Volcano Watch — Island of Hawaii sits at the end of a long, old chain of 125 volcanoes

The Hawaiian Islands form the young end of a chain of more than 125 volcanoes that stretch over 3,720 miles across the north Pacific Ocean. There are two main parts of the chain: the younger Hawaiian Ridge, which includes the Hawaiian Islands, and a string of volcanoes extending west-northwest through Midway Island, and the Emperor Seamounts, a submarine chain of volcanoes oriented almost north-south and connects to the western end of the Hawaiian Ridge.

This unique geologic feature has played an important role in the development of ideas about the history of the Pacific Ocean Basin and the rates and directions of motion of the large crustal plates that make up the surface of the Earth.

The eight principal Hawaiian Islands lie at the southeast end of the chain. The three active volcanoes, Kīlauea, Mauna Loa, and Lo`ihi Seamount are located at the southeast end of the islands. The three dormant volcanoes, Hualālai, Mauna Kea, and Haleakalā, are also located near the southeastern end of the chain. To the west, the islands become more and more eroded, and to the west of Kaua`i, only a few of the volcanoes rise above sea level as small volcanic islets and coral atolls. The western-most island in the chain is Kure Island, located just west of Midway Island. Farther west and to the north along the Emperor Seamount Chain, all the volcanoes are now submarine, although they were once large, subaerial volcanoes like the present-day Hawaiian Islands. They have sunk beneath the sea as they got older, mainly due to their great weight, which bends the underlying almost-rigid outer layer of the Earth.

The greater erosion of the western islands, compared to those farther east, was recognized in the 1800s as indicating that the islands to the west were older than those to the east. It was many years before this inference was quantified using radiometric dating techniques.

The chief technique used to determine the ages of the islands has been the potassium-argon technique. In a nutshell, a certain very small percent of the naturally-occurring potassium in rocks and minerals is radioactive and decays to argon at a measured, slow rate. This radioactive decay emits miniscule amounts of radiation that is naturally present everywhere. If we can measure both the amount of potassium in the rock or mineral and the amount of argon produced by radioactive decay and trapped in the rock or mineral, we can calculate the length of time required for that amount of the potassium to decay to argon. That amount of time represents the age when the lava erupted and cooled. We must assume that the rock contained no argon when it cooled and crystallized (true if it erupted subaerially) and that neither potassium nor argon have been added to, or lost from the rock.

In general, as the minerals in a rock alter, they lose some of the argon formed by radioactive decay or some potassium is added (from groundwater) to the clays that replace the original minerals. Either of these effects will result in a determined age that is younger than the real eruption age of the lava sample. We determine the ages of rocks that have experienced as little of this type of alteration as possible.

Using these techniques, the ages of many of the volcanoes in the Hawaiian-Emperor volcanic chain have been determined. The principal Hawaiian Islands systematically increase in age towards the west-northwest, with Kaua`i being about 5.0 to 5.5 million years old.

Along the Hawaiian Chain, the volcanoes continue to increase in age to the west-northwest, with Midway Island being about 28 million years old and the bend between the Hawaiian and Emperor Chains being about 43 million years old. The Emperor Seamounts continue to get older to the north, with the northern-most submarine volcanoes estimated to be about 80 million years old, although the oldest dated seamount is only 65 million years old.

The systematic increase in ages along the chain has been interpreted as the result of the sequential formation of volcanoes at the same place (the present location of Lo`ihi, Kīlauea, and Mauna Loa Volcanoes). This location has been called a "hot spot" because some scientists think it consists of hotter material inside the Earth or a "plume," because other scientists think it may be caused by a plume-like upwelling of material from deep inside the Earth. In either case, the volcanoes are moved away from where they form by motion of the underlying Pacific crustal plate, much like the motion of a conveyor belt that carries each volcano towards the west-northwest. It has been proposed that the location where the volcanoes have formed has stayed stationary through many millions of years. If this is correct, then the rate of motion of the plate can be calculated directly from the ages of the volcanoes and their distance from the hot spot. For the Hawaiian Chain, this calculated rate is approximately 10 centimeters per year.

In these models, the bend in the chain between the Hawaiian and the Emperor portions of the chain marks a major change in the direction of Pacific plate motion, inferred to have occurred 43 million years ago. Such a change in Pacific Plate motion may have ultimately been caused by a collision of India into Eurasia that began the formation of the Himalayan mountain range.