Volcanic plumbing exposed

Two new studies into the “plumbing systems” that lie under volcanoes could bring scientists closer to predicting large eruptions.

International teams of researchers, led by the University of Leeds, studied the location and behaviour of magma chambers on the Earth’s mid-ocean ridge system – a vast chain of volcanoes along which the Earth forms new crust.

They worked in Afar (Ethiopia) and Iceland – the only places where mid-ocean ridges appear above sea level. Volcanic ridges (or “spreading centres”) occur when tectonic plates “rift” or pull apart. Magma (hot molten rock) injects itself into weaknesses in the brittle upper crust, erupting as lava and forming new crust upon cooling.

Magma chambers work like plumbing systems, channelling pressurised magma through networks of underground “pipes”.

The studies, published in Nature Geoscience, reveal new information about where magma is stored and how it moves through the geological plumbing network. Finding out where magma chambers lie and how they behave can help identify early warning signs of impending eruptions.

Scientists used images taken by the European Space Agency satellite Envisat to measure how the ground moved before, during and after eruptions. Using this data, they built and tested computer models to find out how rifting occurs.

Data in one study showed magma chambers that fed an eruption in November 2008 in the Afar rift of Northern Ethiopia were only about 1 km below the ground. The standard model had predicted a depth of more than 3 km.

It is highly unusual for magma chambers to lie in shallow depths on slow spreading centres such as the Afar rift, where tectonic plates pull apart at about the same speed as human fingernails grow.

Dr Carolina Pagli from the University of Leeds’ School of Earth and Environment, who led the study, says: “It was a complete surprise to see that a magma chamber could exist so close to the Earth’s surface in an area where the tectonic plates move apart so slowly. The results have changed the way we think about volcanoes.”

Dr Pagli also noticed that the ground started “uplifting” (elevating) four months before the eruption, due to new magma increasing pressure in one of the underground chambers. Understanding these precursory signals is fundamental to predicting eruptions.

A wider study of eruptions in Afar and Iceland, two vastly different environments, found remarkable similarities. Many events occurred within a short space of time. Researchers identified multiple magma chambers positioned horizontally and vertically, allowing magma to shoot in several directions. Moving magma triggered earthquakes, and separate magma chambers fed single eruptions.

The 2008 eruption is part of an unusual period of recent volcanic unrest in Ethiopia, and is enabling scientists to learn more about volcanoes at spreading centres. Most spreading centres are under 2 km of water at the bottom of the ocean, making detailed observations extremely challenging. The new knowledge derived from Ethiopian volcanoes will help scientists understand volcanoes in Iceland, where eruptions can have a bigger impact on the UK.

Dr Tim Wright from the School of Earth and Environment, who leads the international Afar Rift Consortium, said: “The dramatic events we have been witnessing in Afar in the past six years are transforming our understanding of how the crust grows when tectonic plates pull apart. Our work in one of the hottest place on Earth is having a direct impact on our understanding of eruptions from the frozen volcanoes of Iceland.”

Pattern of large earthquakes on San Jacinto fault identified with help of LiDAR

The San Jacinto Fault (SJF) Zone is a seismically active, major component of the overall southern San Andreas Fault system. Researchers from San Diego State University (SDSU) and U.S. Geological Survey have mapped evidence of past ruptures consistent with very large earthquakes along the Clark Fault, an individual strand associated with the SJF.

James Barrett Salisbury, now at Arizona State University and formerly a graduate student at SDSU, and his colleagues mapped the terrain by using both LiDAR and traditional field methods in order to determine the usefulness of LiDAR, which is an aerial mapping technology that emits laser pulses from an instrument mounted in an airplane. The laser pulses penetrate dense vegetation, allowing for the vegetation to be removed in data processing to yield high-resolution images of the Earth’s surface. LiDAR is especially useful for analyzing rugged, poorly accessible terrain.

Salisbury and his colleagues identified geomorphic evidence that suggests three large seismic events at evenly spaced intervals along the Clark Fault. This evidence correlates with previous research by co-author Tom Rockwell who dated events at the Hog Lake paleoseismic site near Anza, Calif.

