The Hawaiian Archipelago

A HOT SPOT is an area where superheated rock moves from deep within the Earth’s mantle upward toward the center crust. And while the Pacific Plate moves, the Hawaiian Hot Spot remains fixed in relation to the planet as a whole. Some scientists believe the plate “floats” on a bed of magma that is forced up from deeper in the Earth, and is pushed out laterally in all directions beneath the crust. The Pacific Plate is believed to be moving slowly but steadily at the rate of about three inches a year. When a thermal plume forms a hot spot, magma pushes through the crust to erupt. Initially, as with the Hawaiian hot spot, much of that magma erupted under water millions of years ago. Yet that same process is going on now with the undersea volcano, Lo‘ihi, about 20 miles off the east coast of the Island of Hawai‘i. Geologists theorize that the upwelling of magma finds its way to the surface through cracks that become permanent supply routes, feeding underground magma storage chambers beneath the summits of various volcanoes. There is a huge amount of magma available, and it is capable of supporting several active eruptions. Lo‘ihi, Kilauea and Mauna Loa are all being fed by the same Hawaiian hot spot. Sometimes, two of these erupt simultaneously.

Dating the Hot Spot

Geologists have determined that the Hawaiian Hot Spot has been in place and building volcanoes on the Pacific Plate for 75 to 80 million years. They have calculated this by analyzing the rocks in various volcanic features, including the most ancient, like Meiji Seamount, which has been dated at around 75 million years old. Some believe that our island chain may be much older, but that the evidence has disappeared with the collision of the Pacific Plate and the plates to the north and west. The line of the Hawaiian chain moves roughly northwest from Hawai‘i to the Seamount of Daikakuji, where it changes to the Emperor Seamounts. Seamounts are mountains whose peaks are below the surface of the ocean. Some of them may once have been islands, but due to erosion and subsidence their summits are now below sea level. A few are believed to have spent millions of years as atolls before subsiding. Once sunken, these former atolls are called guyots.

Meiji’s Voyage

The same hot spot that is today producing Lo‘ihi also produced Meiki, 3,700 miles away. Because the Pacific Plate is in constant motion, after the hot spot forms a volcano, the plate’s movement pulls the volcano and the seafloor around it away from the hot spot.
From the time of Meiji’s creation to about 43 million years ago – about the time the Seamount of Daikakuji was formed – the Pacific Plate moved in a northward direction. Then, the direction changed and the chain began moving in a more westerly direction. Today the island farthest from the hot spot is Kure Atoll, whose volcanic parts have sunk below the surface, but whose coral reefs are constantly growing and keeping up with its slow subsidence. Closer to the hot spot one finds Midway Atoll, Pearl and Hermes Reef, Lisianski and Laysan islands, and Maro Reef. These have only the products of the living coral at the surface.

Volcanic Islands

Roughly half the distance from Kure to the current hot spot is the first surface evidence of the chain’s volcanic origins. The basalt, or volcanic rock, remnants called Gardner Pinnacles pierce the ocean’s surface. Nearer the main Hawaiian Islands are the French Frigate Shoals, entirely made up of reefs and sand bars, and accompanied by a fragment of the volcanic island that forms their base: La Perouse Pinnacle, a chunk of rock white with the guano of the seabirds that roost there.

Nearer still are Necker and Nihoa, the islands closest to the main Hawaiian chain that hint at human habitation. The only remaining evidence consists of a few shelters and rock altars left behind. Finally, 300 miles and five or six million years away from the original hot spot are the oldest of the major islands, Kaua‘i and Ni‘ihau. O‘ahu’s most ancient rock has an age of only three million years, and Maui is a relative youngster, just a million years old.

The islands of Maui County – Maui, Moloka‘i, Lana‘i and Kaho‘olawe – although created by several separate volcanoes, are believed to have once been a single island now called Maui Nui. Subsidence of the volcanoes, rising sea levels and erosion have created comparatively shallow ocean channels where at one time there were valleys between the peaks of Maui Nui. In such a natural manner that spans many millennia, groups of islands are created.

On the island of Hawai‘i however, five distinct volcanoes are still connected above the sea’s surface: Kohala, Mauna Kea, Hualalai, Mauna Loa and Kilauea. This volatile area still has a great deal to show for itself, and its story is far from over.

