(see Graphic)
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
Stothers.

Previous attempts to listen for the sound of splitting have proved useless
because it usually occurs too suddenly for drying conditions to be modified
in time to save the wood. Now James Booker, Peter Doe and Arch Oliver of
the University of Tasmania have found that distinctive sounds are emitted
well before final cracking. They liken the process to rope being pulled
apart: initially weaker fibres break under tension, giving off small amounts
of energy, but the whole rope only breaks when the majority of fibres part
suddenly.

Hardwood may either be dried slowly for about 12 months in the open
air, or rapidly in kilns over five or six weeks, at temperatures up to 50
°C and high humidity. Much of the wood splits as moisture is driven
off more quickly from outer sections of a board than from its interior.
Kiln drying renders about 70 per cent of each load useless for furniture
construction, cutting its value from more than £900 per cubic metre
to about £200 per cubic metre.

The Australian Research Council has awarded a £125 000 grant
for the research, which is coordinated through the Australian Furniture
Research and Development Institute in Launceston, Tasmania.

Using data from numerous drying runs, the group has constructed a mathematical
model of the drying process which matches the moisture content of the wood
with the sounds it emits. The noises have proved so consistent that the
researchers can predict how a batch of timber will dry just by monitoring
its first few days in the kiln. With this information, operators can modify
the kiln’s humidity and temperature long before there is a danger of splitting.
The researchers hope to begin commercial trials of the technique early next
year.

Robert Bowell of the Natural History Museum of London has studied elephants
on Mount Elgon, an extinct basaltic volcano on the border between Kenya
and Uganda. The elephants climb a steep and dangerous track to a cave 345
metres above the plain where they live. There they excavate soft lumps of
clay-like minerals called zeolites with their trunks, which they eat.

Bowell believes the elephants mine the zeolites to obtain sodium and
calcium salts they cannot get from other food sources (Mineralogical Society
Bulletin, vol 96, p 3). From the scale of the excavations, he concludes
that this behaviour has continued for hundreds of years.

The African cape porcupine (Hystrix africaeaustralis) also gnaws at
cliffs. In the Makapan Valley of South Africa, there is a 500-metre-long
outcrop of siltstone outcrop where every exposed surface is covered by porcupine
gnaw and scrape marks.

C. Gow of the University of the Witwatersrand in Johannesburg found
that the rock was low in nutrients, so the animals must be using it for
another purpose entirely. He thinks they probably sharpen their incisors
on it (South African Journal of Geology, vol 95, p 74), as the rock outcrop
is more convenient than searching for the bones and roots that these mammals
usually use to sharpen their teeth.

The two scientists have identified features that resemble terminal and
lateral moraines, gouged cirques, vast grooved rock sheets, outwash fans,
deeply scoured valleys and kettled plains, left behind after the melting
of large ice masses. Kargel and Strom also say they have identified eskers
– ridges that are deposited by meltwater streams running beneath glaciers
and ice sheets.

The eskers are similar in size to those on Earth. They appear in the
Martian plains as sinuous ridges between 10 and 200 kilometres long, 0.3
to 3 kilometres wide, and 20 to 160 metres high. Previously, researchers
believed that the ridges were lava flows, sand dunes or the beds of dried-up
streams.

Planetary scientists have long known that Mars has small polar icecaps
made of frozen water and carbon dioxide. The water icecaps are permanent,
but the carbon dioxide is released into the atmosphere every spring.

The ice sheets were far more extensive in the past, say Kargel and Strom,
covering up to 18 per cent of the planet’s southern hemisphere (an area
twice that of Antarctica) and a considerable part of the northern hemisphere.
Most of the ancient glacial landscapes in the south occur farther from the
equator than 33 degrees South.

The two scientists conjure up a picture of global glaciation, glacial
melting and stream flow, with meltwaters collecting in seas and lakes on
the margins of the ice. The glacial features identified suggest that Mars
once had ice sheets up to 2 kilometres thick. For such a large body of ice
to form, water must have circulated around the planet – evaporating from
the oceans and falling as snow in the polar regions, before being transported
as ice and meltwater back to oceans. All this supports the claim by scientists
in the late 1980s that dried-up oceans exist in the planet’s northern hemisphere.

The glaciation on Mars must have occurred relatively recently because
meteorite craters are less common in glaciated areas than in other areas.
The ice must certainly have been laid down after the intense bombardment
of meteorites which Mars suffered between 4600 million and 4000 million
years ago. This dramatic episode created the heavily cratered regions of
the southern hemisphere. Kargel and Strom say the glaciation could have
occurred as long as 2300 million years ago or as recently as 25 million
years ago.

Today the average temperature on Mars is -60 °C, which is too cold
for glaciation to occur. Mars must have warmed up before glaciation took
place, say the researchers. A period of intense volcanic activity could
have caused the warming. Or it may be that the Sun became hotter. In either
case, frozen carbon dioxide and water would have been rapidly released from
the permafrost just beneath the Martian surface. Once the atmosphere became
rich in carbon dioxide, the greenhouse effect would have produced further
warming.

The glaciers may not have been composed of normal water ice, however.
According to Kargel and Strom, carbon dioxide in the atmosphere would probably
have combined with water to form an ice known as a clathrate, which has
a higher melting point (10 °C) than water ice. So a slightly higher
average temperature is required to form clathrate glaciers than to form
water glaciers.