IntroductionPlate tectonic theory has come a longway in its relatively short history.Building on Alfred Wegener’sobservations on continental drift, itwas only in the 1960s thatseismologists and geophysicistsobtained sufficient scientific data toestablish the underlying mechanics ofEarth movement (broadly vindicatingWegener’s ideas). In the succeeding 40years it has since been widely acceptedthat the globe’s oceanic crust (65% ofthe Earth’s surface) and continentalcrust (35% of the Earth’s surface) arebuoyed along on an extremely viscousmass of molten rock (comprising theupper mantle, often described as‘plastic’ in its flow characteristics); andthat underlying convection currentsare the main motive force for themovement of tectonic plates (Figure 1).And yet, while the circumstantial andscientific evidence for plate tectonictheory is almost overwhelming (seeFurther Reading), there is one aspectthat remains potentially problematic –explaining the origins of the world’shot spot volcanism (Figure 2).

BackgroundThe Dictionary of Physical Geographydefines a hot spot as:

‘A small area of the Earth’s crust where anunusually high heat flow is associated withvolcanic activity. Of approximately 125 hot

spots thought to have been active over thepast 10 million years most are located wellaway from plate boundaries.’

The chain of volcanic peakscomprising the Hawaiian Islands andthe volcanic and associatedhydrothermal activity of theYellowstone region in Wyomingprovide two examples of currentlyactive hot spots that fit the definition.There are some exceptions to thegeneral rule – Iceland, for example, isformed by an unusually active hot spotsituated directly below the mid-Atlantic ridge. An understanding ofhot spots can also be used to explainrelict features that, nevertheless, haveprofound geomorphological (andperhaps historical) significance. Onesuch example is the Deccan Traps inNW India, a layered mass of floodbasalt up to 2,000 metres thick andcovering an area of 500,000 sq km. It isknown that these lavas eruptedapproximately 65 million years ago,and some commentators believe thatthe enormous amounts of ash andgases that were ejected into theatmosphere in such a large eventwould have had a global impact on theclimate, perhaps even contributing tothe extinction of dinosaurs atapproximately the same time.There is still some debate as to howhot spots are formed. In part, this

stems from the fact that directobservation of the interior of the Earthsimply isn’t possible, and that mostevidence relies on the interpretation ofhow seismic waves travel through themantle. Even the deepest drillingproject has probed only 12 km or sointo the crust (the Kola SuperdeepBorehole in Russia), at which depththe drill bit was threatening to melt intemperatures of 180° C or more.Nevertheless, the available informationwould seem to indicate the existence ofregions of superheating within themantle where strong convectioncurrents raise mantle plumes, or broaddomes of plastic rock up to 1,000 kmacross. These plumes push upwards tothe surface in much the same way thatthe blobs of translucent wax rise in alava lamp. On nearing the surface theplastic mantle plume encounterssignificantly lower pressures andbecomes molten; and in certaincircumstances this molten rock willpierce the rigid (oceanic orcontinental) crust above. Where a hotspot coincides with the crustalweakness of a divergent margin, thenthe outpourings of lava can besignificant, as was the case with theDeccan Traps and continues to be thecase with Iceland today (situated, as itis, some 2.5 km above the prevailingsummit of the mid ocean ridge andcovering 100,000 sq. km).

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Hot Spots in Plate Tectonics –Evolution of a Theory

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Figure 2: Hot spot distribution

Hot spot theory was first proposed byJ. Tuzo Wilson in 1963, and dozens ofsubsequent studies appeared tosupport his idea of deep origin,relatively stable, heat sources toaccount for volcanism that could notbe otherwise neatly explained by platetectonic theory. Even somewhatunexpected sources of evidenceseemed to bear out his conclusions –for example, satellite imagery whichcan show the swelling of the crust inthe broad region around a hot spot. Inthis way it is known that Iceland andthe Hawaiian Islands, for example, areatop more generalised ‘bulges’ in theoceanic crust which are much largerthan the volcanic islands themselves,just as would be expected above alarge mantle plume.

More recently, however, there havebeen suggestions in the scientificcommunity that Wilson’s beguilinglyneat theory should come undergreater scrutiny. Of particularconcern are several aspects of hot spot‘proof’ that might not stand up tofurther analysis.

