Volcanic Processes
Subduction is the process in which one plate is pushed downward beneath another plate into the underlying mantle when plates move towards each other. The plate that is denser will slide under the thicker, less dense plate. Faulting occurs in the process. It is the process in which rocks break and move or are displaced along the fractures. The subducted plate usually moves in jerks, resulting in earthquakes. The area where the subduction occurs is the subduction zone. A long, narrow, deep depression forms in this area. It is called an oceanic trench. The jerky movement, as well as the friction between the plates causes much heat, and together with the heat from the mantle and from radioactive decay, causes the subducted plate to melt. Magma is produced by the melting plate. It rises through fractures in the crust and reaches the surface to form volcanoes. In the end, the heat may be so intense that large areas of the crust are melted, forming granites just below the surface. The boundary of such plate movements is 'destructive' as part of the plate melts during subduction and is destroyed.
As plates move towards each other, the opposing force between them is so great that the massive folding or the bending of rocks also occurs at the edges of the less dense plate. A range of Fold Mountains, such as the Himalayas, is formed
SEA FLOOR SPREADING
Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called an oceanic ridge. The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. The ocean floor gradually extends and thus the size of these plates
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.
Hotspots
Many of the islands that dot the center of the Pacific Ocean are made up of active, dormant, or extinct volcanoes, whose geologic histories are characteristic of "hot spot" volcanism. The active volcanism is limited to a localized region (or "spot") of the volcanic chain. Hot-spot island chains include the Hawaiian, Marquesas, Society, Pitcairn, Samoan, and Galapagos archipelagos.
Volcanoes are also common around the edge of the Pacific Ocean. Examples include the Cascade volcanoes (e.g., Mt. St. Helens), the Aleutians, the volcanoes in Japan (e.g., Mt. Fuji), the volcanoes of the Philippines (e.g., Mt. Pinatubo), and many others. These volcanoes have earned the Pacific rim the nickname "the Pacific ring of fire". As the name "ring" implies, these volcanic islands are simultaneously active along a line, whereas hot spot volcanism is active only at small "spots." The Pacific rim volcanoes are related to plate boundary processes.
The surface of the Earth is made up of about 14 rigid plates, which move relative to each other like boards floating on a pond. Where two plates collide, one plate usually sinks beneath the other deep into the mantle of the Earth. The sinking plate melts to form magma, which rises and forms volcanoes on the overriding plate. This process forms the ring of volcanoes around most of the Pacific Ocean.
Where two plates move apart, magma rises and fills in the space. This kind of volcanism occurs deep on the ocean floor. Hot spot volcanoes are special because they can be found both within plates (e.g., Hawaii, Marquesas, and Society islands) and along or near plate boundaries (e.g., Galapagos, Iceland, and Azores islands).
Many of the islands that dot the center of the Pacific Ocean are made up of active, dormant, or extinct volcanoes, whose geologic histories are characteristic of "hot spot" volcanism. The active volcanism is limited to a localized region (or "spot") of the volcanic chain. Hot-spot island chains include the Hawaiian, Marquesas, Society, Pitcairn, Samoan, and Galapagos archipelagos.
Volcanoes are also common around the edge of the Pacific Ocean. Examples include the Cascade volcanoes (e.g., Mt. St. Helens), the Aleutians, the volcanoes in Japan (e.g., Mt. Fuji), the volcanoes of the Philippines (e.g., Mt. Pinatubo), and many others. These volcanoes have earned the Pacific rim the nickname "the Pacific ring of fire". As the name "ring" implies, these volcanic islands are simultaneously active along a line, whereas hot spot volcanism is active only at small "spots." The Pacific rim volcanoes are related to plate boundary processes.
The surface of the Earth is made up of about 14 rigid plates, which move relative to each other like boards floating on a pond. Where two plates collide, one plate usually sinks beneath the other deep into the mantle of the Earth. The sinking plate melts to form magma, which rises and forms volcanoes on the overriding plate. This process forms the ring of volcanoes around most of the Pacific Ocean.
Where two plates move apart, magma rises and fills in the space. This kind of volcanism occurs deep on the ocean floor. Hot spot volcanoes are special because they can be found both within plates (e.g., Hawaii, Marquesas, and Society islands) and along or near plate boundaries (e.g., Galapagos, Iceland, and Azores islands).
