2.5: Seamounts, Hot Spots, and Hydrothermal Vents

Seamounts and Hot Spots

The ocean floor is dotted with seamounts, some isolated and some in chains. Seamounts are underwater volcanoes, and most are much younger than the oceanic crust on which they formed. If a seamount gets large enough to break the ocean surface, it becomes a volcanic island. Some seamounts are formed from magma rising at a divergent boundary, and as the plates move apart, the seamounts move with them, which can result in a seamount chain. Other seamounts form from the rising magma at an ocean-ocean subduction zone; these include the Aleutians, extending from Alaska to Russia, and the Lesser Antilles in the eastern part of the Caribbean. Sometimes the crust on which an island or seamount sits will subside, taking the seamount with it. As this happens, the top of the seamount can become eroded flat, and these flat-topped seamounts are then called tablemounts or guyots.

However, some seamounts are formed far away from plate boundaries, in places where we would not usually expect much volcanic activity. Some seamounts and ocean islands are formed above a mantle plume or hot spot — a place where hot mantle material rises in a stationary and semi-permanent plume, and affects the overlying crust. Mantle plumes are thought to rise at approximately 10 times the rate of mantle convection. The ascending column may be on the order of kilometers to tens of kilometers across, but near the surface it spreads out to create a mushroom-style head that is several tens to over 100 kilometers across. Near the base of the lithosphere (the rigid part of the mantle), the mantle plume (and possibly some of the surrounding mantle material) partially melts to form magma that rises to feed volcanoes.

A great example of seamounts created from a hot spot includes the Hawaiian and Emperor Seamount island chains in the Pacific Ocean (Figure \(\PageIndex{1}\)). The oldest of the Hawaiian/Emperor seamounts is dated at around 80 Ma, and it is situated on oceanic crust aged around 90 to 100 Ma. The volcanic rock making up these islands gets progressively younger toward the southeast, culminating with the island of Hawaii itself, which consists of rock that is almost all younger than 1 Ma. It appears that a stationary plume of hot upwelling mantle material is the source of the Hawaiian volcanism, and that the ocean crust of the Pacific Plate is moving toward the northwest over this hot spot. A seamount will be formed through volcanic activity over the hot spot, then the plate will move and displace the seamount before the hot spot produces the next seamount, and so on. In this way, over time, the seamounts are formed in chains. Near the Midway Islands, the chain takes a pronounced change in direction, from northwest-southeast for the Hawaiian Islands to nearly north-south for the Emperor Seamounts. This change is widely ascribed to a change in direction of the Pacific Plate moving over the stationary mantle plume, but it is also possible that the Hawaiian mantle plume has not actually been stationary throughout its history, and in fact moved at least 2,000 km south over the period between 81 and 45 Ma.

Figure \(\PageIndex{1}\) The Hawaiian Islands/Emperor Seamount chain, with ages of selected structures. This chain has formed as the Pacific Plate moved northwest over a hot spot (Steven Earle, “Physical Geology”).

Since most mantle plumes are beneath the oceans, the early stages of volcanism typically take place on the seafloor. Over time, very large islands may form like those in Hawaii. In fact, if you measure it from its base on the seafloor to its summit, Mauna Loa on the island of Hawaii is the largest mountain on Earth, rising 9700 m (in comparison, the elevation of the summit of Mt. Everest is 8848 m). While the island of Hawaii is the youngest in the chain, there is actually a new volcano named Loihi, that is still submerged at a depth of 980 m SE of Hawaii, and may one day become a new Hawaiian island when it emerges 10,000 – 100,000 years from now.

There is evidence of many such mantle plumes around the world. Most are within the ocean basins, including places like Hawaii, Iceland, and the Galapagos Islands, but some are under continents. One example is the Yellowstone hot spot in the west-central United States, and another is the one responsible for the Anahim Volcanic Belt in central British Columbia. It is evident that mantle plumes are very long-lived phenomena, lasting for at least tens of millions of years, possibly for hundreds of millions of years in some cases.

 

Hydrothermal Vents

 

A whole new ecosystem reliant on the processes of plate tectonics was discovered on the deep seafloor of the Galapagos Rift in 1977. The deep sea submersible Alvin was exploring in 2500 m of water when it encountered unusually warm water. Following the temperature gradient, Alvin eventually discovered jets of superheated water coming from out of the seafloor at temperatures up to 350^o C (the normal temperature for water at this depth would be 2-4^o C). The water poured out of cracks in the crust, as well as through tall chimneys up to 20 m high and 1 m wide, and as it emerged it took on the appearance of thick black smoke, These fissures were named hydrothermal vents, and the chimneys “black smokers”.

 

Figure \(\PageIndex{2}\) A black smoker in the High Rise portion of the Endeavour hydrothermal vents (NOAA).

 

To create these vents, water percolates into the crust where there are plumes of magma close to the surface. The water gets superheated by the magma, then moves back to the surface through convection and is released through the vents. The hot water dissolves minerals from the surrounding rock, and as the water emerges and cools, the dissolved minerals and inorganic sulfides precipitate out as small particles and turn the water black, leading to the black “smoke” coming from the vents. Precipitation of these minerals also create the tall chimneys characteristic of many hydrothermal vents.

 

Since their original discovery in the Galapagos Rift, hydrothermal vents have been located across the globe along oceanic ridges where there is shallow crust and a lot of tectonic activity (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{2}\) Distribution of hydrothermal vents (red dots) and their association with plate boundaries (By DeDuijn (Own work) [CC BY-SA 4.0], via Wikimedia Commons).

 

As unexpected as it was to discover these vent systems, even more surprising was the fact that they were teeming with life. The vents are surrounded by a diverse range of previously unknown organisms, including giant tube worms over 2 m long, crabs, shrimp, giant mussels, and mats of bacteria. How is it that such a diverse community can exist in the ocean depths, far removed from the sunlight that supports photosynthesis and primary production in most other ecosystems? The answer is that the water exiting the vents is rich in hydrogen sulfide (H₂S), oxygen and CO₂. The bacteria surrounding the vents use energy from the oxidation of sulfur compounds like H₂S to form carbohydrates from CO₂ and water. This is the process of chemosynthesis, and the bacteria are very productive as these reactions occur faster at high temperatures. The bacteria then represent the base of the food web, as other organisms eat the bacteria, or derive their energy from bacteria living symbiotically within their tissues. Watch the video below for more about hydrothermal vents.

 

 

 

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca