Mountains in the sea

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Mountains in the sea
Tony Watts University of Oxford
A perspective view of predicted bathymetry and
ship track data in the south-central Pacific
Ocean.

• Collaborators David Sandwell (Scripps), Päl
  Wessel (Hawaii), Walter Smith (NOAA), Christine
  Peirce (Durham), Ingo Grevemeyer (IFM-GEOMAR)
• Shell Public Lecture Series, Geological Society
  of London, 11 November 2009

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Outline of the Talk

• The Challenger expedition
• Ocean islands Atolls and Guyots
• Seamounts - Number, distribution and age
• Growth and decay
• Seamounts, the environment and society

H. M. S. CHALLENGER Shortening sail to sound
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The H.M.S. CHALLENGER expedition (1872-76) and
John Murrays 1911 map of the Atlantic Ocean floor

• Sounding and trawling on the CHALLENGER

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The bottom of the Atlantic Ocean along the 40o N
parallel according to Murray (1911)

• Smooth and featureless, except around the Azores

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The Azores

• A group of 0-8 Ma volcanoes that
• rise up gt7 km above the regional
• depth of the surrounding seafloor.
• Global Volcanism Program
• Together make up 9 of the
• worlds 1770 ocean islands.

Pico (2351 m)
San Miguel
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Darwins (1842) theory
Most inactive ocean islands are volcanic in
origin and have coral reefs.
Lagoon with barrier reef
Volcanic island with lagoon and barrier reef
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Ocean island with a barrier reef, lagoon and
(central) volcano
Moorea (Society Islands) 1.5 Ma
Bora Bora (Society Islands) 3.3 Ma
Barrier reef and lagoon Atoll
Aratika (Tuamotus - 42-47 Ma)

• Atolls form by the upward growth of a coral reef
  on a sinking submarine volcano

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Origin of atolls

• The seafloor increases its depth because the
  oceanic crust is created at a mid-ocean ridge and
  gets cooler and, hence, denser with age.

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Harry Hess Discoverer of the first Mountain in
the Sea?
Hess found a large flat-topped feature in the
central Pacific Ocean during WW II which he
called a guyot
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Origin of guyots
Hess suggested that guyots were once volcanic
islands that had been flattened by wave action.
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Mountains in the sea

• Ocean Islands like Hawaii are as tall as Everest
  and as wide as the Alps..

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So, what do we actually find on the ocean floor?

• Ocean islands, atolls and guyots suggest that
  the seafloor should be littered with volcanoes
  some of which are growing upwards and have not
  yet made it to the surface and others which were
  once at the surface and have since sunk below it.

Ocean Island
Seamounts
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Technologies to explore the ocean floor
Ships
Satellites
e.g. multibeam echo-sounder (swath)
Radar altimeter

• 5000 research cruises (400 ship yrs), 6
  satellite altimeter missions (60 satellite yrs)

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Global Topography
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Seamounts of the west-central Pacific Ocean
Mid-Pacific Mountains
Mariana Trench
4 km
100 km
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How many seamounts are there?
Height above the regional seafloor depth (Ben
Nevis 1344 m)
Height 100-750 m 185,901 Seamount census
219,485
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What are seamounts made of?

• Seamounts are made mostly of basaltic lavas,
  volcanoclastic sediments and rock debris (e.g.
  gabbro ejecta). Most are inactive.

Vesicular basalt, Tenerife
Brimstone Pit (550 m), Mariana Arc
Some are active. They are spectacular sights that
involve the jettisoning of rocks, sulfurous
clouds, and volcanic gases.
Pillow lavas, Loihi, Hawaii
Iron oxide crust, Bounty
NOAA - Embley et al. (2006)
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The structure of seamounts(from deep seismic
sounding)
Tenerife Canary islands
Sea-Level
1 sec
Seafloor 3 km
Buried edge of the edifice of the Tenerife volcano
9 km
Top of Oceanic Crust
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Seamount shapes
Marquesas Islands
Line Islands
Marquesas Fracture Zone
Tuamotu Islands
Society Islands
Austral Fracture Zone
Tubai Islands

• Seamount Catalog - http//earthref.org/SBN/

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How do seamounts form?

• The hotspot hypothesis

Hotspot
Hawaiian-Emperor Seamount Chain
86 2 mm/yr
Midway
Kauai
There are 4 main long-lived (gt70 Myr) hotspots in
the Pacific, 3 of which can be backtracked to an
oceanic plateau
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What about the many other seamounts?The crack
hypothesis
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Capturing the growth of a seamount
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Collapse and DecayThe Taney Seamounts, off
Monterey Bay, CA
NW
SE
Clague et al. (2000)
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How old are seamounts?
There are only 350 sample sites!
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What happens to a seamount when it reaches a
trench?
Overriding North American Plate
Trench

Subducting Cocos Plate
Von Huene et al (2000)
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Seamounts may act as barriers during
earthquakes.e.g. the June 2001 Southern Peru,
Mw 8.4 earthquake

• Largest earthquake since 1961
• 75 killed, including 26 by
• a tsunami. 2,687 injured.

Basilica Catedral, Arequipa

• A line of seamounts are entering the trench

Robinson et al. (2006)
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Seamounts scatter tsunami waves and may focus
their run-up along certain segments of a
coastline
Andreanof Island 1996 earthquake
Cape Mendocino
Kodiak-Bowie Seamounts
Hawaiian Ridge
Musician Seamounts
Mofjeld et al. (2004)
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Seamounts are biological hotspotsCatch or
sightings Azores
Big-eye tuna
Dolphin
Corys shearwater (predator)
Skipjack tuna
Vents and turbulence on seamounts provide
nutrients and some of our favourite fish are
found on seamounts

• Pitcher et al. (in press, 2009, Special Issue of
  Oceanography)

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But, there is the (sad) tale of the Graveyard
Seamounts..
Fished
Unfished
Morgue
Gothic
Scroll
Diabolical
Clark Rowden (2009)

• So, we need to manage and protect seamounts

Global census on marine life on seamounts -
http//censeam.niwa.co.nz/
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What limits our exploration of seamounts?Field
data!!
Existing ship track coverage in the South-Central
Pacific Ocean

• We know the surface of the Moon, Mars - and now
  Mercury - better than we do the seafloor of the
  South Pacific Ocean!

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How many seamounts are there that remain to be
discovered?
Satellites have found all the large (gt2
km) seamounts, but few of the small ones
Ships have found most of the small (lt2
km) seamounts and many of the large ones

• Data sources Wessel (2001), Hillier Watts
  (2006)

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Why is it important that we find all the
seamounts?

• On 8 January 2005 the USS San Francisco, a
  nuclear-powered attack submarine, crashed into a
  2 km high uncharted seamount

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What next?We need.

• More shipboard swath bathymetry, sample and deep
  seismic data
• The European Space Agency CRYOSAT satellite
  altimeter Launch - later this month..
• Better collaboration between geologists,
  oceanographers and biologists
• More seamount management and protection
• Public awareness and participation in finding
  seamounts?