The Kilauea volcano on the Island of Hawaii
is one of the most active volcanoes in the world and visitors come to see the
orange rivers of glowing lava that flow usually very peacefully – though
sometimes aggressively and destructively – from the craters and vents and pours
in dramatic cascades into the spitting and steaming ocean. The Island
of Hawaii also includes two much
higher volcanic peaks, Mauna Kea and Mauna Loa ,
whose eruptive activity has continued until fairly recently.
All the Hawaiian
Islands are products of volcanism and the reason for their continued
activity is a phenomenon called a hotspot. Hotspots are thought to be plumes of
rising heat through convection currents within the mantle. The earth has 40-50
identified major hotspots and many minor ones, too. Hotspots remain in a
relatively fixed position and so as the plates drift over them their product
volcanic edifices are moved away from the up-welling magma and the volcanoes
become extinct while a new volcano is born over the hotspot. The Hawaiian Islands form a chain
of volcanoes with the Island of Hawaii comprised of the most recently active
volcanoes, Maui ’s Haleakala having been active until the recent past, and all islands becoming older as one moves northwest. On
the ocean floor, a chain of extinct volcanoes and former islands extends across
the Pacific, makes a change of direction halfway to Asia
and continues on to its subduction zone at the far western end of the Aleutian
archipelago. Other famous hotspots include the Yellowstone Caldera and Mount Etna .
What exactly are hotspots and what causes
them? Two theories are that they are either deep mantle plumes or shallow
plumes. But a new hypothesis is being tested that would suggest the origin of
at least some of earth’s hotspots is large-body impacts. According to the paper
by Johnathan T. Hagstrum, most of the earth’s major hotspots have antipodal hotspots or
large igneous provinces (LIPs) of roughly the same age. The paper examines
evidence that could suggest that a bolide of greater than 20km diameter
striking the earth in an ocean could penetrate into the mantle and disturb it
enough to create long-term volcanic activity where eruptive magma would cover
any evidence of the impact crater. Furthermore, such a large impact would
create Rayleigh shock waves that would spread around the globe and converge at
the antipode of the impact. This convergence of seismic energy could fracture
the crust sufficiently so as to lead to eruptive activity. One significant
point is that most hotspots have antipodal LIPs, which suggests that if a
large-body impact created the hotspot then the seismic energy focused at the
antipode possibly triggered the basalt floods that created the LIPs. Another
point is that in the case of continental impacts, the tensile strength of the thicker
continental crust acted as a shield that prevented the creation of an antipodal
hotspot or basalt flood. Only very large continental impact sites have
antipodal hotspots. The paper offers a lot to consider.
So how about our Hawaiian hotspot? Its
antipode is Lake Victoria in Africa ; however,
the research paper says that some antipodal hotspots are just coincidental and
in the Hawaii/Lake Victoria case this seems to be the situation.
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