Static hot spots The classic explanation for the bendin the Hawaiian Island and EmperorSeamount chain is that the PacificPlate changed directionapproximately 45 million years ago;however, modelling actual platemovements over time suggests that achange in plate direction alone cannotexplain the bend; and that the plume

itself must have shifted location. Thislatter conclusion has been supportedby studies of palaeomagnetic andradiometric age data from ocean corestaken in the vicinity of the EmperorSeamount.

Lithology Analysis of rocks from the Icelandiccrust shows that they formed at atemperature lower than would beexpected from a superheated mantleplume.

Simulations Computer modelling is, as yet, unableto demonstrate how a plume couldaccount for the significant volumes oflava that are found in basalt fields ofthe Deccan Traps.

Depth of plume Insufficient evidence exists to showthat mantle plumes emanate fromdeep within the Earth at the core-mantle boundary, as the theoristswould suggest.

Alternative theoryIn 2003 G. Foulger wrote in a debateon the Geological Society website thatthe number of modifications andelaborations necessary to make thehot spot theory apply in all cases was a‘sign of a hypothesis in trouble’(http://geolsoc.org.uk), and that thereare valid grounds for challenging it.Instead, she proposes that a viableexplanation for anomalous volcanism(incorporating all the currentexemplars of hot spot activity) couldbe found by looking at weaknessesthat exist in the tectonic platesthemselves. It is argued that all plateswill have ‘scars’ from formercollisions or divergence, and thatlarger plates such as the Pacific will bestretched towards the centre as theedges are pulled down intosubduction zones. When these morevulnerable parts of the crust pass overslabs of previously subducted materialthat melt easily in conditions oflowered pressure, the stage is set forvolcanic activity on a scale and of atype observed at what are now knownas hot spots.

Whichever theory provides thecorrect explanation for the volcanicactivity experienced remote fromplate boundaries (or in unexpectedvolumes as in Iceland), there can beno doubt as to the geographicalimportance of the igneous activitythat has taken place there.

Hawaiian IslandsHawaii is the 50th of the United Statesand takes its name from the largestisland (called Hawaii or the BigIsland), but comprises 19 islands andatolls in total (Figure 3). The islandsare all volcanic in origin and show adistinct trend in age the further youtravel from the Big Island, where theonly active volcanoes in the state are tobe found. One of these volcanoes,Kilauea, has been constantly eruptingsince 1983; while 30 km offshore theLoihi Seamount promises to reach sealevel within the next 100,000 years.The basaltic lava flows associated withthese volcanoes prove to be more of aninconvenience than a significanthazard, and examples of a’a andpahoehoe lava flows, as well as lavatubes and other volcanic phenomena,are widespread and accepted as part ofthe landscape.

The benign volcanism of the HawaiianIslands has created conditions that areof considerable interest to thegeographer:

Scientific research Because Hawaii is almost 4,000 kmfrom the nearest continental landmassit offers ideal conditions forastronomical and atmosphericobservations, far from localisedpollution sources. In this way theScripps Institution of Oceanography’sMauna Loa observatory has been ableto log increasing global levels of carbondioxide since the 1950s (Figure 4),making a valuable contribution to ourunderstanding of the enhancedgreenhouse effect (webcam views fromMauna Loa can be seen at :http://www.mlo.noaa.gov/LiveCam/FcamMK.htm). The constant eruptionsince 1983 of Kilauea (the nametranslates as ‘much spewing’) has alsoafforded geophysicists the opportunityto study the chemistry andcomposition of molten rock, therebyfurthering our understanding ofextrusive igneous activity.

Strategic significance Quite apart from the fact that, withoutvolcanism, Hawaii wouldn’t exist at all,the islands play an important role inUS geopolitics. At one time they werea major staging point for trade, andPearl Harbor continues to be the homeport of the US Pacific Fleet.

Agriculture The combination of mild tropicalclimate, high annual rainfall totals anddecomposed volcanic rock has

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produced extremely fertile soils inparts of the islands. Hawaii’s vast sugarand pineapple plantations aretestament to the importance of theprimary sector (agriculture accountsfor 47% of the state’s land area),augmented by the production ofnursery plants, coffee and macadamianuts.

