Vacation Rental in Princeville Kauai

and Kauai Travel Guide

©B.Pruitt 2009-2011

Geology of Kauai and Hawaii

The Hawaiian chain of volcanic peaks–consisting of 132 islands, islets, reefs and shoals–stretches 1,523 miles northwest to southeast across the center of the Pacific Ocean.  There is more to the Hawaiian Islands than the land you see.  If the ocean was drained of its water, this chain would appear as a lofty mountain range.  Kauai, fourth in size, lies at the northwest end of the eight main islands.

Each volcanic cone is built of dark, iron-rich rock that poured out of vents as highly fluid lava in countless eruptions that spanned millions of years.  The result is the tallest mountains in the world, which break the surface of the ocean at about 18,000 feet.

Like most of the world’s volcanoes, the Hawaiian cones spawned from a zone of weakness in the earth's crust.  Weaknesses occur where giant, slow-moving tectonic plates—segments of the earth's crust—rub and collide as they drift like icebergs atop the mantle.  A hot spot is a place where an extraordinary amount of molten rock is generated.  Its source is a huge upwelling column of mantle rock.  The plume of mantle rock ascends because it is hotter than the surrounding rock, chemically different, or both.  When the temperature is high enough, or its pressure has been reduced enough, the rising rock partially melts and pushes through the earth’s crust.

The stationary hot spot in the earth’s mantle that caused Kauai's birth, and later spawned the island of Oahu and the island cluster of Maui, Molokai, Lanai and Kaho‘olawe, now remains beneath the island of Hawaii.  For 70 million years, a plate of the earth’s crust has been moving over that hot spot.  Again and again, molten rock has risen from the hot spot to build volcanic islands, and in the seemingly endless procession of geologic time they have drifted away, riding aboard the plate.  The oldest volcanoes created by this event are the submerged Emperor Seamounts, which now lie north of the island of Midway.  The youngest volcano is a burgeoning seamount named Lo‘ihi, 20 miles southeast of the island of Hawaii.  This embryonic island must grow another 3,180 feet before it emerges from the ocean—60,000 years from now.

The volcanoes of the Hawaiian chain are rounded, dome-shaped masses, broad for their height.  In profile, they resemble the shields of medieval warriors, hence they are known as shield volcanoes.  These volcanoes build atop vents that effuse highly mobile lava that runs swiftly and spreads widely, accreting a cone of gentle slope.

Around the rim of the Pacific Ocean more viscous lava flows have produced the classic upswept form of a composite volcano.  The lava does not flow far before it solidifies, building higher and higher around its vent.  Famous composite volcanoes are Fuji in Japan and Rainier, Hood, Shasta and St. Helens in the United States.

Hawaiian volcanoes generally erupt along cracks in the cone’s flanks.  Movement within the earth’s crust tears open a series of fissures, forming a rift zone.  Pressing through the cracks, magma fountains onto the land as lava.  When an eruption is over, the lava remaining in the fissure hardens into a wall-like mass called a dike.  Hundreds of thin dikes may be seen cutting across the west walls of Waimea Canyon on Kauai.         

Wai‘ale‘ale, the volcano that formed Kauai, had two principal periods of eruptive activity.  Between five and six million years ago, lava flows broke the ocean's surface.  Thin flows of lava, each from 5 to 15 feet thick, piled atop each other to eventually building a broad, smooth, dome-shaped mountain.  Geologists name rock from this island-building period the Waimea Canyon series.  Likely, the neighboring island of Ni‘ihau was linked to Kauai and the same volcano consisted of several craters.  

Fields of rubble in the ocean floor indicate that massive pieces of the island broke off and slid into the ocean.  A slide split the summit of Wai‘ale‘ale along a north-south line.  Part of the island disappeared into the ocean and what remained to the east of present-day Waimea Canyon, dropped in elevation.  

Another catastrophic slide later broke a large portion off of the north flank of Wai‘ale‘ale.  The towering sea cliff left behind eroded into the Na Pali coastline.  Slides of such magnitude must surely have generated big tsunamis.

Eruptions of lava continued after the slides, building a new volcanic shield in the dropped eastern portion of the island.  A caldera formed in the new shield that eventually eroded into the Līhu‘e depression, a broad basin five miles northwest of Līhu‘e.  Some caldera-filling lavas were more resistant to erosion and now stand in relief above the eroded flanks. The Hā‘upu Range, in the southeast corner, is the remains of a caldera that filled with hard, dense lava, which withstood the ravages of weathering that whittled away the softer lava at its flanks.  

Building and rebuilding of the shield that formed Kauai continued until 4.3 million years ago.  Running water shaped the gentle volcanic slopes into a landscape of jagged ridges and deep valleys.  After a time of volcanic quiescence, eruptions began again.  This period of renewed volcanism started 3.65 million years ago and lasted until half a million years ago.  The lava, cinder cones and ash beds produced from these more modern eruptions are identified as the Koloa volcanic series.  The largest volcanic vent of the newer series is Kilohana, the 1,133-foot-high mountain west of Līhu‘e.  Erosion continued relentlessly.  Koloa basalts flowed into valleys, completely filling some of them and diverting streams to erode new channels elsewhere.

Kauai, which appeared late in geological history, is doomed to extinction, but on a time-scale of a magnitude that defies comprehension.  It will be eroded to a shoal as the plate that carried it past the hot spot of its birth inches inexorably toward the edges of the Pacific. Like a conveyor belt, the plate trundles to the Aleutian Trench, where the sea floor bends into the earth’s interior.  Kauai's cycle will be complete as its remnants are reincorporated into the crust of the earth.

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