The Great Mantle Plume Debate

Three conspicuous places on Earth are magmatically (derived from magma or lava) very “hot” yet bear NO relation to “plate tectonics” theory, a problem that a klatch of eager scientists addressed in the 1970s with the “mantle plume” theory. Earth’s three prototypic “hotspots” are shield-type volcano-rich Hawaii, which sits in the middle of an oceanic plate; Yellowstone, which bubbles and foams in the middle of a continental plate; and the tempestuous Iceland, which strides the mid-Atlantic Ridge. Note that Hawaii and Yellowstone are not located anywhere near the edge of a tectonic plate. Iceland, by contrast, is located at the junction of two tectonic plates but spews magma at a ferocious rate that is of a different nature than what is seen elsewhere on the mid-Atlantic Ridge.

Mauna Loa, the largest volcano on Earth. Source: http://spider.ipac.caltech.edu/staff/ cd/Pictures/mauna_loa.jpeg.	Mauna Loa from space: Source: http://www.jsc.nasa.gov/images/eol/2002/maunaloa.html.
Thermophilic (heat-loving) bacteria in a Yellowstone hot spring	Diagrammatic view of Yellowstone from space photo. “ Yellowstone currently lies above a source of magma called a hot spot or a plume. The giant volcanic depression that contains the park and its geysers and hot springs (for example, Old Faithful) formed about 100,000 to 5 million years ago. Over the last 16 million years, the North American Plate has moved westward over this plume, creating the Snake River Plain. The Yellowstonecaldera (indicated by the red outline) is a large crater-like feature covering more than 1300 square miles. It formed when an underground magma chamber collapsed 630,000 years ago.” Source: http://tapestry.usgs.gov/features/35yellowstone.html.
Laki is a volcanic system, belonging to Katla central volcano in Iceland, and was at the origin of the biggest volcanic eruption on earth in historical times (1783), in the form of a flood basalt or “ Large Igneous Province” (see SEMP Biot #217 at: http://www.semp.us/biots/biot_217.html).	A map of Iceland astride the mid-Atlantic Ridge. Source: http://earthquake.usgs.gov/4kids/ learning/images/iceland.gif.

Plate tectonic theory, developed in the 1960s and 1970s, states that the Earth's outermost layer, the lithosphere, is broken into seven large, rigid pieces called plates: “the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates. The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year--about the speed at which your fingernails grow) in relationship to each other. The plates are moving around like cars in a demolition derby, which means they sometimes crash together, pull apart, or sideswipe each other. The place where the two plates meet is called a plate boundary. Boundaries have different names depending on how the two plates are moving in relationship to each other

• crashing: convergent boundaries,
• pulling apart: divergent boundaries,
• or sideswiping: transform boundaries.”*

Tectonic plates.
Source: http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html

The evidence for plate tectonics includes the nature of the ocean floor, Earth's ancient magnetism, the distribution of volcanoes and earthquakes, the flow of heat from Earth's interior, and the worldwide distribution of plant and animal fossils.*

Mantle plume theory, which developed to address the weaknesses of plate tectonic theory, postulates the existence of hot upwellings called plumes that form somewhere deep in the mantle, perhaps as deep as the core-mantle boundary. The heated material rises because of its buoyancy, then stops rising and begins to accumulate somewhere in the asthenosphere because of similar its buoyancy. The asthenosphere is a zone of the Earth's mantle that lies beneath the lithosphere and consists of several hundred kilometers of deformable rock. The lithosphere is the outer part of the Earth, consisting of the crust and upper mantle, approximately 100 km (62 mi.) thick. Basaltic magma is derived from the spreading plume through the process of partial melting. Eventually the magma makes its way to the surface at hotspots including Hawaii, Yellowstone and Iceland.

Anatomy of a mantle plume.
Source: http://faculty.weber.edu/bdattilo//shknbk/notes/htsptplm.htm

Hawaii: Underneath oceanic crust (the situation illustrated by Hawaii), the plume spreads at the asthenosphere-lithosphere boundary and partially melts basaltic magma there. The basaltic magma then rises by buoyancy to the ocean floor where it erupts as a volcano. The oceanic plate moves while the plume stays in the same place, thereby creating a chain of volcanoes.

Oceanic volcanic activity associated with a mantle plume.
Source: http://faculty.weber.edu/bdattilo//shknbk/notes/htsptplm.htm

Magmatism associated with continentally-based mantle plume.
Source: http://faculty.weber.edu/bdattilo//shknbk/notes/htsptplm.htm

Yellowstone: Underneath continental crust (the situation at Yellowstone) the p lume spreads at the asthenosphere-lithosphere boundary where it partially melts basaltic magma. The basaltic magma rises by buoyancy to the base of continental crust where it forms pools at the base of continental crust. The continental crust melts to form granitic magma. Large magma chambers accumulate below surface. Eruptions release large amounts of lava. Emptied magma chambers may form large calderas. Basaltic magma pools eventually seep upward through fractures forming dikes and filling up calderas. This leaves a trail of progressively older granitic-type flows covered by basalt flows with active granitic flow above mantle plume, according to one source.*

Iceland: Iceland is a volcanic island built almost completely of cooled basaltic lava. According to one source, “[g]eologists believe that Iceland formed because of an unusual geological coincidence. Iceland is one of the few places in the world where a mid-ocean ridge is exposed above sea level. A mid-ocean ridge forms along the boundary of two oceanic plates, which are spreading slowly apart. As the gigantic plates move apart cracks, called fissures, form and provide a path for molten rock to move to the surface. Along the axis of the mid-ocean ridge is usually a rift valley.

“So why is Iceland above sea level? Geologists think that Iceland is above sea level because at this one place on Earth, the position of the mid-ocean ridge coincides with the position of a hotspot. A hotspot is a plume of hot mantle material that is rising up and heating the outermost layer of the Earth. The hot spot causes the crust under Iceland to bulge up and therefore raises the mid-ocean ridge well above sea level. Another famous hot spot is beneath the Hawaiian Islands. Here there is not a mid-ocean ridge, but just normal ocean crust. This hot spot has been building a series of volcanoes as the oceanic plate moves over the mantle plume.

Much of the surface of Iceland was created in eruptions of fluid lava similar to that seen erupting in the picture above. This fluid basaltic lava spreads out (think “Large Igneous Provinces”) and forms flat lava flows and gently sloping shield volcanoes (think Mauna Loa in Hawaii).”***

Disputing the Mantle Plume Theory: Without disputing the weakness of explaining Earth’s hotspots by the theory of plate tectonics, many researchers, including seismologists Don Anderson and James Natland at California Institute of Technology**, do not believe the evidence supports mantle plume theory. They concede, however, that, despite some problems with the plume theory, it will likely remain the reigning paradigm in the near future. Meanwhile, a younger generation of Earth scientists, including Ian Campbell at the University of Australia, is championing mantle plumes as the source of “Large Igneous Provinces” described in Biot #217 at: http://www.semp.us/biots/biot_217.html.

Editor’s Note: Well, at least we aretrying out new ideas without proper evidence instead of holding on to old ideas without proper evidence.

Sources:

* “Earth Floor: Plate Tectonics” at: http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html.

** “A brief history of the plume hypothesis and its competitors: Concept and controversy” by Don L. Anderson and James H. Natland at California Institute of Technology, available online at: http://www.dur.ac.uk/maple.plumes/Penrose/BookChapterPDFs/AndersonHistory_Accepted.pdf#
search='don%20l%20anderson%20james%20h%20natland'.

***Volcanoes & Iceland by Patrick M. Colgan, Northeastern University. Source:http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/volcanos.htm.