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| Enlarge ImageHow is the seafloor paved with lava?
The development of ABE, the Autonomous Benthic Explorer, made it possible to fly close enough to the seafloor to measure magnetic intensities of young seafloor lava. Such measurements will give scientists the ability to unravel the convoluted processes by which lava carpets the seafloor. Scientists are investigating whether fresh lava (red) erupts from a central point in the mid-ocean ridge (as depicted above) and cascades downhill to overlay older lava flows (gray), or whether lava erupts from several isolated, outlying magma chambers to create discrete patches of seafloor. Or perhaps both processes occur. (Illustration by E. Paul Oberlander, WHOI) |
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| Enlarge ImageBy superimposing magnetic measurements on detailed seafloor topography maps like this one, scientists can distinguish how, when, and where individual lava flows occurred. Younger lava has the highest magnetic intensities (red and yellows). Above, the most recent lava flow erupted from the ridge axis, overlaid older lava flows, and pooled to the left of the axis. |
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| Enlarge ImageMagnetic ‘zebra stripes’
Seafloor lavas have built-in magnetic “clocks” that reveal their age. When seafloor lava solidifies at the seafloor, its magnetic crystals are quenched in alignment with Earth’s magnetic field, and the rocks’ magnetic “polarity” is preserved. But Earth’s magnetic field has reversed many times over the planet’s history—with the magnetic north sometimes facing south, or vice versa, as it is today.
New seafloor is created at mid-ocean ridges (with the prevailing magnetic signature) and spreads out in both directions, creating a symmetrical “zebra-stripe” pattern of alternating rocks with either “reversed” or “normal” polarity. |
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Related Multimedia |
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New Ocean Crust
This animation shows a possible sequence of how lava flows might erupt on the seafloor and create new ocean crust. It is based on conceptual models and observations derived from collected data, at the East Pacific Rise near 9°50'N in the Pacific Ocean.
Courtesy: Dan Fornari, WHOI | » View Video (Quicktime) LAN
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Maurice A. Tivey, Associate Scientist
Geology and Geophysics Department
Woods Hole Oceanographic Institution Most of Earth’s crust is manufactured at the bottom of the sea. Deep
beneath the waves and beyond our view, magma erupts along a 40,000-mile
volcanic mountain chain that bisects the ocean floors and encircles the
globe. The lava flowing from these mid-ocean ridges solidifies into new
ocean crust that spreads out and paves the surface of our planet.
That’s the “big picture.” But our ability to understand all the subtle
and complex details of this fundamental, planet-shaping process is
blocked by the oceans themselves. Miles of water prevent us from seeing
the seafloor directly, and we can’t survive the darkness and high
pressures at the seafloor to explore it firsthand.
Exploring from afar
Imagine explorers from another planet hovering in a spaceship high
above a large American city. From this lofty perspective, the explorers
would deduce that the urban landscape was not natural, but constructed
somehow in particular ways.
To understand this landscape,
they would want to know: When were individual structures made? Of what
materials? In what sequence? How are the roads, buildings, and
sidewalks erected? Why are they located where they are? Which came
first? The extraterrestrials would have to zoom down to get a closer
look and perhaps take some close-up photos and samples to untangle the
myriad factors that cumulatively result in cities.
For ocean
scientists to zoom down and take a closer look at the seafloor requires
specialized equipment. In the past decade, we have taken big
technological leaps. New undersea vehicles have given us unprecedented
access to the seafloor and new abilities to collect previously
unattainable data.
Our goal is to understand the seafloor
processes that turn molten lava into fresh crust. In particular, we are
developing an innovative technique, measuring the magnetic properties
of very young seafloor rocks. It is revealing new details of how,
where, and when lava erupts, and how it flows and accumulates on the
ocean floor.
Telltale magnetic clues
As
hot lava erupts, magnetic iron oxide crystals within it (magnetite)
orient themselves to align with Earth’s magnetic field. When the lava
cools and solidifies, its magnetic crystals are “flash frozen”—pointing
“north.” The rocks’ magnetic direction, or “polarity,” is preserved.
But the magnetic north pole has not always been where it is now.
Throughout Earth’s 4.6-billion-year history, its magnetic field has
flip-flopped several times—with the magnetic north sometimes facing
south, or vice versa as it is today.
The time periods when
many of these magnetic reversals occurred are well documented. So
seafloor lavas provide a built-in chronometer or calendar that we can
use to determine when they were created. Thus, the continually forming
ocean crust is kind of a tape recorder of Earth’s magnetic field
history. Magnetic ‘zebra stripes’
In the late 1960s, this phenomenon provided crucial evidence confirming
that the seafloor was indeed spreading apart. This concept is at the
core of the revolutionary theory of “plate tectonics.”
