coring bits into a smooth stump.
 Laying out a fishing tool on the rig floor. An illustration of the hard work by the drill ship crew. (Credit: IODP) |
Expedition 335 operations also succeeded in clearing Hole 1256D of drill cuttings, much of which appear to have been circulating in the hole since earlier expeditions.
"We recovered a remarkable sample suite of granoblastic basalts along with minor gabbros, providing a detailed picture of a rarely sampled, yet critical interval of the oceanic crust," Ildefonse observed. "Most importantly," he added, "the hole has been stabilized and cleared to its full depth, and is ready for deepening in the near future."
About IODP
The Integrated Ocean Drilling Program (IODP), is an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring, and monitoring the subseafloor. The JOIDES Resolution is a scientific research vessel managed by the US Implementing Organization of IODP (USIO). CHIKYU is a drilling vessel operated by JAMSTEC/CDEX (Japan), and mission-specific platforms are supplied by ECORD (the European Consortium for Ocean Research Drilling). IODP is supported by two lead agencies: the US National Science Foundation (NSF) and Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT). Additional program support comes from ECORD, the Australian-New Zealand IODP Consortium (ANZIC), India's Ministry of Earth Sciences, the People's Republic of China (Ministry of Science and Technology), and the Korea Institute of Geoscience and Mineral Resources.
Useful Websites:
For more information about IODP Expedition 335, Superfast Spreading Rate Crust 4, visit http://iodp.tamu.edu/scienceops/expeditions/superfast_rate_crust.html.
For more information about the JOIDES Resolution, visit www.joidesresolution.org.
For more information about the Integrated Ocean Drilling Program, visit www.iodp.org.
Source
--
Got Penguins?
Penguin News Today
The Science of Penguins
Gentoo Penguins of Gars O'Higgins Station, Antarctica
Wolves & Werewolves/Science, Myth & News
http://canislupus101.blogspot.com/
http://dances-with-werewolves.blogspot.com/
http://throughgoldeneyes.blogspot.com/
__._,_.___
No comments:
Post a Comment