Hawaii hotspot
Country | United States |
---|---|
State | Hawaii |
Region | North Pacific Ocean |
Coordinates | 18°55′N 155°16′W / 18.92°N 155.27°W—Kamaʻehuakanaloa Seamount (formerly Loihi), actual hotspot lies about 40 km (25 mi) southeast |
The Hawaiʻi hotspot is a
While most volcanoes are created by geological activity at tectonic plate boundaries, the Hawaiʻi hotspot is located far from plate boundaries. The classic hotspot theory, first proposed in 1963 by John Tuzo Wilson,[3] proposes that a single, fixed mantle plume builds volcanoes that then, cut off from their source by the movement of the Pacific Plate, become increasingly inactive and eventually erode below sea level over millions of years. According to this theory, the nearly 60° bend where the Emperor and Hawaiian segments of the chain meet was caused by a sudden shift in the movement of the Pacific Plate. In 2003, fresh investigations of this irregularity led to the proposal of a mobile hotspot theory, suggesting that hotspots are mobile, not fixed, and that the 47-million-year-old bend was caused by a shift in the hotspot's motion rather than the plate's.
The hotspot has since been tomographically imaged, showing it to be 500 to 600 km (310 to 370 mi) wide and up to 2,000 km (1,200 mi) deep, and olivine and garnet-based studies have shown its magma chamber is approximately 1,500 °C (2,730 °F). In its at least 85 million years of activity the hotspot has produced an estimated 750,000 km3 (180,000 cu mi) of rock. The chain's rate of drift has slowly increased over time, causing the amount of time each individual volcano is active to decrease, from 18 million years for the 76-million-year-old Detroit Seamount, to just under 900,000 for the one-million-year-old Kohala; on the other hand, eruptive volume has increased from 0.01 km3 (0.002 cu mi) per year to about 0.21 km3 (0.050 cu mi). Overall, this has caused a trend towards more active but quickly-silenced, closely spaced volcanoes — whereas volcanoes on the near side of the hotspot overlap each other (forming such superstructures as Hawaiʻi island and the ancient Maui Nui), the oldest of the Emperor seamounts are spaced as far as 200 km (120 mi) apart.
Theories
Wilson's stationary hotspot theory
Wilson proposed that
This cycle of growth and dormancy strings together volcanoes over millions of years, leaving a trail of volcanic islands and seamounts across the ocean floor. According to Wilson's theory, the Hawaiian volcanoes should be progressively older and increasingly eroded the further they are from the hotspot, and this is easily observable; the oldest rock in the main Hawaiian islands, that of Kauaʻi, is about 5.5 million years old and deeply eroded, while the rock on Hawaiʻi island is a comparatively young 0.7 million years of age or less, with new lava constantly erupting at Kīlauea, the hotspot's present center.[1][5] Another consequence of his theory is that the chain's length and orientation serves to record the direction and speed of the Pacific Plate's movement. A major feature of the Hawaiian trail is a "sudden" 60-degree bend at a 40- to 50-million-year-old section of its length, and according to Wilson's theory, this is evidence of a major change in plate direction, one that would have initiated subduction along much of the Pacific Plate's western boundary.[6] This part of the theory has recently been challenged, and the bend might be attributed to the movement of the hotspot itself.[7]
Geophysicists believe that hotspots originate at one of two major boundaries deep in the Earth, either a shallow interface in the lower
Arguments for the validity of the hotspot theory generally center on the steady age progression of the Hawaiian islands and nearby features:[10] a similar bend in the trail of the Macdonald hotspot, the Austral–Marshall Islands seamount chain, located just south;[11] other
Shallow hotspot hypothesis
Another hypothesis is that melting anomalies form as a result of lithospheric extension, which allows pre-existing melt to rise to the surface. These melting anomalies are normally called "hotspots", but under the shallow-source hypothesis the mantle underlying them is not anomalously hot. In the case of the Hawaiian–Emperor seamount chain, the Pacific plate boundary system was very different around 80
Moving hotspot theory
The most heavily challenged element of Wilson's theory is whether hotspots are indeed fixed relative to the overlying tectonic plates. Drill samples, collected by scientists as far back as 1963, suggest that the hotspot may have drifted over time, at the relatively rapid pace of about 4 centimeters (1.6 in) per year during the late Cretaceous and early Paleogene eras (81–47 Mya);[18] in comparison, the Mid-Atlantic Ridge spreads at a rate of 2.5 cm (1.0 in) per year.[1] In 1987, a study published by Peter Molnar and Joann Stock found that the hotspot does move relative to the Pacific Ocean; however, they interpreted this as the result of the relative motions of the North American and Pacific plates rather than that of the hotspot itself.[19][20]
