L earning Objective 16

Information from Volcanoes in the Sea: The Geology of Hawaii, by Macdonald, Abbott and Peterson, 2nd edition, published by University of Hawaii Press, 1990.

A. Earth's internal structure. (See Figure 16-A)

1. Crust is a thin solid outer layer of rock on earth's surface. It is thicker under continents (30-70 km.) and thinner under oceans (5 km.).

2. The mantle is below the crust. Its total composition is not known but it is partly composed of the mineral peridotite. The earth's interior is more dense than the crust so the crust "floats" on the mantle.

3. The core is partly liquid and it is made up of molten iron-nickel.

1. The crust is made up of many "plates" - each about 50 miles thick. Since they are less dense, they float on the earth's mantle. They are in constant motion relative to each other due to convection currents under the crust in a process called Plate Tectonics.

2. Plate Tectonics is believed to be responsible for such geologic phenomena as mountain-building, volcanoes, and earthquakes.

1. Continents at one time formed one super continent called Pangaea. Slowly Pangaea spread apart and the pieces that drifted away are now recognizable as the world's major continents.

One indication for the original fit is demonstrated by the remarkable matching symmetry between the Atlantic coasts of South America and Africa.

2. The slow movement of continents is explained by convection currents inside the earth's mantle.

3. According to the plate tectonic theory, the ocean plates ride up under continental plates. This gives rise to volcanic and earthquake activity at the edges, particularly where the oceanic Pacific plate meets continental plates. The "ring of fire" reference is due to all the volcanic activity at the edges of the Pacific plate. (See Figure 16-B)

A. Explanation of the Hawaiian Island Chain by a "Hot Spot"

1. Since the Hawaiian Islands are formed by volcanic activity but are in the center of the Pacific Plate and not at the edges, a different explanation is required for the volcanic activity which created the islands. (See Figure 16-B)

2. T. Wilson in 1963 gave the following explanation which is broadly accepted today. He suggested that the Hawaiian Islands were formed as Pacific plates moved slowly northwest over some melting point, currently described as a convection "plume." These plumes arise deep within the mantle and, like torches, super-heat the section of crust with which they intersect. The high-temperature plume create the "hot spot", the melting of the crust, as it is popularly called today.

3. The "hot spot" is hypothesized to be a melted area of the earth's crust on the ocean floor which is in the middle of the Pacific plate. The hot spot forming the Hawaiian Island chain is presently under part of the Big Island.

4. Magma from the earth's interior (called lava when released) comes out of the "hot spot", building a mound of cooled lava on the ocean floor. After many thousands of years, it breaks surface and becomes an island.

For further information on Hot Spots, click here.

For another view of

Hawaiian volcanoes, visit this site.
Lo'ihi is the ocean floor structure to the south-east of the Big Island which eventually will become high enough to rise above the surface of the ocean. This demonstrates that the process of island building in the Hawaiian Island chain is ongoing! (See Figure 16-C)

Lo'ihi is one of the only volcanoes in the world with its own Web page.
How do scientists visit Lo'ihi? Click here to find out.
How will Lo'ihi be monitored in the future? Click here to find out.
For photos of Hawaii's volcanoes and another diagram of hot spots , click here.
How much do you really know about the Hawaiian Islands? Take this quiz to find out.

B. The actual extent of the Hawaiian Island Chain.

1. What we commonly call the Hawaiian Islands, from Kaua'i to the Big Island of Hawai'i, are really part of a longer island chain called the Northwestern or Leeward Island chain which is about 1,600 miles long.



This island chain includes small islands, atolls and pinnacles north of Kaua'i, from Kure Atoll in the far north to Nihoa in the south. (See Figure 16-D)

For a sea-floor view of the islands, click here.

1. While the hot spot is releasing lava on the ocean floor, the Pacific plate is moving slowly to the north-west. It carries the forming island along with it and eventually away from the source of lava. (Figure 16-C)

Eventually the island no longer increases in size, becomes dormant (unlikely to erupt) and then extinct (no longer able to erupt). In terms of volcanic activity Kaua'i is extinct and O'ahu dormant .

2. If you trace the path of the Hawaiian Island chain, the oldest, Kure is at the north-west tip, and the youngest, the Big Island, is at the south-east tip of the chain.

This also correlates with the age of the islands: Kure about 30 million years old, Kaua'i 5-6 million years old, and the Big Island 1 to half a million years old. (See Figure 16-D)

1. The present rate of movement of the Pacific Plate, in relation to the Hot Spot, is about 8 centimeters a year. The direction of the plate movement is clearly from the north-west to the south-east orientation of the island chain.

2. It is possible to speculate that the Emperor Seamount chain is an even more ancient part of the chain and was formed when the Pacific plate was going in a more northerly direction. Later, the Pacific plate rotated and changed direction, causing the later island chain to be in the north-west direction. (See Figure 16-f.)

1. For 38 million years, or even longer, large islands could have been present at the current position of the island of Hawaii, the Big Island. Explain.

2. As these islands moved north-west on the Pacific Plate and gradually became eroded, their plants and animals could have had time to move "down" the island chain to newer islands that were forming.

3. The ideas expressed above are logical but speculative. Because the older islands above Kaua'i are extremely reduced in size and, because lava is a poor preservative of fossils, strong data to support this explanation is not easy to come by.

On the other hand, there is clear evidence today that native Hawaiian plants and animals are (and were) able to disperse between the lower-island chain in several ways.

One resulting pattern has older plant species (called relict species) more common on Kaua'i, and younger species more frequent on the Big Island. Also, the substantial number of Hawaiian native plants endemic to Kaua'i only indicates its antiquity and isolation as compared to the other islands - a direct result of its position on the hot spot formation "train."