Maui County’s four islands were once one

The four islands of Maui, Moloka`i, Lana`i, and Kaho`olawe were once all connected as a vast landmass known as Maui Nui, literally “big Maui.” This concept was first proposed 60 years ago by geologist Harold Stearns, who recognized the geologic evidence for repeated episodes of island submergence and reemergence. Recently, Jonathan Price of the Smithsonian Institution modeled the area’s submergence history using a computer-based geographic information system (GIS). The model examines the effects of global sea-level change, age of volcanism, and likely rates of volcano subsidence. Using Price’s model, we can watch the response of Maui Nui to subsidence and sea-level change as we move the clock forward or back.

Maui Nui in its heyday, about 1.2 million years ago, encompassed about 14,600 square kilometers (5,640 sq mi), larger by 50 percent than today’s Big Island of Hawai`i. Included in this landmass was Penguin Bank, a broad shoal west of Moloka`i. Penguin Bank originated as a volcano that completed its shield-building stage about 2.2 million years ago, when it was briefly connected to O`ahu. Now bearing a thick coral cap and lying in water shallower than 200 m, the once-emergent volcano covered an area eight times that of present-day Kaho`olawe. From Penguin Bank on the west, Maui Nui reached 250 km (160 mi) eastward, its distant shore lying 70 km (40 mi) east of today’s Maui village of Hana.

Enter the agent of subsidence. Hawai`i’s volcanoes build by a vast outpouring of lava from the Earth’s upper mantle. The oceanic crust bows under the added weight, subsiding at rates exceeding 3 mm per year (0.1 inch per year). As Maui Nui’s upbuilding volcanoes waned, the saddles between them were submerged, isolating the different volcanoes as distinct islands.

Another factor that isolates islands is sea level change resulting from the growth and wasting of continental glaciers. Unlike subsidence, which works to diminish an island’s extent, sea-level changes may either diminish or enlarge an island. Glacial expansion, for example, leads to falling sea level, which enlarges an island. Today our world has a greatly reduced ice volume. As a consequence, sea level stands higher now than almost any other time in the past 200,000 years.

Within the past 1.2 million years, the first Maui Nui connection to be broken was that between Penguin Bank and West Moloka`i volcanoes. A permanent embayment separated East Moloka`i from West Maui by about 700,000 years ago, but these two volcanoes remained connected via Lana`i. The saddle between East Moloka`i and Lana`i submerged about 600,000 years ago, then reemerged whenever glaciers grew and sea level fell.

The connection between Kaho`olawe and Maui disappeared permanently, probably between 200,000 and 150,000 years ago. But Maui, Lana`i, and Moloka`i were connected intermittently thereafter, most recently about 18,000 years ago. Since then, sea level has risen as the amount of ice stored in glaciers worldwide has waned, except during a few short-lived cooling episodes. Moloka`i became isolated, and then the land bridging Lana`i and Maui was submerged, creating the four islands as we know them today.

The island of Maui itself comprises two volcanoes-West and East Maui. The isthmus between them has always been above sea level. No known sea stand has been high enough to submerge it.

Maui Nui comprised extensive lowlands dotted by upland areas. Plants and animals could disperse readily across this terrane. Even flightless or slowly dispersing species could spread widely, making the biologic communities on these islands more similar than might be expected for islands always isolated from each other. Also, Maui Nui’s vast extent increased the chance that airborne or waterborne species would make landfall on the ancient landmass. Once there, those species had a wealth of environments in which to evolve. Thus Maui Nui provided a rich storehouse for subsequent colonization by plants and animals onto the island of Hawai`i, the youngest island in the chain.

Activity Update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Several tongues of lava from breakouts of the main Mother’s Day tube system are coursing their way down Pulama pali and giving visitors a great view. Brilliant crimson streams can be seen from the top of the pali down to the coastal flats at night. Scattered surface breakouts are found throughout the inflating Kohola flow, and the National Park Service has marked trails out to the closest activity. The only lava entering the ocean is at the West Highcastle delta.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

Two earthquakes were reported felt during the past week. Residents of the Volcano Golf Course subdivision felt an earthquake at 9:27 p.m. on April 6. The magnitude-3.1 temblor was located 5 km (3 mi) west of Kilauea Summit at a depth of 6.7 km (4 mi). The second earthquake was felt by residents of Leilani Estates and Lani Puna Gardens subdivisions at 6:17 a.m. on April 8. The magnitude-2.6 event was located 2 km (1.2 mi) east of Pu`ulena Crater at a depth of 4.5 km (2.7 mi).

Mauna Loa is not erupting. Inflation of the summit region has slowed down, and seismic activity remains low, with only three earthquakes located in the summit area during the last seven days.

This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory and is republished by HawaiiNews.com with permission.