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Marie Tharp and the Mid-Atlantic Ridge,
9" x 12" linocut on Japanese paper, by Ele Willoughby, 2015 |
Happy birthday to
American geologist and oceanographic cartographer Marie Tharp (July 30, 1920- August 23, 2006), whose pioneering, thorough and complete ocean floor maps made with her partner in science
Bruce Heezen
revealed the Mid-Atlantic Ridge. The mid-ocean ridge itself, based on
their 1957 physiographic map, is illustrated behind her, along with the
sort of echo sounder or precision depth recorder tracks she used, in
front of her.
Tharp had struggled to find the the right university major; she wanted
something she could do, and enjoy, but there were not many options for
women in her day. More opportunities opened up during WWII and she took
the chance to return to school and study geology and then math.
Looking for something challenging (but not tedious) she contacted
Maurice 'Doc' Ewing
at Lamont-Doherty Earth Observatory at Columbia, who hired her to
draft data, including the thousands of echo sounder profiles they were
gathering. Women were still not allowed to participate in research
cruises, but they could work with the data. Before long, Heezen came to
Lamont and required so much drafting work that Tharp worked exclusively
with him.
Scientists once imagined the ocean floor as a largely featureless
plain. Early depth measurements were taken with lead weights (such as
canon balls) and a whole lot of rope! As early as the late 19th
century, such laboriously collected datasets began to hint at a broad
rise in the centre of the Atlantic. By the mid 20th century, there was a
push to try and map these submarine mountains.
Tharp spent months painstakingly "plotting, drawing, checking,
correcting, redrawing and rechecking" profiles of the North Atlantic.
The ship tracks across the Atlantic were a sparse web, but when Tharp
compared half a dozen more or less parallel transects she noticed no
only the general similarities of the ridge, but a V-shaped notch in the
centre of all the profiles. She suspected they coincided because they
indicated a rift valley all along the ridge crest. The early ideas
about plate tectonics or the "continental drift" theory were still
quite controversial and unpopular. Heezen dismissed Tharp's observation
as "girl talk" for looking too much like continent drift - as in fact
it was indeed a vital piece of the plate tectonics puzzle. We now know
that surface of the Earth is itself a jigsaw puzzle of pieces known as
tectonic plates, jostling one another at a stately, geological pace.
Mid-ocean ridges are underwater volcanic mountain chains which roughly
bisect all ocean basins. They are all cut by a rift valley which is the
spreading centre. These rifts are where new crust is born, pushing
upwards and outward. This drives the two plates on either side slowly
apart over geological time. On our own timescales of everyday life, we
notice the bumps in this slow ride: the sporadic earthquakes, rather
than the slow creep (though today, we can meticulously measure both).
Tharp believed the rift was real though her contour maps hadn't
convinced Heezen. In 1952, they began working on physiographic maps,
which would show seafloor topography as if you were flying just above
it, and the water were drained away. These had the advantage of really
giving a sense of the variety of geology, from plains to mountains,
seamounts to trenches. Also, unlike detailed contour maps,
physiographic maps were not US Navy classified information, so Tharp
and Heezen would be able to publish what they produced. Further, they
were beginning to gather much better precision depth recorder data,
which revealed far more features, along with better navigation to plot
ships' positions along tracks more accurately. A second project in
their research group involved plotting earthquakes, and Heezen insisted
they work at the same scale. Heezen then noticed that ocean earthquake
epicentre data also formed long lines - and in fact, when one map was
placed above the other on a light table they found the earthquakes
formed near continous lines along the Mid-Atlantic ridge right where
Tharp had indicated there was a rift valley. Using the earthquake data
to extrapolate and plot the rift position where there was no seafloor
sounding data, they found that the rift extend landward into the Rift
Valley of East Africa - a well-known, easy to observe terrestrial rift
valley. Heezen was then convinced. They had discovered a worldwide
mid-ocean ridge system, tens of thousands of kilometres long. Tharp was
able to mine existing data to show the Mid-Atlantic Ridge extended to
the south Atlantic and found similar features in other oceans. These
all similarly lined up neatly with earthquake epicentres. Ewing and
Heezen announced their findings in 1956. In 1957 Tharp and Heezen
published their North Atlantic physiographic map; I've shown my version
of their map behind her. The ridge snakes from top to bottom (north to
south-south-west), above and almost mimicking the line of her arm.
They continued this work, extending to other oceans over the next 25
years, ultimately producing detailed physiographic maps of the world
oceans. Their pioneering work mapping the oceanic plate boundaries, and
showing their clear alignment with seismic data helped fuel the
revolution in geology and geophysics, the paradigm shift of plate
tectonics.
Tharp's work was largely in the background during her university
career, though she won a number of prizes during her retirement and has
continued to gain posthumous recognition for the importance of her
work and observations. I was very pleased to see her recognized
recently in Neil DeGrasse Tyson's
Cosmos
reboot. I want to bring her incredible insight and excellent work to a
wider audience as both artist and marine geophysicist myself.
(cross posted from
the minouette blog)
