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Roger Weller, geology instructor
Diamondhead, Oahu, Hawaii
by Svetlana Melton
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
(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
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.
(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.
The broad, saucer-shaped crater covers 350 acres, its width being greater than its height.
The southwest rim is
highest as winds blew ash in this direction during the eruption.
(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.
(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
(Crater interior as viewed from summit)
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
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.
(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.
(Photo courtesy of Wikipedia)
(View of Waikiki from Diamond Head summit)
· Diamond Head State Monument Travel Guide; Hawaii State Parks
· All photos by Svetlana Melton except as indicated