In 2021 researchers proposed a three stage Hawaii hotspot model.[21] The first stage has ridge plume interaction in which the Hawaii hotspot either fed the Izanagi-Pacific or Kula-Pacific ridge. This period involved the creation of young oceanic crust and the formation of the Meji and Detroit seamounts. The second stage involved the mutual movements of the Pacific plate and the Hawaii hotspot. It is possible, as supported by gravitational modelling, that during this period that the Hawaii hotspot drifted about 4-9 degrees to the south, in contrast to the northward Pacific Plate movement. The third stage has continued movement of the Pacific plate, with stagnation of the Hawaii hotspot.[21]
In 2001 the
The Hawaii bend was used as a classic example of how a large plate can change motion quickly. You can find a diagram of the Hawaii–Emperor bend entered into just about every introductory geological textbook out there. It really is something that catches your eye."[24]
Despite the large shift, the change in direction was never recorded by
History of study
Ancient Hawaiians
The possibility that the Hawaiian islands became older as one moved to the northwest was suspected by ancient Hawaiians long before Europeans arrived. During their voyages, seafaring Hawaiians noticed differences in erosion, soil formation, and vegetation, allowing them to deduce that the islands to the northwest (Niʻihau and Kauaʻi) were older than those to the southeast (Maui and Hawaiʻi).[1] The idea was handed down the generations through the legend of Pele, the Hawaiian goddess of volcanoes.
Pele was born to the female spirit
Modern studies
Three of the earliest recorded observers of the volcanoes were the Scottish scientists Archibald Menzies in 1794,[28] James Macrae in 1825,[29] and David Douglas in 1834. Just reaching the summits proved daunting: Menzies took three attempts to ascend Mauna Loa, and Douglas died on the slopes of Mauna Kea. The United States Exploring Expedition spent several months studying the islands in 1840–1841.[30] American geologist James Dwight Dana was on that expedition, as was Lieutenant Charles Wilkes, who spent most of the time leading a team of hundreds that hauled a Kater's pendulum to the summit of Mauna Loa to measure gravity. Dana stayed with missionary Titus Coan, who would provide decades of first-hand observations.[31] Dana published a short paper in 1852.[32]
Dana remained interested in the origin of the Hawaiian Islands, and directed a more in-depth study in 1880 and 1881. He confirmed that the islands' age increased with their distance from the southeasternmost island by observing differences in their degree of erosion. He also suggested that many other island chains in the Pacific showed a similar general increase in age from southeast to northwest. Dana concluded that the Hawaiian chain consisted of two volcanic strands, located along distinct but parallel curving pathways. He coined the terms "Loa" and "Kea" for the two prominent trends. The Kea trend includes the volcanoes of
Dana's work was followed up by the 1884 expedition of geologist
In 1912 geologist
In the 1970s, the Hawaiian seafloor was mapped using ship-based
Characteristics
Position
The Hawaiʻi hotspot has been imaged through
Temperature
Indirect studies found that the magma chamber is located about 90–100 kilometers (56–62 mi) underground, which matches the estimated depth of the Cretaceous Period rock in the oceanic lithosphere; this may indicate that the lithosphere acts as a lid on melting by arresting the magma's ascent. The magma's original temperature was found in two ways, by testing garnet's melting point in lava and by adjusting the lava for olivine deterioration. Both USGS tests seem to confirm the temperature at about 1,500 °C (2,730 °F); in comparison, the estimated temperature for mid-ocean ridge basalt is about 1,325 °C (2,417 °F).[44]
The surface heat flow anomaly around the Hawaiian Swell is only of the order of 10 mW/m2,[45][46] far less than the continental United States range of 25–150 mW/m2.[47] This is unexpected for the classic model of a hot, buoyant plume in the mantle. However, it has been shown that other plumes display highly variable surface heat fluxes and that this variability may be due to variable hydrothermal fluid flow in the Earth's crust above the hotspots. This fluid flow advectively removes heat from the crust, and the measured conductive heat flow is therefore lower than the true total surface heat flux.[46] The low heat across the Hawaiian Swell indicates that it is not supported by a buoyant crust or upper lithosphere, but is rather propped up by the upwelling hot (and therefore less-dense) mantle plume that causes the surface to rise[45] through a mechanism known as "dynamic topography".