Salisbury et al., infer that Nov. 22, 1800 is the date of the most recent surface rupturing earthquake event on the Clark fault. It correlates to the poorly located historic event recorded in southern California, and generally corresponds to the most recent event date at Hog Lake, which is radiocarbon dated at ca. 1790.

“LiDAR and Field Observations of Slip Distribution for the Most Recent Surface Ruptures along the Central San Jacinto Fault,” is published in the April issue of the Bulletin of the Seismological Society of America (BSSA). The authors are J. B. Salisbury, T. K. Rockwell and T.J. Middleton at San Diego State University; and K.W. Hudnut at U.S. Geological Survey.

New seismic hazard assessment for Central America

A new study evaluates the seismic hazards for the entire Central America, including specific assessments for six capital cities, with the greatest hazard expected for Guatemala City and San Salvador, followed by Managua and San José, and notably lower in Tegucigalpa and Panamá City.

The study, published in the April issue of the Bulletin of the Seismological Society of America (BSSA), included input from seismic hazard experts from Costa Rica, Guatemala, Honduras, Nicaragua, El Salvador, Panama, Norway and Spain. All seismic experts from Central American countries, except Belize, agree with the study’s assessments.

The paper outlines the work carried out as part of the cooperation project named RESIS II, under the auspices of the Norway Cooperation Agency (NORAD), with a contribution of the Technical University of Madrid (UPM). A new regional seismic catalog and a strong motion database updated up to December 2010 have been developed.

A seismogenic zonation has been proposed for the entire region, considering the three tectonic settings (crust, subduction interface and inslab), with zones defined at a national level and coherent at a regional scale, avoiding discontinuities along the national boundaries.

This is the first study developed in Central America at a regional scale this century and the first done in terms of peak ground acceleration (PGA) and different spectral acceleration values for the entire region. The study provides new information that is being considered in the revision of national seismic codes and it is also supported by the Coordination Centre for Natural Disasters Prevention in Central America (CEPREDENAC).

Scientists refine Earth’s clock

New research has revealed that some events in Earth’s history happened more recently than previously thought. Scientists from the British Geological Survey and the Massachusetts Institute of Technology, publishing this week in the journal Science, have refined the data used to determine how much time has passed since a mineral or rock was formed. They report uranium isotopic composition of minerals, used to date major geological events, which are more accurate than previously published. The major effect of this is to reduce previous age determinations by up to 700,000 years.

Minerals naturally capture uranium when they form, which in turn undergoes a chain of radioactive decays to other elements, ending with lead. This new research has shown that, by more accurately measuring the relative amount of the uranium isotopes 238U and 235U, we now have a better understanding of how much time has passed since a mineral or rock has formed.

A major effect of this work will be to decrease all previous uranium-lead (U-Pb) age determinations, by up to 700,000 years for samples that are about 4.5 billion years old – the age of the Earth. In particular, the new 238U/235U ratio will allow geologists to place more accurate limits on the exact timing of a broad range of geological processes, from the initial formation of our planet, continents and economic mineral deposits, to past evolutionary events and climate change.

Blair Schoene, a geologist from Princeton University said “This new determination will not only improve the accuracy of each U-Pb age but ultimately our understanding of events in Earth history.”

For over 35 years, a 238U/235U ratio of 137.88 has been used to calculate U-Pb dates, from the oldest rocks that formed four billion years ago, to much younger rocks that are hundreds of thousands of years old. When scientists recently evaluated the measurements used to arrive at the 137.88 value, they came to a dead end: the value could not be traced back to standard units such as the kilogram. This new study shows that many naturally occurring uranium-rich minerals, such as zircon, actually have a lower 238U/235U value with an average of 137.818 ± 0.045 (the uncertainty assigned to this value relates to the variation observed between different samples). Agreement between these results, other rocks, and meteorites indicate the new average 238U/235U value and uncertainty may also be representative of the Earth’s ‘bulk’ uranium isotopic composition.</P