In a few places on Earth, blocks of oceanic crust (called ophiolites) have been thrust onto the continents, giving scientists the unusual chance to get a firsthand look at rock formations that were once beneath the seafloor. The largest ophiolite is in Oman near the Persian Gulf. (Photo by Peter Kelemen, Woods Hole Oceanographic Institution )
WHOI scientists Peter Kelemen (top arrow) and Greg Hirth (about 50 meters directly below) are walking on rocks that once were in the upper mantle beneath the seafloor. In this photomosaic of a mountainside in Oman (and in photo on above), light-colored rocks (dunite) are ancient channels through which melt once flowed through the mantle. (Photo by Mike Braun, WHOI)
A computer model simulating the location of channels formed by localized melt flow in the mantle provides this picture: The melt dissolves minerals in rock minerals to form small porous channels that subsequently coalesce into larger ones. Similar flow patterns may be at work in other natural systems. (Courtesy of Marc Spiegelman/Lamont-Doherty)
By studying how water flows on a beach, scientists can make parallels with the flow of magma beneath the seafloor. On a beach, small water channels flowing downhill erode sand in front of them and coalesce into one larger channel. But when the slope decreases downstream, the flow slows down. Sand grains that were carried in suspension begin to become deposited, creating a barrier that block the flow. Smaller channels begin to diverge again. (Photo by Peter Kelemen)
Peter Kelemen, Senior Scientist Geology and Geophysics Department Woods Hole Oceanographic Institution
Most people know that oceans cover about 70 percent of Earth’s
surface. Fewer people realize that the crust beneath oceans and
continents is fundamentally different. Why this is so remains a mystery
that scientists are still trying to solve.
Oceanic crust is
generally composed of dark-colored rocks called basalt and gabbro. It
is thinner and denser than continental crust, which is made of
light-colored rocks called andesite and granite. The low density of
continental crust causes it to “float” high atop the viscous mantle,
forming dry land. Conversely, dense oceanic crust does not “float” as
high—forming lower-lying ocean basins. As oceanic crust cools, it
becomes denser and ultimately sinks back into the mantle under its own
weight after about 200 million years.
Earth’s continental
crust, on the other hand, is up to 4 billion years old, and it is
thought to be the product of geologic recycling processes far more
complicated than those that create ocean crust. If we can decode and
read the relatively simple story of how oceanic crust is formed, we may
someday be able to decipher the more complex record of how the
continents developed.
Sounding out seafloor structure
Because most oceanic crust is hidden from view beneath many kilometers
of water, our research must be conducted “remotely,” often using
acoustic techniques. Sound—emanating from an earthquake, an explosion,
or a relatively benign source known as an airgun—travels through
different rocks at different speeds. Geophysicists infer the basic
geologic structure of underlying rocks by measuring the time it takes
for sound to travel from one source to many different receivers, or
from many sources to a single receiver.
In the oceans, this
technique has yielded a simple picture of a basaltic, layered crust
about 7 kilometers (4.3 miles) thick, underlain by the mantle. Rock
samples obtained via dredging, submersible operations, and drilling
confirm that the top of the oceanic crust, where it is not obscured by
sediments, is composed of basaltic lava that originates in the mantle.
At the dawn of the modern theory of plate tectonics in the 1960s,
geologists and geophysicists realized that the entire oceanic crust was
created from basaltic lava along linear chains of seafloor volcanoes
known as mid-ocean ridges, or spreading ridges. Seafloor spreading
carries older oceanic crust away from the ridges over tens of millions
of years, until it cools, becomes denser, and “falls” back into the
mantle in areas known as subduction zones.
Seafloor clues in the desert
In a few places on Earth, blocks of oceanic crust, called “ophiolites,”
have been thrust, relatively intact, onto the continents during
collisions between tectonic plates. Tilting and subsequent erosion
allow scientists to walk through a section that once extended 25
kilometers (15 miles) into Earth’s interior. The largest and best
exposed of these, the Oman ophiolite near the Persian Gulf, comprises
about ten blocks that together cover roughly the same area as
Massachusetts.
The great extent of these ophiolites, once
deep beneath the seafloor but now exposed, provides a comprehensive
view of the internal geometry of oceanic plates that is unmatched by
any sampling or imaging technique at sea. Like pot shards covered with
hieroglyphics, ophiolites open a window onto an ancient, largely
vanished world, and provide a rare avenue for systematic investigation.