Biogeography Hawaii’s extreme isolation meant thatcolonisation of the islands by newspecies took many thousands of years.One estimate suggests that new speciesbecame established in Hawaii (undertheir own power, or blown by storms)only once every 35,000 years.Evolutionary adaptation and selectionresulted in plants and animals thatwere highly adapted to theenvironment that they encountered,and there are many species that arenow unique to the islands.Considerable variation in flora andfauna has been further encouraged bythe close proximity of diverse biomes(from tropical rainforest at sea level tohigh alpine deserts on the volcanoflanks).

One of the major biological concernsabout Hawaii is the introduction ofnon-native animals that hasaccompanied recent humancolonisation. With native species ill-prepared to compete with invasivealiens, the state Department of Landand Natural Resources claims that‘hundreds or possibly thousands ofunique species are already extinct’,with hundreds more criticallyendangered.

Tourism Hawaii is ideally positioned to benefitfrom domestic US tourism. Offering atropical climate, few security concerns,and a familiar currency, it is littlesurprise that over 65% of the islands’visitors come from the continentalUnited States. The second largestgroup, at an average of 20% of visitors,comes from Japan. The tourism sectoraccounts for more than 25% of thelocal economy and is a significantsource of employment. Suchdependence on one industry leaves thestate vulnerable to changes inconsumer trends and the globaleconomy, but tourism in Hawaii hasshown remarkable resilience over time– not least due to the unique volcaniclandscape that the islands offer.

Yellowstone National ParkYellowstone National Park forms thecore of the Greater YellowstoneEcosystem, which is one of the largestintact temperate zone ecosystems onEarth. It also sits atop a colossalchamber of molten rock which isunder incredibly high pressure. Thelast significant eruption to occur herewas 650,000 years ago, at which time acaldera 30 miles wide by 45 miles longwas formed, and much of the westernUnited States was covered inpyroclastic material (enough material

was ejected – 1,000 cubic km – tocover the entire country in a layer 12cm thick).

The United States Geological Survey(http://www.usgs.gov) explains howthe hot spot underneath the NorthAmerican plate has built up an

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Figure 4: Atmospheric carbon dioxidelevels, 1959-2004. A clear trend ofincreasing concentration of carbondioxide in the atmosphere since 1959 isshown by this data from Mauna Loa(rising from 315ppmv to 379ppmv in2004). The cyclic fluctuation is causedby the seasonal variation in the growthof vegetation in the NorthernHemisphere.

Figure 5: Taro production. The taro/kalo, also known as the ‘potato of the tropics’,was once the staple crop of the indigenous Hawaiians. Labour intensive wet-planted kalo (cultivated in a fashion similar to padi rice), such as still exists inpockets on the islands, provided a good source of vitamins and protein and was alsoknown for its medicinal value.

Figure 6: A 2-metre high Haleakalasilversword – a highly adapted species.The Haleakala silversword is aremarkable plant that grows only athigher altitudes on the flanks of theHaleakala Crater on the island ofMaui. The sword-shaped leaves havesilvery hairs that reflect light and heatand insulate the plants from the strongsolar rays and dryness in the area.They are similar to the Yucca plant, inthat they live for many years and thenbloom at the end of their lifespan. Theplants can live for 30 to 40 years, so it isfairly rare to see a silversword in bloom.

accumulation of magma near the baseof the plate, where it has melted rocksfrom the lower regions of the crust.These melts, in turn, rise closer to thesurface to form large reservoirs ofpotentially explosive rhyolite magma.And it is rhyolitic magma which hascaused the episodic cataclysms notedin the geological record (withapparently regular eruptions at 2.1million years ago, 1.3 million years agoand 650,000 years ago).

The potential destruction that wouldfollow an equivalent eruption atYellowstone today has excited theinterest of hazards expert Bill McGuireof University College London. As partof his global geomorphological riskassessment, and especially in light ofunusual hydrothermal activity notedin and around Yellowstone in 2003, hepredicted that a similar-sized eventcould bring about a volcanic winter,with global temperatures dropping byas much as 10° C as sunlight wasblocked by thick volcanic clouds. Thiswould prevent plants fromphotosynthesising, disrupting foodchains and could even lead to man’sextinction. The USGS agrees that, atsome unspecified point in the future,another eruption is inevitable. Such anoccurrence would bring with it ‘globalconsequences that are beyond humanexperience and impossible toanticipate fully’. Given that the USGSestimates the force of Yellowstone’slast eruption at 10,000 times aspowerful as that of Mount SaintHelens in 1980, the threat is certainlyworth monitoring.

