Koolau Range
Koolau Range is a name given to the fragmented remnant of the eastern or windward shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...

 of the Hawaiian
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...

 island of Oahu
Oahu or Oahu , known as "The Gathering Place", is the third largest of the Hawaiian Islands and most populous of the islands in the U.S. state of Hawaii. The state capital Honolulu is located on the southeast coast...

. It is not a mountain range
Mountain range
A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...

 in the normal sense, because it was formed as a single mountain called Koolau Volcano (koolau means "windward
Windward and leeward
Windward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...

" in Hawaiian
Hawaiian language
The Hawaiian language is a Polynesian language that takes its name from Hawaii, the largest island in the tropical North Pacific archipelago where it developed. Hawaiian, along with English, is an official language of the state of Hawaii...

, cognate
In linguistics, cognates are words that have a common etymological origin. This learned term derives from the Latin cognatus . Cognates within the same language are called doublets. Strictly speaking, loanwords from another language are usually not meant by the term, e.g...

 of the toponym Tokelau
Tokelau is a territory of New Zealand in the South Pacific Ocean that consists of three tropical coral atolls with a combined land area of 10 km2 and a population of approximately 1,400...

). What remains of Koolau is the western half of the original volcano that was destroyed in prehistoric times when the entire eastern half—including much of the summit caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

—slid cataclysmically into the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

. Remains of this ancient volcano lie as massive fragments strewn nearly 100 miles (160.9 km) over the ocean floor to the northeast of Oahu. The modern Koolau mountain forms Oahu's windward coast and rises behind the leeward coast city of Honolulu
Honolulu, Hawaii
Honolulu is the capital and the most populous city of the U.S. state of Hawaii. Honolulu is the southernmost major U.S. city. Although the name "Honolulu" refers to the urban area on the southeastern shore of the island of Oahu, the city and county government are consolidated as the City and...

 — on its leeward slopes and valleys are located most of Honolulu's residential neighborhoods.
The volcano is thought to have first erupted on the ocean floor more than 2.5 million years ago. It eventually reached sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 and continued to grow in elevation until about 1.7 million years ago, when the volcano became dormant. The volcano remained dormant for hundreds of thousands of years, during which time erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 ate away at the initially smooth slopes of the shield-shaped
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...

 mountain; and the entire mass subsided considerably. The highest elevation perhaps exceeded 3000 metres (9,842.5 ft); today, the summit of the tallest peak, Puu Konahuanui is only 3,100 feet.

After hundreds of thousands of years of dormancy, Koolau volcano began to erupt again. Some thirty eruptions over the past 500,000 years or so have created many of the landmarks around eastern Oahu, such as Diamond Head
Diamond Head, Hawaii
Diamond Head is the name of a volcanic tuff cone on the Hawaiian island of Oahu and known to Hawaiians as Lēahi, most likely from lae 'browridge, promontory' plus ahi 'tuna' because the shape of the ridgeline resembles the shape of a tuna's dorsal fin...

, Hanauma Bay
Hanauma Bay
Hanauma is a marine embayment formed within a volcanic cone and located along the southeast coast of the Island of Oahu in the Hawaiian Islands....

, Koko Head
Koko Head
Koko Head is the headland that defines the eastern side of Maunalua Bay along the southeastern side of the Island of Oahu in Hawaii. On its western slope is the community of Portlock, a part of Hawaii Kai...

, Punchbowl Crater
Punchbowl Crater
Punchbowl Crater is an extinct volcanic tuff cone located in Honolulu, Hawaii. It is the location of the National Memorial Cemetery of the Pacific....

, Tantalus, and Āliapaakai
Salt Lake, Hawaii
Salt Lake is a suburban neighborhood of Honolulu, Hawaii on the island of O‘ahu. The area is also known as Āliamanu after a nearby crater, although Salt Lake itself is in a crater called Ālia pa‘akai — meaning "salt pond" in the Hawaiian language...

. Geologists do not always agree on the dates of these more recent eruptions, some dating them to around 32,000 years ago, others to as recently as 10,000 years ago. Geologists believe that there is at least a remote possibility that Koolau volcano will erupt again.

There are three roads that tunnel through the southern part of the Koolau Range, connecting Honolulu to the Windward Coast. From south to north:
  • Hawaii Route 61 (Pali Highway)
  • Hawaii Route 63 (Likelike Highway)
  • Interstate H-3
    Interstate H-3
    Interstate H-3 is an intrastate Interstate Highway located on the island of O'ahu in the state of Hawai'i in the United States. H-3 is also known as the John A. Burns Freeway...

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