CCD   Geography 105 - World Regional Geography


USGS

Pacific basin gif

History of Concepts

In the 1960s, geologists were seeking ways to prove or disprove the new idea of moving plates. Exploration of magnetic anomalies at mid-ocean ridges provided strong support for seafloor spreading . Geologists studied other ocean features to see how they related to plate tectonics. While visiting Hawaii, Tuzo Wilson, one of the founders of the theory of plate tectonics, noticed some interesting features about ocean islands. On a map of the Pacific basin, he found three linear chains of volcanoes and submarine volcanoes (seamounts).

 

Although separated by thousands of miles, the three linear chains are parallel to each other. Of the three, the Hawaii-Emperor seamount chain was the most well known. Wilson reviewed the reports that had been published on these island chains and recorded the age of each island. An interesting pattern emerged. For each chain, the islands become progressively younger to the southeast. The extreme southeast end of each chain is marked by active volcanoes.

 

Wilson proposed that the Hawaiian islands formed successively over a common source of magma called a hot spot. The Island of Hawaii is currently located above the hot spot.


Image Source: Eruptions of Hawaiian Volcanoes: Past, Present, and Future: U.S. Geological Survey General Interest Publication.

 

Hot, solid rock rises to the hot spot from greater depths. Due to the lower pressure at the shallower depth, the rock begins to melt, forming magma. The magma rises through the Pacific Plate to supply the active volcanoes. The older islands were once located above the stationary hot spot but were carried away as the Pacific Plate drifted to the northwest .

The long trail of the Hawaiian hotspot

Over the past 70 million years, the combined processes of magma formation, volcano eruption and growth, and continued movement of the Pacific Plate over the stationary Hawaiian "hot-spot" have left a long trail of volcanoes across the Pacific Ocean floor. The Hawaiian Ridge-Emperor Seamounts chain extends some 6,000 km from the "Big Island" of Hawaii to the Aleutian Trench off Alaska. The Hawaiian Islands themselves are a very small part of the chain and are the youngest islands in the immense, mostly submarine mountain chain composed of more than 80 volcanoes. The length of the Hawaiian Ridge segment alone, from the Big Island northwest to Midway Island, is about equal to the distance from Washington, D.C. to Denver, Colorado (2,600 km). The amount of lava erupted to form the Hawaiian-Emperor chain is calculated to be at least 750,000 cubic kilometers-more than enough to blanket the entire State of California with a layer of lava roughly 1.5 km thick.

Pacific basin gif

Map of part of the Pacific basin showing the volcanic trail of the Hawaiian hotspot-- 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain. (Base map reprinted by permission from World Ocean Floor by Bruce C. Heezen and Marie Tharp, Copyright 1977.)

A sharp bend in the chain indicates that the motion of the Pacific Plate abruptly changed about 43 million years ago, as it took a more westerly turn from its earlier northerly direction. Why the Pacific Plate changed direction is not known, but the change may be related in some way to the collision of India into the Asian continent, which began about the same time.

As the Pacific Plate continues to move west-northwest, the Island of Hawaii will be carried beyond the hotspot by plate motion, setting the stage for the formation of a new volcanic island in its place. In fact, this process may be under way. Loihi Seamount, an active submarine volcano, is forming about 35 km off the southern coast of Hawaii. Loihi already has risen about 3 km above the ocean floor to within 1 km of the ocean surface. According to the hotspot theory, assuming Loihi continues to grow, it will become the next island in the Hawaiian chain. In the geologic future, Loihi may eventually become fused with the Island of Hawaii, which itself is composed of five volcanoes knitted together-Kohala, Mauna Kea, Hualalai, Mauna Loa, and Kilauea.

The Hawaiian - Emperor Volcanic Chain

The Hawaiian - Emperor volcanic chain, one of the most dramatic features on the Earth, is the result of the drifting of the Pacific Plate over a fixed hot spot.

 

The Hawaiian - Emperor volcanic chain extends nearly 3,750 miles (6,000 km) across the northwest Pacific Ocean. The volcanic chain is made of the Hawaiian Ridge and the Emperor Chain. The Hawaiian Ridge extends from Kilauea Volcano to Daikakuji Seamount, 2,183 miles (3,493 km) to the northwest, and includes the eight main islands and numerous smaller islands and seamounts. The Emperor Chain extends 1,454 mi (2,327 km) from Daikakuji Seamount to Meiji Seamount and trends almost directly north. The oldest seamount, Meiji, is about 75-80 million years. Since Meiji formed above the hot spot we know that the hot spot has been active for at least 75-80 million years, probably longer. The bend in the volcanic chain is the result of a major change in the direction of movement of the Pacific Plate about 43 million years ago. Since this change, the Pacific Plate has been moving to the northwest an average of 3-3.5 inches (8-9 cm) per year.