Where on Earth...? MISR Mystery Image Quiz #16
Here's another chance to play geographical detective! These natural-color images were acquired by the
Multi-angle Imaging SpectroRadiometer (MISR) during October and November 2003. The two images
represent about 310 kilometers x 141 kilometers (left panel) and 310 x 184 kilometers (right panel). This
mystery concerns two coral atoll ecosystems located in different parts of the globe.
These images depict atoll groups of the Maldives in the Indian Ocean (left) and Tuamotu, French Polynesia
(right). Answers to the 4 questions are below.
1. Three of these four statements about the geomorphology of the coral reefs are true. Which one is false?
(A) Their modern shape has been influenced by sea level changes over geologic
(B) The reefs have grown over millions of years atop ancient volcanic mountain
(C) Unless there is too much sediment present in the water, some of the
reefs can grow at depths of up to 170 meters.
(D) The reefs tend to grow in density and thickness where the water flow
is greatest due to prevailing ocean currents.
Answer: C is false
Today’s coral reefs were formed over the last 500 million years and are influenced by climactic and geologic
processes. Coral reefs can typically only accumulate at a rate of several millimeters per year. If the Earth’s crust is subsiding or sea level rising slowly enough, reefs are able to construct their protective calcium carbonate encasements at a rate fast enough to keep abreast of the diminishing coastline. Volcanoes that form near mid ocean spreading centers gradually subside as the crust on which they stand cools and moves away, and the
coral reefs that fringe their edges grow upward as the volcanic rock sinks, leaving behind coral atolls such as
those pictured here. Reef-building corals benefit from a symbiotic relationship with photosynthetic algae, and
since the algae rely on sunlight, reefs do not grow at depths greater than 70 meters below the ocean’s surface.
Coral reefs are also restricted to the relatively nutrient-poor tropical oceans, where their growth is affected by
prevailing ocean currents. The currents agitate the water and provide the coral and their algal partners
(zooxanthellae) with more calcium, nutrients, and food particles. This stimulates faster coral growth on one side
of the outer reef, so that the reefs become wider and denser over time near the locations where the water flow
is the greatest.
2. Three of the following four statements accurately describe coral atoll ecosystems. Which one does not?
(A) Other living organisms besides coral help to build the atolls
through calcium carbonate production.
(B) Symbiotic zooxanthellae provide reef corals with the calcium and
nutrients they need to grow.
(C) Atolls can be very open, entirely closed, or have a number of entry
points by which a lagoon area is open to the ocean.
(D) The diversity and abundance of corals is greater along the protected
inner atoll and lagoon areas than along the outer reef slopes.
Answer: D is false
Various types of corraline algae help to produce calcium carbonate, including zooxanthellae which live in the
tissues of modern reef-building corals. Zooxanthellae are intimately related to the rate of reef building in that
they help these coral to produce calcium carbonate encasements faster during the day when photosynthesis
occurs. Hard corals obtain calcium from the seawater and carbon dioxide from cell respiration, and bring these
products into the internal tissues of the coral polyp. The zooxanthellae provide nutrients for the coral community
via photosynthesis and assist in calcium carbonate formation by exchanging the products of photosynthesis and
cell respiration. Atoll shapes are extremely varied, and range from the classic closed ring and inner lagoon, to a
completely open reef rim. The diversity and abundance of corals is greater along the outer atoll edges, where
the flow of relatively nutrient-rich water is greater.
3. Three of these four statements about the atolls of the left-hand
image are true. Which one is false?
(A) The many small saucer-shaped reef structures within the lagoonal
areas make this reef system different from all others.
(B) The word "atoll" is a term that comes from a language native to this
region, and has been expanded for use worldwide.
(C) The two largest cities in these atolls are home to about 8000 people
(D) At one atoll, construction of an artificial reef began in November 2001.
Answer: C is false
AThe many small saucer-shaped closed oceanic atolls that occur within the larger lagoonal areas are called
"faros" and are part of the nation of the Maldives. These atolls grew upon a submerged mountain range named
the Laccadives Chagos Ridge, which runs north-south along 2000 kilometers of the Indian Ocean. While "faro"
is simply the Maldives name for a reef with a central enclosed lagoon or depression, the processes that created
faros in the Maldives are different to the processes that created atolls worldwide. The native language of the
region is Dhivehi and the word "atoll" comes from their word, atholhu. The capital city, Male, only covers
about 2 square kilometers, but is home to about 70,000 people. The maximum elevation of the atolls is only 5
meters above sea level, and the very existence of these atolls and their inhabitants depends on healthy coral
reefs. In the last few decades the reefs have been damaged by higher than average sea-surface temperatures
and unsustainable resource management practices. One method used to restore damaged reef structures is to
construct artificial reefs and artificially stimulate the coral growth (e.g., the Vabbinfaru Lotus Structure on North Male Atoll).
4. Three of these four statements about the atolls of the right-hand
image are true. Which one is false?
(A) In this region the prevailing ocean current, or swell, comes from
(B) The chemical character of the inner lagoons is not significantly
influenced by the size of the openings in the reef rims.
(C) Within the image area there is a designated biosphere reserve and
breeding grounds for at least 3 species of sea turtle.
(D) The most extensive freshwater is found at the small, round, green
atoll that appears along the left-hand image edge.
Answer: either A or B accepted as false
The right-hand image includes (in whole or part) 11 of the 77 atolls that make up the Archipelago of Tuamotu in
French Polynesia. The Tuamotus are located within the ocean current known as the South Pacific Gyre, and the
surface hydrological characteristics here are particularly stable, warm, clear and salty. Although the dominant
wind and ocean currents are from the east, the dominant wave direction (swell) is from the south. Since wave
direction (or swell) and ocean current (or circulation) are different attributes, statement (A) is accepted as false. Statement (B) is also false since the openings of the atoll rims enable, to a greater or lesser degree, water exchange between the lagoon and the open sea, and thereby influence the chemical character of the lagoon
waters. Taiaro Atoll (the small circular atoll at the right-hand image edge) was designated as a biosphere
reserve in 1977. There are three turtle species (green, hawksbill and leatherback) at Taiaro. The most extensive
fresh-to-brackish-water marshes in the Tuamotus are thought to be at Niau, the small green atoll at the
left-hand image edge.
45 people from all over the world sent in responses before the deadline.
Individuals who answered all four questions correctly are listed below
in the order in which responses were received. The prize winners are
indicated by an asterisk.
1. Regina Seiler, Lucerne, Switzerland*
2. Matthias Knirr, Tuebingen, Germany*
3. JL, Malaysia*
4. Yu Wei, Xiamen, China
5. Rachel Mapson, London, UK
6. Rob Hale, Norman, OK, USA
7. Jim Armstrong, Potter Valley, CA, USA
8. Christoph Janda, Vienna, Austria
9. Neki Frasheri, Tirana, Albania
10. Thomas Franz
11. Ivan P Anderson, Ditton, Kent, England
12. Steve Gaiser, Sierra Madre, CA, USA
MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The Terra satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of the California Institute of Technology.
Credit: Image credit: NASA/GSFC/LaRC/JPL, MISR Team.
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Text acknowledgment: Clare Averill MISR Team, Geographic Interpretation and Science Outreach, c/o Raytheon, Australia.