Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Research Reveals Limitations of Seismic Data for Mapping Rock Units in Young Oceanic Crust

31.01.2007
Researchers report that an approach used for years to understand the structure of Earth's oceanic crust is flawed and geoscientists will have reconsider the correspondence between seismic data and rock units when mapping formations of young oceanic crust.

The new finding alters the view of how new crust is formed at mid-ocean ridges, how heat and chemicals flow through oceanic crust and how life can exist in the hot, inhospitable environment deep below the seafloor.

Scientists Gail Christeson and Kirk McIntosh from the Institute for Geophysics at The University of Texas at Austin’s Jackson School of Geosciences, and Jeffrey Karson from Syracuse University, publish their findings in this week's edition of Nature. Their research reveals that seismic data, widely used by geoscientists to create a picture of the geology below the seafloor, cannot reliably map the boundaries between rock units in young oceanic crust. Despite this limitation, seismic data may hold keys to understanding how fluids reside and circulate through the crust and the limits of the subsurface biosphere.

Oceanic crust makes up two-thirds of the Earth’s surface. It is made of igneous rock and formed at mid-ocean ridges, the largest volcanic system on our planet, from melting in the mantle. Eventually, oceanic crust is consumed at major earthquake-generating deep-sea trenches.

The oceanic crust has been studied by geophysicists and geologists for many years—geophysicists using remote sensing techniques such as seismic exploration, and geologists with samples and direct observations. In part because of their different perspectives and techniques, geologists and geophysicists have been at odds over the basic definitions of oceanic crustal structure.

Geologists typically describe an upper layer of basaltic lavas, a middle layer of basaltic intrusive rock units, known as dykes, and a lower layer of gabbroic rocks. The top layer of lavas formed when magma, or molten rock, erupted onto the seafloor. The middle layer of dykes were created as molten rock from the underlying magma chamber intruded into incrementally opening cracks at a spreading center, with younger dykes cross-cutting older dykes, eventually creating a massive collection of rock units referred to as a sheeted dyke complex. The incremental in-filling of cracks above a magma chamber is the essence of seafloor spreading.

Geophysicists divide oceanic crust (beneath any sedimentary material) into two basic layers, layer 2 and layer 3. Layer 2 is typically subdivided further into layers 2A and 2B. Layer 2A, the subject of the report in Nature, is a commonly imaged horizon in the seismic data, known as the 2A reflector, which numerous studies have mapped over extensive regions of young oceanic crust.

The problem with trying to reconcile the two models of the structure of oceanic crust is that while seismic methods can be used to probe deeply into the crust and gather data over unlimited distances, the interpretation of results is usually by inference. Geologic methods, on the other hand, provide direct evidence and observation, but are limited by the few outcrops and drill holes where samples from the crust can be collected, and by the limitation of observing outcrop data with submersibles.

“Our work addressed the extent to which seismic boundaries within the crust correlate with rock units at the Hess Deep rift and the Blanco transform fault,” explained co-investigator McIntosh, “where nature offers a rare glimpse of what lies beneath the seafloor and the Earth’s crust-making processes.”

“Places like Hess Deep rift and the Blanco transform provide windows into the internal structure of the oceanic crust in cliffs that are on the scale of the walls of the Grand Canyon— more than a mile high,” said Karson, co-investigator on the project and a professor in the Department of Earth Sciences at Syracuse University.

Because of the exposures at Hess Deep and the Blanco transform fault, the researchers were able to compare the seismic structure of upper oceanic crust with the known geology of the crust exposed and mapped by previous submersible dives.

“Prior to our study, there were no links between the geologic and seismological structure of oceanic crust except at a few deep drill holes,” said Christeson.

“Many researchers interpret seismic reflector 2A as the geologic boundary between the upper layer of lavas and the underlying sheeted dykes,” said Christeson. “Our work shows that we can’t reliably use seismic methods to map the boundary between lavas and dykes in young oceanic crust.”

“However, the seismic data maps porosity,” said Karson. “Microbes live in this pore space—a very exciting frontier of geology/biology.”

The results also undermine an alternative hypothesis that the 2A reflector is associated with a chemical alteration boundary zone within the upper lava unit, raising the question: What are these seismic differences mapping?

“We propose that the 2A reflector corresponds to a chemical alteration front associated with a feature—possible a crack where minerals can precipitate as a result of increased temperature and decreased porosity,” said Christeson. “Such a hydrothermal alteration zone can occur either within the lava section or near the top of the sheeted dyke complex of oceanic crust.”

The work carried out at Hess Deep and the Blanco transform fault expands geologists and geophysicists’ understanding of the relationship between the seismic boundaries and the rock units of oceanic crust and provides a new avenue of research to learn more about the porosity structure of the upper oceanic crust.

The research was supported by the National Science Foundation.

The article, titled “Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust” is available online (by subscription) at:

http://www.nature.com/nature/journal/v445/n7126/full/nature05517.html

J.B. Bird | EurekAlert!
Further information:
http://www.utexas.edu

More articles from Earth Sciences:

nachricht Novel method for investigating pore geometry in rocks
18.06.2018 | Kyushu University, I2CNER

nachricht Decades of satellite monitoring reveal Antarctic ice loss
14.06.2018 | University of Maryland

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

Im Focus: Water is not the same as water

Water molecules exist in two different forms with almost identical physical properties. For the first time, researchers have succeeded in separating the two forms to show that they can exhibit different chemical reactivities. These results were reported by researchers from the University of Basel and their colleagues in Hamburg in the scientific journal Nature Communications.

From a chemical perspective, water is a molecule in which a single oxygen atom is linked to two hydrogen atoms. It is less well known that water exists in two...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Novel method for investigating pore geometry in rocks

18.06.2018 | Earth Sciences

Diamond watch components

18.06.2018 | Process Engineering

New type of photosynthesis discovered

18.06.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>