Movement
Hawaiian volcanoes drift northwest from the hotspot at a rate of about 5–10 centimeters (2.0–3.9 in) a year.[18] The hotspot has migrated south by about 800 kilometers (497 mi) relative to the Emperor chain.[25] Paleomagnetic studies support this conclusion based on changes in Earth's magnetic field, a picture of which was engrained in the rocks at the time of their solidification,[48] showing that these seamounts formed at higher latitudes than present-day Hawaii. Prior to the bend, the hotspot migrated an estimated 7 centimeters (2.8 in) per year; the rate of movement changed at the time of the bend to about 9 centimeters (3.5 in) per year.[25] The Ocean Drilling Program provided most of the current knowledge about the drift. The 2001[49] expedition drilled six seamounts and tested the samples to determine their original latitude, and thus the characteristics and speed of the hotspot's drift pattern in total.[50]
Each successive volcano spends less time actively attached to the plume. The large difference between the youngest and oldest lavas between Emperor and Hawaiian volcanoes indicates that the hotspot's velocity is increasing. For example, Kohala, the oldest volcano on Hawaiʻi island, is one million years old and last erupted 120,000 years ago, a period of just under 900,000 years; whereas one of the oldest, Detroit Seamount, experienced 18 million or more years of volcanic activity.[23]
The oldest volcano in the chain, Meiji Seamount, perched on the edge of the Aleutian Trench, formed 85 million years ago.[51] At its current velocity, the seamount will be destroyed within a few million years, as the Pacific Plate slides under the Eurasian Plate. It is unknown whether the seamount chain has been subducting under the Eurasian Plate, and whether the hotspot is older than Meiji Seamount, as any older seamounts have since been destroyed by the plate margin. It is also possible that a collision near the Aleutian Trench had changed the velocity of the Pacific Plate, explaining the hotspot chain's bend; the relationship between these features is still being investigated.[25][52]
Magma
The composition of the volcanoes' magma has changed significantly according to analysis of the
Almost all magma created by the hotspot is
Eruptive frequency and scale
There is significant evidence that lava flow rates have been increasing. Over the last six million years they have been far higher than ever before, at over 0.095 km3 (0.023 cu mi) per year. The average for the last million years is even higher, at about 0.21 km3 (0.050 cu mi). In comparison, the average production rate at a mid-ocean ridge is about 0.02 km3 (0.0048 cu mi) for every 1,000 kilometers (621 mi) of ridge. The rate along the Emperor seamount chain averaged about 0.01 cubic kilometers (0.0024 cu mi) per year. The rate was almost zero for the initial five million or so years in the hotspot's life. The average lava production rate along the Hawaiian chain has been greater, at 0.017 km3 (0.0041 cu mi) per year.[25] In total, the hotspot has produced an estimated 750,000 cubic kilometers (180,000 cu mi) of lava, enough to cover California with a layer about 1.5 kilometers (1 mi) thick.[5][18][54][55][56]
The distance between individual volcanoes has shrunk. Although volcanoes have been drifting north faster and spending less time active, the far greater modern eruptive volume of the hotspot has generated more closely spaced volcanoes, and many of them overlap, forming such superstructures as Hawaiʻi island and the ancient Maui Nui. Meanwhile, many of the volcanoes in the Emperor seamounts are separated by 100 kilometers (62 mi) or even as much as 200 kilometers (124 mi).[55][56]
Topography and geoid
A detailed topographic analysis of the Hawaiian–Emperor seamount chain reveals the hotspot as the center of a topographic high, and that elevation falls with distance from the hotspot. The most rapid decrease in elevation and the highest ratio between the topography and geoid height are over the southeastern part of the chain, falling with distance from the hotspot, particularly at the intersection of the Molokai and Murray fracture zones. The most likely explanation is that the region between the two zones is more susceptible to reheating than most of the chain. Another possible explanation is that the hotspot strength swells and subsides over time.[37]
