The Tallest Mountain In The World!

It makes for an eye-grabbing headline, but the reaction of a reader who doesn't know about Mauna Kea at this point would be to say something along the lines of "Everyone from the age of five years and up knows that Mount Everest is the tallest mountain in the world." Well, not exactly - Everest has the highest summit. The conventional measurement of a mountain's height is the vertical distance of its summit from sea level. However, it makes just as much logical sense - possibly more - to measure a mountain's height from its base to its summit, and by that measure Mauna Kea ("White Mountain" in the Hawaiian language), one of the five volcanos that make up the island of Hawaii, is the tallest mountain in the world. In fact, by this criterion, Denali (Mount McKinley) in Alaska is also taller than Everest.

Mauna Kea's height from its base on the Pacific ocean floor to its summit is approximately 10,200 metres (estimates vary according to source, but most are very close to this figure). Its height above sea level is only 4,205m, but the ocean depth here is such that its base is lost in staggering depths, fully six kilometres down. Add to this the fact that it's a volcano and you have one of the coolest geological objects on Earth by any small child's standards.

Geologically speaking, Mauna Kea, about a million years old in total, stopped being a shield volcano somewhere between 200,000 and 250,000 years ago, and is now in its postshield phase. Shield volcanoes generally rise smoothly and gradually towards a large caldera at the summit, with eruptions tending to be gentle flows of lava that add layer upon layer to the volcano's mass. In the postshield stage, the main type of magma changes (from tholeiitic basalt to alkalic basalt, if that means anything to you) and eruptions become both rarer and more explosive. The caldera stops developing, and on top of the volcano's gentle shield slopes grow steep cinder cones and deep fissure vents. This has been happening for some time on Mauna Kea - however, its height is now gradually decreasing, as its enormous weight causes the ocean floor beneath it to subside1. Currently classed as dormant, Mauna Kea last erupted 4,500 years ago - however, it is almost certain to erupt again at some point. The onset of such an event could be signalled by frequent earthquakes, but not infallibly. A more reliable indicator would be small changes in the tilt of the ground on the summit plateau, something that the numerous sensitive telescopes located there would pick up almost immediately.

A fortuitous combination of factors has led to Mauna Kea becoming the largest astronomical site in the world, with several prominent observatories located on its summit plateau. The altitude greatly reduces atmospheric interference due to the reduced volume of air, and the even more reduced volume of water vapour, through which telescopes must peer. Being above the inversion layer, most nights of the year are perfectly clear, and because of its gentle shield slopes, the summit observatories are easily supplied. Add to this the fact that there is very little light pollution due to the low population of Hawaii, and that its location close to the equator allows both the northern and southern skies to be seen, and this is about as good as it gets for non-orbital telescopes. The observatories and surrounding research facilities are all contained in a 500 acre zone called the Astronomy Precinct, established in 1967 and located in the Mauna Kea Science Reserve, a site administered by the University of Hawaii. Perhaps the most famous of the observatories is the W. M. Keck Observatory, two huge telescopes with 10-metre primary mirrors which can operate together as a single astronomical interferometer.

There are, of course, some problems associated with the site - almost every large mountain is sacred to some sect or other, and Mauna Kea is considered by certain native Hawaiian groups to be the abode of the snow goddess, Poliahu. In fact, the Hawaiian Creation Chant cites Mauna Kea as the progenitor of the Hawaiian people, and for this reason some feel that building observatories on the summit amounts to a desecration. There has been a certain amount of inevitable environmental damage due to the buildings and habitation in the summit's pristine environment, and further building is likely to be increasingly controversial.

In terms of its ecology, Mauna Kea contains several zones linked to changes in altitude. Between 450m and 1,600m on the lower slopes on the east, the windward side, rain forest flourishes, just above agricultural lands where huge quantities of sugar cane were once grown, before the sugar cane industry of Hawaii collapsed in the 1990s. This agricultural land is now used for various different purposes, among them eucalyptus farming, cattle grazing, or timber. Higher up, between 1,600m and 2,400m there is a band of pasture land which was once a forest of koa and mamane. This area has suffered from the proliferation of invasive species such as gorse and feral sheep which have had a devastating impact on the natural ecology at this level. Above this altitude is mostly higher than the timberline, and consists of alpine tundra and bare rock, with snow possible for almost half the year above 3,400m. If it's been a particularly cold and wet winter, the cinder cones of Mauna Kea become a popular ski destination. It's not completely without danger however - the air is thin enough to lead to the possibility of altitude sickness, and there can be strong winds at the summit, gusting 70mph or more.

1. The reasons for subsidence at the ocean floor in the area of Hawaii are in fact not straightforward, and the mechanisms underlying hot-spot volcanism are still controversial and too complicated to go into in this writeup. Suffice it to say that Mauna Kea's weight probably contributes to the subsidence in its immediate area in some way.

References and Further Reading:

Wikipedia (Mauna Kea):
Wikipedia (Mauna Kea Observatory):
Wikipedia (Keck Observatory):
Volcanic Activity:
Tallest Mountain: