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Roger Weller, geology instructor

wellerr@cochise.edu

Diamondhead, Oahu, Hawaii
by Svetlana Melton
Physical Geology
Fall 2011
  

Diamondhead Crater, Hawaii
 

 

            This semester we studied volcanoes, and I was fortunate to be able to visit Hawaii during this time. It was an amazing feeling to see a volcano from my hotel room, to swim in a crater (Hanauma Bay), and to hike up to the summit of Diamond Head, which are all dormant shield volcanoes.
 

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(View from my hotel room)

 

Hawaii is the only state in the US which is built entirely of volcanic materials. Over the past 70 million years, the combined processes of magma formation, volcanic eruption, and the continual movement of the Pacific plate over the stationary Hawaiian “hot-spot” have left a long trail of volcanoes across the Pacific Ocean floor. Many of these volcanoes formed islands that have either eroded and subsided below sea level, or never reached sea level at all. Each Hawaiian island is made up of one or more massive shield volcanoes, which first erupted on the ocean floor, emerging above the ocean’s surface after countless eruptions.
 

The most famous volcanic crater in the world is Diamond Head (in Hawaiian “Le’ahi”), located on the southeast coast of O’ahu at the end of Waikiki, overlooking the Pacific Ocean.

 

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(View of Diamond Head from Waikiki Beach)

 

The creation of O’ahu began around 2.5 to 3 million years ago with volcanic eruptions from two shield volcanoes. A period of extensive erosion followed, leaving the Ko’olau and Wai’anae Mountain Ranges as the remnants of the two volcanoes. Then, after two million years of volcanic inactivity, the southeastern end of the Ko’olau Mountain Range erupted. The eruptions occurred near the ocean where the magma was broken down into ash and fine particles by the water and steam. Blown into the air, these particles settled to be cemented together into a rock called tuff, which built tuff cones such as Le’ahi. The hydro magmatic explosions also ripped through 200,000 year old coral reefs. As a result, large pieces of coral and basalt are mixed in the tuff and magmatic debris of the cone. The eruptions occurred about 300,000 years ago in a very short period of time; from days to perhaps a month, as suggested by the symmetry of the cone.


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 The broad, saucer-shaped crater covers 350 acres, its width being greater than its height.
 

(Crater interior)

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The southwest rim is highest as winds blew ash in this direction during the eruption.
 

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(Southwest rim of crater, highest peaks)
 

Since then, the slopes of the crater have been eroded and weathered by wind, rain, and pounding waves. A coral reef now protects the seaward slopes of the crater.
 

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(Eroded seaward slopes and protective coral reef)
 

The crater is 3,520 feet in diameter, with a 760 foot high summit. The type of eruption that built Diamond Head tends to be monogenetic, and geologists believe that it will not erupt again.
 

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(Crater interior as viewed from summit)
 

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The Hawaiian name Le’ahi comes from lae (“ridge”) and ahi (“tuna”) because the shape of the ridgeline resembles the shape of a tuna’s dorsal fin. The English name was given to the crater by British sailors and western traders in the 1800s. The calcite crystals embedded in the rock glimmered in the sunlight and they mistook them for diamonds. Thus the name Diamond Head came into common usage.
 

When the US annexed Hawaii in 1898, harbor defense became a main responsibility, and the Le’ahi summit was an ideal site for the coastal defense of O’ahu. In 1904 Diamond Head was purchased by the US Federal Government and designated for military use. An observation post was built on the summit, and a battery of cannons was located within the crater, providing complete concealment and protection from invading enemies. Diamond Head was prepared to defend O’ahu from attack, but no artillery was ever fired from there during a war.

 

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(Bunker atop tuff cone)

  

 

Today Le’ahi is the most recognizable landmark in Hawaii. It was designated a National Natural Landmark in 1968 as an excellent example of a tuff cone.
 

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(Photo courtesy of Wikipedia)

 

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(View of Waikiki from Diamond Head summit)

Resources

·         http://en.wikipedia.org/wiki/Diamond_Head,_Hawaii

·         http://www.soest.hawaii.edu/coasts/publications/hawaiiCoastline/oahu.html

·         http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_hawaii.html

·         http://www.hawaiistateparks.org/

·         Diamond Head State Monument Travel Guide; Hawaii State Parks

·         All photos by Svetlana Melton except as indicated