In the late 1960s and early 1970s, geologists and geophysicists
observed similarities between the layered structure of oceanic crust,
as interpreted from sound velocities, and the layering in ophiolites. A
thin, upper layer in oceanic crust (with low sound velocities)
corresponds to a layer of sediments and lava flows in ophiolites. A
deeper layer (with faster sound velocities) corresponds to an ophiolite
layer of “gabbro,” which formed when molten basalt solidified beneath
Earth’s surface. In both oceanic crust and ophiolites, the gabbro layer
is underlain by the mantle, which extends thousands of kilometers down
to Earth’s core.
A striking feature of well-exposed
ophiolites is a continuous layer of “sheeted dikes,” which lies between
the lava and the gabbro. These are tabular rock formations, about a
meter wide, created by periodic bursts of molten rock. The dikes stand
side-by-side, like soldiers in formation, each dike adjacent to
neighboring dikes, or sometimes leaning or intruding into them.
This recurring structural pattern occurs because all oceanic crust is
newly created at spreading mid-ocean ridges on a kind of continuous
conveyor belt: Each dike, in a simple view, forms directly at the
center of a ridge. It then spreads out from the ridge center, as
another dike forms behind it, in an ongoing process that creates the
continuous layer observed in ophiolites. Nothing like that happens in
continental crust, where new dikes more randomly intrude older rock.
Going with the flow
During the 1970s and 1980s, geophysicists and geologists strove to
understand how basaltic lava forms beneath spreading ridges. They
theorized that because the oceanic plates pull apart at the surface,
new material must rise to fill the gap. As the material rises, the
pressure that helps keep it solid decreases. This allows hot mantle
rocks to partially melt and produce basaltic liquid. This so-called
“melt” is less dense than surrounding solids, and so it buoyantly rises
to the surface to form the crust.
However, this theory raises
as many questions as it answers. From lava compositions, we know that
from an enormous volume of mantle rock, only small amounts of rock
partially melt to create oceanic crust. Melt forms in micron-size pores
along the boundaries of innumerable crystal grains across a mantle
region that is 100 to 200 kilometers wide and 100 kilometers deep. From
this vast region, however, the melt somehow is focused into only a
5-kilometer-wide zone at the spreading ridge. How is lava channeled
from tiny pores in a broad region of melting into a narrow region where
it forms new oceanic crust topped by massive lava flows?
My
colleagues in exploring this mystery, working in various combinations,
have included Greg Hirth, Nobu Shimizu, and Jack Whitehead at Woods
Hole Oceanographic Institution (WHOI), Marc Spiegelman of the
Lamont-Doherty Earth Observatory, French geologists Adolphe Nicolas and
Françoise Boudier, Massachusetts Institute of Technology graduate
student Vincent Salters, and MIT/WHOI Joint Program students Einat
Aharonov, Mike Braun, Ken Koga, and Jun Kornaga. Our research has been
funded by the U.S. National Science Foundation, the WHOI Interdisciplinary and Independent Study Award program, and the Adams Chair at WHOI.
We have shown that melt travels through the mantle in porous channels,
similar to channels filled with gravel that provide permeable pathways
through clay-rich soil. Melt rising through the hot mantle can
partially dissolve minerals around them and gradually enlarge the pores
along the boundaries between individual crystal grains. This, in turn,
creates a favorable pathway through which more melt can flow—in a
positive feedback loop that spontaneously creates channels that focus
the flow.
Small channels formed in this fashion coalesce to
form larger channels, in a network analogous to a river drainage
system. The number and size of melt flow channels we observe in the
mantle section of ophiolites supports these theories.
Melt lenses and periodic bursts
New questions arose. If melt flows through the mantle in micron-scale
pores along the boundaries of crystal grains, where does it accumulate
to form massive lava flows at spreading ridges? And, if porous flow is
a continuous, gradual process, what causes the periodic bursts of
molten rock that create new dikes?
Once again, the Oman
ophiolite provided clues. Embedded in the shallowest mantle rocks,
Nicolas and Boudier found small formations of gabbro, called sills.
Chemical analyses of these sills indicated that they crystallized from
the same melt that formed gabbro, sheeted dikes, and lava flows in the
crust. In addition, the gabbro, dikes, and lava flows all had an
identical, distinctive pattern of alternating bands of dark and light
minerals.
It seemed to us that the entire gabbro layer in the
Oman ophiolite crust, from uppermost mantle to the surface, could have
formed when melt material periodically collected in relatively small
pools that subsequently crystallized into solid “melt lenses.” Over
time, a myriad of these melt lenses accumulates—embedded within each
other and stacked atop each other or side by side—to produce gabbro’s
rocky, banded fabric.
Clogged pores build up pressure
Why would melt lenses first appear in the uppermost mantle, immediately
beneath the base of the crust? We propose that such lenses form where
melt, approaching the seafloor, begins to cool. Melt rising through the
hot mantle can dissolve minerals surrounding it to create pore spaces,
but cooling melt will begin to crystallize and clog pores.
