2. Hawaiian Island Formation
° Plate Tectonic Theory
° Missing Link
°What Are Hot Spots?
° Stationary Thermal Plumes
° Effusive Eruption
When you think of islands, especially Hawaii, the first thought that crosses one’s mind is warm weather, sunny beaches, and coconuts. But have you ever wondered how islands such as the Hawaiian chain islands were formed? It is an amazing concept that a piece of land is located out in the middle of a large ocean.
Hawaiian Island Formation
When discussing the formation of islands, especially the Hawaiian Islands, the theory of plate tectonics must be reviewed. The earth is broken into 12 major plates 50-100 kilometers thick, which move horizontally. These plates float and slide along the earth’s mantle, which explains why most of the world’s volcanic eruptions and earthquakes occur here.
How does the theory of plate tectonics relate to formation of the Hawaiian Islands? It doesn’t really, and that is the amazing concept behind theses Hawaiian Islands. 95 percent of the world’s known volcanoes occur close to these plate margins, but what about the remaining 5 percent of volcanoes that aren’t occurring at the plate boundary? Hawaiian volcanoes occur well within the Pacific Plate, close to 4,000 kilometers from the nearest plate boundary. How then could this chain of islands form in the middle of a plate away from its boundaries if the center of volcanic activity occurs at plate boundaries?
This was a major puzzle in relation to plate tectonics. A Canadian scientist, J. Tuzo Wilson, developed this answer. Wilson, who was one of the co-authors of the plate tectonic theory, suggested in the 1960’s that the convection in the underlying mantle produced what we now know as hot spots, and that the islands arose from magma poking through the sea floor as the plate moved over the hot spot. Wilson looked to the Hawaiian Island Chain as dramatic proof of plate movement. Stretching both to the west and north of Hawaii, there is a string of smaller islands and submerged volcanoes about 3,700 miles long. Each of these islands or seamounts (a volcanic mountain found below sea level) is said to have formed in the location of Hawaii presently. Adding to Wilson’s theory, Jason Morgan furthered the idea of the hot spot. Morgan suggested a large amount of excess energy was welling up from deep within the planet. Many scientists questioned Morgan’s theory, stating the Hawaiian chain of islands has a bend in it. Morgan claimed the reason for the bend was because the hot spot is stationary, concluding that the Pacific Plate changed direction around the age of the islands at the bend.
In simple terms, hot spots appear to be plumes of molten material that well up from a fixed position deep within the earth as the plates move above them. But it’s not that simple. Earthquakes play a major role in the hot spot formation. The location of earthquakes at depth indicates the process through which molten rocks well upward. The force comes from lighter molten rock surrounded by dense crystalline rocks. This, in return, changes the temperature and pressure, breaking the hard surrounding rock, which causes earthquakes.
The magma comes from a layer 30-37 miles below the plate. The magma then rises to the shallow chamber passing through the mantle and the crust and finally reaching the surface. As the magma reaches the surface, it cools quickly and forms pillow lava. This pillow lava rests on the contact edges of the volcano growing quickly as magma. With time, these developing volcanoes-sea mounts- become complex until finally breaking the surface of the ocean and become an island. Wilson then explained the Hawaiian volcanic chain of islands. He proposed that there was an upwelling of hot plastic rock originating deep beneath the plate. This plume rises by very slow convection and causes some melting because of decompression as the hot rock rises into the mantle. This plume and its hot spot stay in one location while the plate slides over this magma. A volcano then develops over the hot spots and then drifts away with the moving plate. This process is repeated continuously as long as the plates remain in motion and the hot spot remains active.
So how do we explain the continuous thermal plume at a constant spot? The heat source comes from the mantle or even the core. This indicates that the magma used in hot spots comes from a deeper source than volcanoes at plate boundaries. In fact, the Hawaiian hot spot has been active for 80 million years. If the theory of hot spots holds true, the island furthest away from the hot spot should be the oldest and the island closest to the hot spot should be the youngest. Also, the aging of volcanoes away from the hot spot should match the rate of plate movement. The Hawaiian Island chain indicates that the plate moved 10 cm per year. The hot spot theory also explained why the Hawaiian Islands only have active volcanoes at the southeast end of the chain. The hot spot explanation suggests that another island will soon form. The next to-be island, which is in the works, is the Loihi seamount.
Most hot spot volcanoes are effusive. This is concept explains how the islands are formed. An effusive eruption refers to the type of eruption that characterizes Hawaii. Effusive is a type of eruption in which the gas content in the magma is low and the magma is less cohesive causing the gases to boil out less violently.
New discoveries are made every day and it becomes more evident that the study of volcanoes, plate tectonics and hot spots are all very closely related to the formation of the Hawaiian Islands. So the next time you think about Hawaii, don’t just consider the sun, sand, and beaches. Think about this amazing creation of island formation through the process of hot spots and realize just how complex the world really is.
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Krista Lindemann, April 18, 2001