Earth’s mega moun­tains

Australian Geographic - - Geo Buzz -

EARTH’S BIG­GEST moun­tain range is one very few of us will ever see. The Oceanic Ridge sys­tem is a near-con­tin­u­ous un­der­wa­ter range, more than 60,000km long, that winds, like a ten­nis-ball seam, around the Earth on the ocean floor.

Up to thou­sands of kilo­me­tres wide, it trav­els through ev­ery ocean basin.

In 1855 the US Navy’s Matthew Fon­taine Maury pro­posed its ex­is­tence as a shal­low “mid­dle ground” in his then-new At­lantic Ocean chart. It was orig­i­nally called the Mid-Oceanic Ridge, but later dis­cov­ered to have some of its sec­tions, such as the East Pa­cific Rise, not in the mid­dle of an ocean. It was not un­til the 1950s, when sys­tem­atic ocean floor sur­veys be­gan, that the Oceanic Ridge’s ex­is­tence was proved. These sub­ma­rine moun­tain ridges rise from about 5km deep, on the ocean floor, to a fairly uni­form depth of about 2.6km be­low the sur­face. At var­i­ous lo­ca­tions along the crests of these moun­tains are ‘spread­ing cen­tres’, re­gions where new ocean crust is cre­ated. The crust is lit­er­ally spread apart by new crust pushed up from be­low. The spread­ing rate varies from 10mm to 160mm/year.

The ge­og­ra­phy (the dip at the top) around the ac­tual spread­ing cen­tre is re­lated to how fast the crust pulls apart. If the rate is high, as it is at the East Pa­cific Rise, the ac­tual spread­ing cen­tre

is in a nar­row and sharp dip. But a slower spread­ing site, such as the Mid-At­lantic Ridge, tends to sit inside a larger rift val­ley that can spread as wide as 20km.

The source of the new crust could be vast un­der­ground cham­bers of hot molten magma. In the East Pa­cific Rise these cham­bers seem to be about 2km be­low the ocean floor, about 1–4km across and about 2–6km thick. Across the whole planet, about 4cu.km of brand new ocean crust is formed each year.

It seems that about 20 per cent of all heat ris­ing from the core of the Earth comes out through these spread­ing cen­tres that sit on top of world’s long­est (and un­seen) moun­tain range.

DR KARL

is a pro­lific broad­caster, au­thor and Julius Sum­ner Miller fel­low in the School of Physics at the Univer­sity of Syd­ney. His latest book, The Doctor, is pub­lished by Pan Macmil­lan. You can fol­low him on Twit­ter: @Doc­torKarl

The Oceanic Ridge.

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