Two scenarios are possible: When the supply of melt from below is low,
conduits become narrower. The melt is forced outward around impermeable
barriers, migrating via diffuse porous flow along crystal grain
boundaries throughout surrounding rock.
But when melt supply
is large, as it is immediately beneath a spreading ridge, buoyant melt
accumulates beneath impermeable barriers and creates excess pressure.
Eventually, the melt bursts through the barriers and creates a
melt-filled fracture that intrudes the overlying crust. If the fracture
propagated high enough in the crust, it would form a sheeted dike, and
if it reached even higher, it would spill out onto the seafloor and
feed a lava flow.
In this cycle of buildup and release,
minerals alternately crystallize and melt under conditions of higher
and lower pressure. At relatively high pressure, much less of the
light-colored mineral (plagioclase) is formed, compared to
darker-colored minerals. At lower pressure, the proportion of
plagioclase is larger. Thus, periodic pressure changes result in the
light-and-dark banding observed in ophiolite gabbros.
Paths of most resistance
Working from geological evidence in ophiolites, together with physical
and chemical theory, we hypothesize that there are two distinct ways to
transport melt that forms oceanic crust. Within the melting region in
the mantle, melt can dissolve minerals and create additional pore
space. As a result, continuous, high-porosity conduits form a
coalescing drainage network that focuses melt transport to the
spreading ridge.
At shallow levels beneath the ridge, cooling
melt begins to crystallize, clogging pore space along crystal grain
boundaries. As a result, flow becomes diffuse, melt accumulates beneath
impermeable barriers. Pressure builds up until the melt periodically
bursts through overlying barriers, and melt-filled fractures are
injected into overlying rocks to feed dikes and lava flows. Together,
these processes form a highly organized system that consistently
produces new oceanic crust with a regular structure along spreading
ridges.
In our ongoing research, we are more rigorously
testing theories about how porous conduits form in the mantle. We seek
to understand in more detail how melt lenses form beneath spreading
ridges. And we want to figure out the factors that determine why and
when diking and eruption events occur.
Exploring a universal pattern of
fluid flow There are intriguing parallels between the mechanisms that lead to the creation of seafloor and to erosion on Earth’s surface.
Consider water flowing over a sandy surface. Where the slope is steep
enough (but not too steep), water begins to move sand grains downward
and form channels. As the channels grow, water flows faster, leading to
more vigorous erosion of sand at the leading edge of the flow. An
analogous process occurs beneath the seafloor, as rising, hot melt
dissolves minerals in rocks to form porous channels.
When the
slope decreases downstream in an erosional system, water begins to
deposit sand grains that were carried in suspension. The deposited
grains begin to construct barriers that block flow and force it to
diverge away from the main channel. Water accumulates behind these
barriers to form temporary lakes. These lakes periodically overflow the
old channel and create transient, new pathways, which in turn are
clogged and abandoned. A delta or alluvial fan forms.
Analogous processes occur beneath the seafloor as rising melt cools,
precipitates crystals that block pore spaces, causes flow to diverge
and accumulate, and periodically bursts through impermeable barriers to
form dikes and fractures.
Optimizing fluid flow
What lies behind these apparent fundamental similarities between fluid
transport during erosion on Earth’s surface and melt transport in the
mantle?
Basically, where energy is available for fluid to
create new pathways—via physical erosion or chemical
dissolution—drainage networks evolve from relatively inefficient, slow
moving, diffuse flow to faster, focused, steady flow in well-defined
channels. Where energy is lost—via a decrease in slope angle in erosion
or a decrease in temperature of melt—the drainage network becomes
inefficient and disorganized, with quick shifts in flow rate and
location.
Scientists working on the evolution of river
drainage systems propose that erosion tends to produce an “optimal”
drainage network that maximizes flow velocity and minimizes loss of
energy via friction. This is an intriguing idea, offering the vision of
a systematic, “thermodynamic” theory of drainage morphology. (It is
also controversial theory, since river drainages inherit much of their
complicated structure from the prior geologic history of a watershed.)
It is difficult to use ophiolites to explore a themodynamic theory of
drainage morphology for mantle melt transport mechanisms—because
ophiolites constitute a “frozen” system. So I began to look elsewhere
for an active fluid transport system that developed channels within an
initially diffuse flow pattern.
Finally, I realized that
erosional channels form twice a day as the tide falls on beaches all
over Cape Cod. Cautiously, Dan Rothman, a geophysicist at MIT, and I
are learning about beach erosion and making observations on channel
formation. We hope to determine whether the evolving channel network
gradually approaches an “optimal” geometry that allows water to flow
over the beach surface with minimal frictional energy loss.