Individual Islands

ONCE MAGMA CRACKS through the crust at a new location over the hot spot and starts building an island, it begins a process of volcanic activity that may last millions of years. The initial eruptions release comparatively small amounts of lava, then in the shield-building stage most of a volcano’s volume is deposited. Finally, the activity slows down. Eruptions which occur later may be hundreds or thousands of years apart. Haleakala is close to a million years old and looks like an extinct volcano, but erupted as recently as about 200 years ago.
The initial structure of a Hawaiian volcano is believed to be a cone with steep sides. Lo‘ihi, the undersea volcano off the southeast coast of the Island of Hawai‘i has this shape and is an example of the early stage of activity, which geologists call the preshield phase.

Rift Zones

A volcano begins developing rift zones early in its life. These are interior areas of weakness generally leading outward from the summit, and are the site of many and varied eruptions. Rift zones develop into major sources of lavas that broaden the growing volcanic structure. Lo‘ihi already has two rift zones, one extending to the south of its summit and a smaller one to the north.

When a new volcano’s pre-shield phase is over – after a few hundred thousand years – its eruptions increase in frequency and produce more and more lava. The flows spread out, pile on top of each other, and the shape of the volcano changes from a steep-sided cone to a shield, whose sides run out at a shallow angle. Most of the Hawaiian islands are believed to have been in the shield-building phase when they reached the ocean’s surface, a period geologists feel was associated with a great deal of explosive activity of the Hawaiian volcanoes, is in its shield-building period. It has extensive rift zones to the east and southwest.

By the time a major Hawaiian volcano nears the end of its shield-building phase, its peak can be thousands of feet above sea level, and the frequency of its eruptions tapers off. Mauna Loa, which still erupts on average every 10 years or so, is believed to be near the end of its shield-building phase.

Erosion and Subsidence

Hawaiian volcanoes may be nearing a million years in age when most eruptive activity stops. By this time, the moving Pacific Plate has pulled the volcano 30 or 40 miles away from where the hot spot first began feeding it, and it no longer has a short, direct source to the magma under the Earth’s mantle. Now erosion and subsidence become the major factors in geologic change. On the Island of Hawai‘i, Hualalai volcano is past its major shield-building phase; it erupts every few hundred years. Mauna Kea is considered dormant, since it hasn’t erupted for more than 4,000 years, and there is no indication it will erupt again any time soon. Kohala Mountains represent the remainder of a volcano which last erupted perhaps 60,000 years ago during a post-shield building session. Mahukona was the first Hawai‘i island volcano, but never reached the height of others. Most of it is now underwater or buried under the lavas of Kohala and Hualalai eruptions.

Hawai‘i is still evolving and illustrates both ends of the volcano spectrum. While Mahukona is gone, Lo‘ihi is still underwater and growing. Scientists at the Volcano Observatory figure that Lo‘ihi will eventually develop a surface connection with Hawai‘i island. In several hundred thousand years, the Kohala Mountains are expected to have subsided to form a separate island.

Maui Nui

Some separation of land has already occurred in what is now Maui County which includes four islands. At its largest the land mass called Maui Nui was on and a half times larger than Hawai’i Island is today. Years ago, it encompassed the volcanoes that presently form West Maui, East Maui, Kaho‘olawe, East Moloka‘i, West Moloka‘i, Lana‘i, and shallow waters between Moloka‘i and O‘ahu called Penguin Bank. Over eons, because of island subsidence and the effects of various ice ages, parts of Maui Nui were either exposed or submerged, creating the separate islands as they are today.

Once the shield-building phase of activity is over, some volcanoes stop erupting all together. Others enter a post-shield phase, during which they can pour out thick flows of lava that obscure many of their distinctive summit features from the shield-building phase. Kaua‘i, for example, has canyons thousands of feet deep that some geologists feel were formed by a separate volcano and suggests millions of years of erosion. But the island also has comparatively uneroded volcanic cones, and even spatter cones with fresh looking cinder. Lavas in the Po‘ipu area, on the dry leeward side of Kaua‘i, still have the appearance of fresh flows. Modern techniques for dating volcanic rocks imply that while the majority of the rocks on Kaua‘i are in the range of five million years old, other cones scattered around the island erupted only half million to 1.5 million years ago. This apparent contradiction is termed rejuvenation. However, it isn’t particularly common. Rejuvenated phase features are not found on O‘ahu’s Wai‘anae coast range, or in the West Moloka‘i mountains. West Maui has only a few rejuvenation features, around the Lahaina area.

However, researchers say East Maui’s Haleakala, still in its rejuvenated stage, is a potentially dangerous volcano. It probably erupts every few hundred years, and it’s been more than 200 years since the last one. Nobody is guessing when the next Haleakala eruption will occur. Maybe in a few years, perhaps in the next century, or the one after that.