Hot Spot HazardsIt has been shown that hot spots arewidely distributed across the Earth, andthat hot spot activity has beenexperienced throughout the geologicalrecord. In the more recent past, ashuman settlements have become moredispersed and therefore in greaterproximity to hot spots, these naturallyoccurring processes have come torepresent a significant natural hazard,owing to the potential threat that theynow pose to life and property. Themagnitude of the hazard posed variesfrom place to place, and the humanresponse has been correspondinglydiverse.

The Volcanic Explosivity Index (VEI)attempts to classify the destructive forceof a volcano, ranging from a VEI of 0(‘non-explosive’) to the maximum of 8(‘very large . colossal’), and suggests that

the frequency of a given event declineswith its magnitude. Thus a VEI 0volcano erupts on an almost daily basis,and the human response to this is usuallyavoidance. Occasionally humansettlement might be exposed to a threatwhen a long dormant volcano springs tolife. In such circumstances there havebeen some successful attempts to divertlavas from properties (Mt Etna in Sicily,1983), but more frequently the most cost-efficient course of action is to yield to theadvancing lava flows. This was certainlythe case when Kilauea began to erupt in1983 and engulfed a number of villageson the southern flanks of the volcano. Inany case, the hazard posed to people isextremely limited due to thepredictability of the eruption – to theextent that intrepid visitors can evenchoose to walk to the current lavachannels (though the US National ParkService advises against it). This level andtype of hazard, with its associated humanresponses, is most typically associatedwith hot spots.

A volcano (or volcanic system) with aVEI of 8, such as underlies Yellowstone,entails very different priorities. Here the‘return period’ of each eruptive event ismeasured in hundreds of thousands ofyears, with consequences that aremeasured in global, rather than local,terms. It is also statistically unlikely thatYellowstone will erupt in the near future(despite the fact that it is ‘overdue’according to the frequency of pastevents), and so the most cost-efficientcourse of action is to ignore the hazardaltogether. Encouraging a state of hazardpreparedness might be considered asensible course of action; but the costs ofmaking this effective, combined with thefact that most people have much morepressing concerns, means that the USgovernment does not consider this asensible use of funds. Geothermalactivity throughout the western UnitedStates is therefore seen as something tobe exploited – for tourism, for heating,for agriculture and aquaculture – ratherthan to be avoided for the risk posed.

ConclusionHot spot theory has provided avaluable ‘final piece to the puzzle’ inour overall understanding of platetectonics. However, there are somescientists who are now seeking tochallenge the governing paradigm,arguing for alternative explanations inthe light of new evidence. No doubtthere will continue to be discoveriesthat push forward our detailedunderstanding of the globaldistribution of volcanoes, and perhapshot spot theory will be finally provedor disproved in due course. Until thistime J. Tuzo Wilson’s ideas are likelyto dominate the agenda, as man seeksto learn more about this restless planet.

Further readingA. Jones (2006) ‘Seismic/Eruption: Aprogram for the visualization ofseismicity and volcanic activity inspace and time’,http://www.geol.binghamton.edu/faculty/jones.G. Nagle (2005) ‘Island Arcs’, Geofileunit 492.L. Newstead (2004) ‘Geological Slanton Plates’, Geofile unit 477.D. Thomas & A. Goudie (2000)Dictionary of Physical Geography,Blackwell.http://en.wikipedia.org/wiki/Main_Page.http://www.mantleplumes.org.http://www.nps.gov/yell/.http://www.sciencemag.org/cgi/content/abstract/301/5636/1064.http://vulcan.wr.usgs.gov/Volcanoes/Yellowstone/Publications/OFR95-59/OFR95-59.html.

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1. Using examples, compare and contrast oceanic and continental hotspot activity.

2. What steps can be taken to reduce the hazards posed by hot spotvolcanoes?

3. Describe the advantages and disadvantages of hot spot volcanism for theresidents of Iceland.

F o c u s Q u e s t i o n s