In 1953, Robert S. Dietz and his colleagues first identified the swell behavior. It was suggested that the cause was mantle upwelling. Later work pointed to tectonic uplift, caused by reheating within the lower lithosphere. However, normal seismic activity beneath the swell, as well as lack of detected heat flow, caused scientists to suggest dynamic topography as the cause, in which the motion of the hot and buoyant mantle plume supports the high surface topography around the islands.[45] Understanding the Hawaiian swell has important implications for hotspot study, island formation, and inner Earth.[37]
Seismicity
The Hawaii hotspot is a highly active
Volcanoes
Over its 85 million year history, the Hawaii hotspot has created at least 129 volcanoes, more than 123 of which are
Volcanic characteristics
Hawaiian volcanoes are characterized by frequent
Landslides
The Hawaiian islands are carpeted by a large number of landslides sourced from volcanic collapse. Bathymetric mapping has revealed at least 70 large landslides on the island flanks over 20 km (12 mi) in length, and the longest are 200 km (120 mi) long and over 5,000 km3 (1,200 cu mi) in volume. These debris flows can be sorted into two broad categories:
Slumps tend to be deeply rooted in their originators, moving rock up to 10 km (6 mi) deep inside the volcano. Forced forward by the mass of newly ejected volcanic material, slumps may creep forward slowly, or surge forward in spasms that have caused the largest of Hawaii's historical earthquakes, in 1868 and 1975. Debris avalanches, meanwhile, are thinner and longer, and are defined by volcanic amphitheaters at their head and hummocky terrain at their base. Rapidly moving avalanches carried 10 km (6 mi) blocks tens of kilometers away, disturbing the local water column and causing a tsunami. Evidence of these events exists in the form of marine deposits high on the slopes of many Hawaiian volcanoes,[65] and has marred the slopes of several Emperor seamounts, such as Daikakuji Guyot and Detroit Seamount.[23]
GPS measurements on the eastern flank of Hawaii Island over a 5 year epoch show the pattern of collapse with velocities of up to 15 cm/year (5.9 in/year) relative to the Pacific Plate[67]
Evolution and construction
Hawaiian volcanoes follow a well-established life cycle of growth and erosion. After a new volcano forms, its lava output gradually increases. Height and activity both peak when the volcano is around 500,000 years old and then rapidly decline. Eventually it goes dormant, and eventually extinct. Weathering and erosion gradually reduce the height of the volcano until it again becomes a seamount.[61]
This life cycle consists of several stages. The first stage is the
As the seamount slowly grows, it goes through the
The volcano enters the subaerial subphase once it is tall enough to escape the water. Now the volcano puts on 95% of its above-water height over roughly 500,000 years. Thereafter eruptions become much less explosive. The lava released in this stage often includes both pāhoehoe and ʻaʻā, and the currently active Hawaiian volcanoes, Mauna Loa and Kīlauea, are in this phase. Hawaiian lava is often runny, blocky, slow, and relatively easy to predict; the USGS tracks where it is most likely to run, and maintains a tourist site for viewing the lava.[61][69]
Mechanical collapse, indicated by large submarine landslides adjacent to landslide scars on the islands, is an ongoing process that shapes the early phases of volcano construction for each of the islands.
After the subaerial phase the volcano enters a series of postshield stages involving mechanical collapse creating subsidence and erosion, becoming an atoll and eventually a seamount. Once the Pacific Plate moves it out of the 20 °C (68 °F) tropics, the reef mostly dies away, and the extinct volcano becomes one of an estimated 10,000 barren seamounts worldwide.[61][70] Every Emperor seamount is a dead volcano.
Coral reef development on Hawaiian Hotspot islands
See also
- List of volcanic hotspots
- List of volcanoes in the Pacific Ocean
- List of volcanoes in the United States
- Maui Nui
- Types of volcanic eruptions
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External links
- Pele — Goddess of Fire Archived 26 March 2017 at the Wayback Machine: Details Pele's full story, according to Hawaiian myths.
- The long trail of the Hawaiian hotspot: USGS article on the Hawaiian island chain.
- Evolution of Hawaiian Volcanoes: USGS article on the evolution of Hawaiian volcanoes over time.
- The short film Inside Hawaiian Volcanoes (1989) is available for free viewing and download at the Internet Archive.