What do sunspots and Hawaiian volcanoes have in common? According to a NASA
scientist, both are caused by convection, a heat transfer process which
drives flows of material deep in the interior of the Sun and the Earth. The
difference between the two phenomena may simply be the speed of the cycles
of convection, with a cycle on Earth lasting about 180 million years while a
solar cycle averages 22 years.
Volcanoes, such as those in Hawaii, form above ‘hotspots’ or ‘mantle
plumes’, which are thought to be narrow conduits of magma and fluid in the
Earth. The volcanoes are arranged across the ocean floor in chains which are
parallel to the motion of a tectonic plate. Geologists explain this by
saying that lava from a stationary hotspot source is being pumped up to
volcanoes on a moving plate.
The ocean crust forming the plate is about 10 kilometres thick, but hotspots
probably come from far deeper in the Earth, possibly from the core-mantle
boundary that lies 2900 kilometres down. Hotspots on land are linked with
the formation of large areas of flood basalt – relics of volcanic events so
huge that they may have caused mass extinctions.
Sunspots, on the other hand, are conspicuous dark regions on the surface of
the Sun which may be thousands of kilometres across. They are more common
during active periods of the Sun’s magnetic cycle. During these periods,
solar flares shower the Earth with ionised particles, which cause the
aurorae and magnetic storms when they meet the Earth’s magnetic field.
Richard Stothers of NASA has found some remarkable similarities between
hotspots and sunspots. He has made a geological study of the past 250
million years, and discovered that hotspots have not been constant features
of the Earth (Earth & Planetary Science Letters, vol 116, p 1).
Nevertheless, they have been long-lived, with the only complete cycle
identified being about 180 million years long.
Stothers found that the latitudes of hotspots, like sunspots, had a
distinctive pattern: there were two parallel belts of spots on either side
of the equator, from about 10 degrees to 50 degrees North and South. These
were distributed in two broad regions about 120 degrees apart, one in the
Pacific and the other in Africa. And during a cycle, new hotspots, like
sunspots, were concentrated initially at middle to high latitudes, before
appearing preferentially at lower latitudes, and creeping slowly in the
direction of rotation.
However, Stothers does not think hotspots and sunspots are identical
features. ‘Sunspots are minor magnetic phenomena and cannot possibly be
construed as analogues of terrestrial hotspots,’ he says. ‘Nevertheless,
they essentially reflect the behaviour of the large active regions (of the
Sun) and can be used as surrogates for them.’
Stothers’s observations support the recent idea that the Sun’s mantle of
hydrogen-helium plasma convects in sheets very much like the Earth’s
semi-fluid lower mantle, but about 10 million times faster. If the idea is
right, geologists may be able to use the Sun as a guide to the patterns of
the flow of heated rock in the deep Earth over the past hundreds of millions
of years to thousands of millions of years. In a human lifetime, it would be
possible to watch several ‘experiments’ using the solar sunspot cycle, says