Volcano Watch — Evolution of Hawaiian volcanoes

The 15 volcanoes that comprise the eight principal Hawaiian Islands are the youngest in a linear chain of about 125 volcanoes that stretches for about 3,600 miles across the north Pacific.

The chain is remarkable for its length and the number of volcanoes that comprise it, the bend that separates the older Emperor Seamount Chain from the younger Hawaiian Ridge, and the age of the volcanoes, which is systematically younger towards the southeast. The age of the oldest dated volcano, located near the northern end of the Emperor Seamount Chain, is 81 million years, the bend between the two chains, 43 million years, and the oldest of the principal islands, Kaua'i, a little more than 5 million years.

The assembly line that forms the volcanoes is driven by a "hot spot," or plume of hot material, deep within the Earth that partially melts to produce magma as it rises beneath the Pacific Plate. As the plate moves west-northwest, each volcano moves with it from its place of origin above the hot spot. The age and orientation of the volcanic chain records the direction and rate of movement of the Pacific Plate.

The pronounced 43-million-year-old bend between the Hawaiian Ridge and the Emperor Seamount Chain marks a dramatic change in plate direction.

Each of the volcanoes in the chain evolved through a sequence of life stages. When a new volcano forms, eruption rates gradually increase over a period of several hundred thousand years, attain their peak for perhaps 500,000 years, then decline rapidly. It may be several million years before the final activity at any volcano is over. As the eruption rates change, so does the composition of the lavas.

The relationship of the top of the volcano to sea level determines the style of eruptive activity, e.g., submarine eruptions form pillow lava; explosive eruptions occur in shallow water, may continue after the volcano has grown above sea level, and form ash deposits; and subaerial (above sea level) eruptions form ropey pahoehoe and blocky 'a'a flows.

The initial stage of volcano growth is the submarine preshield stage, characterized by infrequent, small-volume eruptions. Pillow lava constructs a steep-sided volcanic edifice with a shallow summit caldera and two or three rift, or fissure, zones radiating from the summit.

Rift zones are a prominent feature of Hawaiian volcanoes during all but the final eruptive stage. This stage lasts for perhaps 200,000 years, yet produces only a small portion of the final volume of the volcano. As the volcano grows, the composition of the lava changes, and eruptions become more frequent and voluminous. When the transition is complete, the volcano enters the shield stage.

The shield stage is subdivided into three substages. (1) The submarine substage is characterized by the continuing eruption of pillow basalt. (2) The explosive substage begins when the summit of the volcano reaches shallow water and magma and sea water mix to produce explosive eruptions. These ash-generating eruptions occur intermittently for a period of several hundred thousand years. During this and the subsequent subaerial substage, summit calderas repeatedly collapse, are filled with thick, ponded flows, only to collapse again.

When the volcano has risen enough above sea level, explosive eruptions cease, and (3) the subaerial substage begins. The low-profile "shield" shape of Hawaiian volcanoes is formed (named after the shape of a warrior's shield). Eruption rates and frequencies peak during the shield stage, and about 95% of the volcano's volume forms during a period of roughly 500,000 years.

The postshield stage caps the shield with a carapace of lavas that have low silica and high alkali contents. Not all Hawaiian volcanoes go through this stage (e.g., Ko'olau and Lana'i), whereas other volcanoes have erupted thick lava sequences (e.g., Wai'anae and Mauna Kea) during this stage. The high fountains of these eruptions produce large amounts of cinder and short, thick, pasty 'a'a flows that steepen the slopes of the volcano as they pile up near the vents around the summit region and along the previously active rift zones. These lavas commonly fill and overflow from the final caldera. During this stage, the eruption rate gradually decreases to zero over a period of 250,000 years or less.

The end of the postshield stage is followed by a period of erosion and subsidence, during which deep canyons may form along the flanks of the volcano. As the islands subside, fringing coral reefs grow. Following this erosional period, the volcanoes may go through one last eruptive phase, the rejuvenated stage. During this stage, lavas with very low silica and high alkali contents may erupt, commonly through the reefs formed during the erosional period; the flows are funneled down stream valleys, such as Manoa Valley on Oahu. Lavas erupted during the rejuvenated stage are abundant on Kaua'i and Ni'ihau, common on Ko'olau Volcano, Oahu, and occur on East Moloka'i, West Maui, and Haleakalā, East Maui. Rejuvenated-stage activity has very low eruption rates and may occur over several million years.

Following the rejuvenated stage, the islands undergo a long period of erosion and subsidence that eventually reduces them to sea level. With continued subsidence, the islands become coral atolls, such as those that lie west of La Perouse Pinnacles, the westernmost and oldest subaerial remnant of a former Hawaiian island. These coral reefs may die if conditions are not favorable. After the reef dies, the volcano continues to subside.

Once below sea level, these flat-topped, coral-capped volcanoes are called "guyots." Although a few volcanoes apparently never grew above sea level, nearly all the volcanoes in the Hawaiian-Emperor volcanic chain older than 30 million years are guyots. Most of these volcanoes stood thousands of feet above sea level in their youth, as the volcanoes that make up the present-day islands do today.