Blog#1

Map of Belize

Great Blue Hole, Lighthouse Reef Atoll, Belize (Photographed by David Doubilet, National Geographic)

Believed to be the largest feature of its kind in the world, the Great Blue Hole is located in Belize’s Lighthouse Reef Atoll, a large coral reef, or biologic sedimentary rock formed by the accumulation of organic material (i.e., coral).  The bedrock in the region is limestone, a chemical sedimentary rock formed by calcite.  The Great Blue Hole is a giant sea-hole, or a sinkhole found in the ocean, measuring 984 feet in diameter and 407 feet deep.  The interesting feature of this site is an underwater cave.  

Stalactites, Half Moon CayeGreat Blue Hole, Lighthouse Reef Atoll, Belize

 

The presence of stalactites, stalagmites and columns shows the cave was once above sea level.  As sea levels rose, the caverns grew, and the roof weakened and collapsed, forming the sinkhole.  At some point, Lighthouse Reef Atoll experienced some tectonic movement along the continental/oceanic transform fault, as the North American Plate and Caribbean Plate moved against each other, and shifted at an angle of approximately 12 degrees, shown by the angle of the stalactites found in the sea cave, which only form in a perpendicular pattern due to gravity.  

 Sources:

http://ga.water.usgs.gov/edu/earthgwsinkholes.html

http://www.physicalgeography.net/physgeoglos/s.html

http://ambergriscaye.com/pages/town/blueholebottom.html

http://geology.com/nasa/great-blue-hole/

http://photography.nationalgeographic.com/photography/photos/pod-best-of-2007/blue-hole-belize-reef_pod_image.html

http://soundwaves.usgs.gov/2001/06/

http://ga.water.usgs.gov/edu/earthgwsinkholes.html

http://www.ecophotoexplorers.com/images/belizemap.gif

http://monkeybusinesstours.com/diving-blue-hole-half-moon-caye-lighthouse-reef-full-day

http://en.wikipedia.org/wiki/File:Tectonic_plates_boundaries_detailed-en.svg

http://en.wikipedia.org/wiki/Great_Blue_Hole

 

Blog#2

The Great Blue Hole is a sinkhole that was caused by the weathering of limestone dissolved by subsurface drainage.  The rock found in the cave of the Great Blue Hole is limestone karst.  It is believed that at the end of the Great Ice Age, as glaciers melted, sea levels rose, forcing more groundwater through the open spaces in the limestone karst.  As the rock dissolved, caverns developed underground.  The stalactites, stalagmites and columns were formed by calcite deposits from the chemical dissolution of the limestone.  

Illustration of how Caves/Caverns form

Illustration of how Sinkhole forms

 

The depth and isolation of the sea floor at the bottom of the Great Blue Hole has resulted in an interesting soil profile.  In 1997, divers took core samples from the bottom of the Great Blue Hole.  The 2-foot long core samples showed a long history of sedimentary layers. There is no oxygen near the bottom of the hole, and high levels of hydrogen sulfide prevent bottom dwellers from disturbing the sediment.  The samples showed layers of atmospheric fallout such as pollen, spores, as well as mercury, and arsenic.  The clay-rich sedimentary layers resulted in a gleization process, reducing the porosity of the soil, which caused the slow deposition and accumulation of organic material.  

Diver exploring the reef in the Great Blue Hole, approx. 60-80 feet underwater

 

Sources:

http://soundwaves.usgs.gov/2001/06/

http://www.news-world.us/pics/tag/coral-reef/

http://virtual.yosemite.cc.ca.us/chudelson/online/soils.htm

http://www.tulane.edu/~sanelson/geol111/groundwater.htm