The basic characteristics of hot spot volcanism can be observed using the best scientific tool there is--your eyes! Think about the different Hawaiian islands you have visited. Which one appeared to be the youngest? (The Big Island appears to be the youngest because Kilauea is erupting now and Mauna Loa last erupted in 1984.) Which island is the oldest? (This is a trick question because there are many ways to answer it. Of the islands we can visit, Kauai has the deepest eroded valleys and the most coral reefs, which are indicative of age in the process of island formation and degradation. The oldest rocks outcropping on Kauai are about 5 million years old--older than the oldest hominid fossils in Africa! The extinct volcano underlying the coral atoll of Midway is 26 million years old. If you could remove all of the water in the Pacific Ocean, you would find that the Hawaiian islands are peaks on a 10,000-foot-high ridge that extends 2,100 miles west-northwest from Hawaii, and then bends northward and extends for another 1,400 miles. The "elbow" in the ridge is about 43 million years old. The northernmost end is about 80 million years old and was formed during the time of the dinosaurs.)
The Society Islands follow the same pattern as Hawaii, with the youngest volcano at the southeast end of the island group and the oldest volcano at the northwest end. These observations led to the idea that deep within the earth, well below the "plate traffic," are some sources of magma, called "hot spots", that are fixed relative to each other. Volcanoes form on the plate above the hot spot. As the plate moves over the hot spot, new volcanoes form, and the old ones, which no longer overlie the hot spot, become extinct.
In Hawaii, the hot spot is currently under the active volcanoes of Mauna Loa, Kilauea, and Loihi, a submarine volcano southeast of the Big Island. From the geometry of the islands and submarine ridges on the Pacific Plate, we know that the Pacific Plate has been moving west-northwest for the last 43 million years. The Pacific Plate moved directly northward from about 43 to 80 million years ago. Given the distances and times mentioned in this article, what is the average speed of the Pacific Plate relative to the Hawaii hot spot during the two time intervals?
A hot spot volcano making news recently is Fogo in the Cape Verde Islands off of the west coast of Africa. Fogo is a volcanic island that began erupting last week after lying dormant since 1951. One of our HVO scientists is part of the international team assessing the volcanic hazards of Fogo. The age relations in the Cape Verde Islands are somewhat confusing, but a basic observation is that Fogo and the other westernmost Cape Verde islands have active or dormant volcanoes. The easternmost islands are nearly flat and their volcanoes are extinct. Which direction is the African Plate moving relative to the Cape Verde hot spot? Now when you travel around Polynesia and other hot spot islands or just travel with a map and your imagination, you can observe the degree of erosion, reef growth, and the recency of volcanic activity and estimate which way the plates are moving relative to the hot spots. Note, this doesn't work well for Samoa, perhaps because the islands are near a place where the plates are coming together
The Society Islands follow the same pattern as Hawaii, with the youngest volcano at the southeast end of the island group and the oldest volcano at the northwest end. These observations led to the idea that deep within the earth, well below the "plate traffic," are some sources of magma, called "hot spots", that are fixed relative to each other. Volcanoes form on the plate above the hot spot. As the plate moves over the hot spot, new volcanoes form, and the old ones, which no longer overlie the hot spot, become extinct.
In Hawaii, the hot spot is currently under the active volcanoes of Mauna Loa, Kilauea, and Loihi, a submarine volcano southeast of the Big Island. From the geometry of the islands and submarine ridges on the Pacific Plate, we know that the Pacific Plate has been moving west-northwest for the last 43 million years. The Pacific Plate moved directly northward from about 43 to 80 million years ago. Given the distances and times mentioned in this article, what is the average speed of the Pacific Plate relative to the Hawaii hot spot during the two time intervals?
A hot spot volcano making news recently is Fogo in the Cape Verde Islands off of the west coast of Africa. Fogo is a volcanic island that began erupting last week after lying dormant since 1951. One of our HVO scientists is part of the international team assessing the volcanic hazards of Fogo. The age relations in the Cape Verde Islands are somewhat confusing, but a basic observation is that Fogo and the other westernmost Cape Verde islands have active or dormant volcanoes. The easternmost islands are nearly flat and their volcanoes are extinct. Which direction is the African Plate moving relative to the Cape Verde hot spot? Now when you travel around Polynesia and other hot spot islands or just travel with a map and your imagination, you can observe the degree of erosion, reef growth, and the recency of volcanic activity and estimate which way the plates are moving relative to the hot spots. Note, this doesn't work well for Samoa, perhaps because the islands are near a place where the plates are coming together
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