Seeking The Elusive 'Moho'
By Robert Roy Britt - Senior Writer - LiveScience.com - 7 April
2005
Scientist said this week they had drilled into the lower section
of Earth's crust for the first time and were poised to break through
to the mantle in coming years.
The Integrated Ocean Drilling Program (IODP) seeks the elusive "Moho,"
a boundary formally known as the Mohorovicic discontinuity. It marks
the division between Earth's brittle outer crust and the hotter,
softer mantle.
The depth of the Moho varies. This latest effort, which drilled
4,644 feet (1,416 meters) below the ocean seafloor, appears to have
been 1,000 feet off to the side of where it needed to be to pierce
the Moho, according to one reading of seismic data used to map the
crust's varying thickness.
The new hole, which took nearly eight weeks to drill, is the third
deepest ever made. The rock collection brought back to the surface
is providing new information about the planet's composition.
"It will provide important clues on how ocean crust forms,"
said Rodey Batiza, program director for ocean drilling at the National
Science Foundation (NSF).
Already the types of rocks recovered show that conventional interpretation
of Earth's evolution are "oversimplifying many of the features
of the ocean's crust," said expedition leader Jay Miller of
Texas A&M University. "Each time we drill a hole, we learn
that Earth's structure is more complex. Our understanding of how
the Earth evolved is changing accordingly."
The latest drilling was done at the Atlantis Massif, located at
the intersection of the Mid-Atlantic Ridge and the Atlantis fracture
zone, two plates of the planet's broken crust. The seafloor is shallower
at the center of this region and therefore easier to reach.
It's not clear yet whether drilling should continue at the new hole
or if another one should be started in the effort the reach the
mantle. Such work isn't likely to begin again in the next year,
said Barbara John, a University of Wyoming geologist and one of
the co-chief scientists on the expedition.
"We need to evaluate all the data we have from the cruise and
re-analyze the seismic data, to determine whether it's better to
deepen the current hole or drill elsewhere, or maybe even collect
additional seismic data to better constrain where to drill,"
John told LiveScience. "Our major result is that we've recovered
the lower crust for the first time and have confirmed that the Earth's
crust at this locality is more complicated than we thought."
John said mantle material will be evident when and if it's brought
up because it will have different texture and chemistry and will
contain different proportions of minerals compared with rock in
the crust.
Drillers use the vessel JOIDES Resolution. The 10-year, $1.5 billion
program is funded by the NSF and Japan's Ministry of Education,
Culture, Sports, Science, and Technology.
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