The mysterious geologic forces powering the current eruptions of Hawaii's Mount Kilauea are quietly creating the newest piece of American territory.

Climbing with agonizing slowness from the warm tropical waters just southeast of the Big Island of Hawaii is an undersea mountain named Loihi. Volcanologists believe Loihi eventually could emerge above the surface as the ninth Hawaiian island."Loihi has grown about 10,100 feet from the Pacific floor," Thomas L. Wright, scientist-in-charge of the Hawaiian Volcano Observatory, told National Geographic. "It seems to be tapping the same source of magma (lava still underground) as Kilauea. Loihi needs about 3,100 more feet to break the surface."

Perched on the very rim of Kilauea's summit caldera, or lava lake, the observatory is operated by the U.S. Geological Survey.

Volcanoes have been creating and modifying the Earth's environment over eons of geologic time. More than 80 percent of the Earth's surface is of volcanic origin. Volcanic eruptions have formed fertile soils, sculpted mountains, plateaus and other majestic landscapes. Gaseous emissions from volcanic vents even helped form the early atmosphere and oceans where scientists believe life first emerged.

Yet, the volcanic machine powering Kilauea and building Loihi may rank as the most visible and most persistent example of this little-known facet of volcanoes.

For Kilauea has been in the news time and again since its current eruption began Jan. 3, 1983. Lava oozing at the rate of 635,000 cubic feet each day has spread 20 miles east to the Pacific Ocean.

Lava flows from the eruption, 50 feet thick in some places, already have destroyed 140 homes and caused more than $60 million in damage. Last May 5, a 2,000-foot-wide flow cut off an entire village on the island's southeast coast, causing Gov. John D. Waihee to declare the town a disaster area.

These highly publicized lava flows, and those silently lifting Loihi toward the surface, result from the peculiar form of volcanic activity occurring in Hawaii.

Most of the 500 active surface volcanoes on Earth straddle the margins of continents and the huge plates of rock that are fitted together like pieces of a jigsaw puzzle to make up the planet's surface.

Earth's surface consists of about a dozen of these plates, each of which is about 50 miles thick. The plates move relative to each other a few inches a year. Active volcanoes tend to occur in zones of underthrusting or "subduction," where one plate slides beneath another.

But how can volcanic activity occur in the Hawaiian Islands, which are more than 2,000 miles from the nearest plate margin?

Hawaiian volcanoes are believed to result from a different phenomenon termed "hot spot" volcanism. Geologists theorize that deep under the Pacific plate is an exquisitely hot, stationary region termed the "Hawaiian hot spot."

Like a propane torch placed under a moving sheet of plastic, the hot spot melts rock in the overriding Pacific plate. The molten rock, or magma, is less dense than surrounding solid rock and rises through fracture zones to spill onto the surface or ocean floor.

Over a span of about 70 million years, the continuous movement of the Pacific plate over this stationary "fountain of fire" sputtered a trail of more than 80 big volcanoes over the floor of the Pacific Ocean. Called the Hawaiian Ridge-Emperor Seamount Chain, the islands and undersea mountains, or seamounts, stretch northwest about 4,000 miles, from the Big Island of Hawaii to the Kamchatka Peninsula in the Soviet Union.

The five main Hawaiian islands are the tops of volcanic mountains formed as the Pacific plate moved over the Hawaiian hot spot.

A scientific record of the plate's movement to the northwest is written in the age of solidified lava left on each of the islands. The age decreases from northwest to southeast. Kauai in the extreme northwest is 5.6 million years old; Oahu, 3.4 million; Molokai, 1.8 million; Maui, 800,000; Hawaii, less than 700,000. Lava ages even decrease in the same northwest-to-southeast fashion on each individual island.

Continued movement of the Pacific plate promises to give Pele a new home, and Hawaii a new island.

If theories about hot spot volcanism are correct, plate movement eventually will carry the Big Island off the Hawaiian hot spot. The two active volcanoes on the island, Kilauea and Mauna Loa, will become extinct. The hot spot then will pour molten rock onto the surface through the next volcano in the Hawaiian chain.

Experts do not know exactly when Loihi will break the surface and emerge as a new island. Assuming Loihi's growth continues at current rates, it could take anywhere from a few thousand to a few tens of thousands of years.

But other fates also may await the infant volcano.

Infusions of molten rock from the hot spot, for instance, may halt unexpectedly. Loihi's growth would be stunted, leaving it just another seamount in the chain. The ninth Hawaiian island thus may form in an entirely different place.

There is also the possibility a sudden major undersea eruption could raise Loihi to the surface sooner than anyone expects.