In
the early 1960s, scientists analyzed the magnetic properties of rocks
in terrestrial outcrops and developed a timescale that chronicled the
reversals of Earth’s magnetic field. At sea, scientists collected
magnetic data from sensors towed from ships and found remarkable
magnetic patterns in seafloor rocks that were subsequently correlated
with the magnetic reversal “clock” established in terrestrial rocks.
On both sides of mid-ocean ridges, scientists found a pattern of
alternating magnetic “zebra stripes.” “Black” stripes represented rocks
imprinted when Earth’s magnetic field was in a normal direction, as it
is today, and “white” represents rocks imprinted when the field was in
a reversed direction.
The stripes ran parallel to the
ridges, and the pattern was astonishingly symmetrical on either side of
the ridges. This mirror image could form only if new seafloor was
created at the crests of ridges (with the prevailing magnetic
signature) and then spread outward in both directions.
A new way to measure ‘young’ rock
Historically, magnetic data were collected with magnetometers towed at
the ocean surface by research ships. They provided a large-scale
picture (like the view of a city from a high-flying spaceship) of how
ocean crust forms over many millions of years and hundreds of miles.
To get a more detailed view, we began to look at rocks less than
100,000 years old and within just a few miles of the ridge crest. This
was problematic, because the most recent magnetic reversal occurred
780,000 years ago. We had no way of finding the age of lava younger
than this.
To solve this dilemma, we began to examine not only
the polarity of Earth’s magnetic field, but also the strength of the
field, or intensity. The intensity of Earth’s magnetic field has also
varied dramatically through time, and ocean floor sediments have
preserved a record of Earth’s magnetic field intensity over the past
few hundred thousand years.
So while magnetic polarity
measurements give us ages within millions to hundreds of thousands of
years, intensity measurements give us ages within thousands to tens of
thousands of years—a minute hand on the magnetic clock. Thus, within a
patch of young lava with the same magnetic orientation, we can
distinguish older and younger rocks, and we can begin to unravel the
sequences in which they were deposited on the seafloor. Unprecedented seafloor access
To detect magnetic intensities, we must take measurements within meters
of the rocks—something we just could not do until recently. In 1993, we
received an opportunity to use a magnetometer aboard the submersible
Alvin, and we confirmed for the first time that we could detect strong
magnetic intensity signals in freshly erupted lava. The invention and
development at WHOI of the Autonomous Benthic Explorer (ABE) in the
mid-1990s made it possible to measure magnetic intensities over wide
swaths of seafloor lava.
ABE can maintain a stable ride and
a constant altitude over changing seafloor contours—features that make
it well suited for collecting high-resolution magnetic measurements.
Over several hours, it can survey 20 to 25 kilometers of seafloor. At
the same time, its sonar can take measurements to create fine-scaled
topographical maps.
The maps give us detailed,
three-dimensional pictures of contorted amalgamations of lava flows. By
superimposing magnetic measurements on these maps, we can distinguish
the extent and volume of individual lava flows in the upper crust and
tell when they erupted. We can begin to unravel how flows are buried by
subsequent flows like a deck of cards. Alvin plays a crucial
complementary role, giving us an essential visual picture of the
seascape and the ability to sample fresh lava for precision dating.
Many bursts or a ‘crack of doom’?
My colleagues in this National Science Foundation-sponsored
research—Hans Schouten, Dan Fornari, and Ken Sims at WHOI, and Jeffrey
Gee at Scripps Institution of Oceanography—focus on the ocean bottom
because seafloor lava flows are young and not yet buried by thick muds.
They also have not lost their magnetic intensity due to chemical
alteration with seawater, so they accurately record Earth’s recent
magnetic field history.
The lavas found at mid-ocean ridges
erupt more often, are better preserved, and are less disturbed than
those found on continents. These attributes give us an opportunity to
get a more “brick-by-brick” understanding of how the surface of Earth
is paved.
For example, does lava erupt at a central “crack
of doom” atop a ridge, and then spill over and cascade downhill to bury
older lava flows (as portrayed in the right side of the top diagram)?
Does it burst from several outlying magma chambers through narrow
channels to create discrete patches of seafloor (as in the left side of
the top diagram)? Or are both processes at work?
Answers to
these questions will delineate the width of the zone along mid-ocean
ridges in which new seafloor crust is formed. It will also reveal how
frequently and periodically ridge eruptions take place—which, in turn,
will tell us about the heartbeat of magma movements deeper in the earth.
Posted: March 22, 2004 [top] |
