Geoscience Reference
In-Depth Information
and in Germany had to be abandoned after about 11,000 metres. Not only was the rock dif-
ficult to drill, but the heat and pressure tended to soften the drill components and squeeze
the hole shut again as soon as it was drilled.
Messengers from the deep
There is one way in which we can sample the mantle directly: in the outpourings of deep-
rooted volcanoes. Most of the magma that erupts from volcanoes comes from only partial
melting of the source material, so basalt, for example, is not a complete sample of mantle
rock. It does, however, carry isotopic clues to what lies beneath. For example, basalt from
some deep-rooted volcanoes, such as that in Hawaii, contains helium gas with a high ratio
of helium 3 to helium 4, as the early solar system is believed to have had. So this is thought
to come from a part of the Earth's interior that is still pristine. The helium gets lost in vol-
canic eruptions and is slowly replaced by helium 4 from radioactive decay. The basalt in
ocean ridge volcanoes is depleted in helium 3. This suggests that it is recycled material that
lost helium gas in earlier eruptions and does not come from so deep in the mantle.
Violent volcanic eruptions do sometimes carry in their magma more direct samples of
mantle rocks. These so-called xenoliths are samples of mantle rock that have not been
melted, just carried along in the flow. They are typically dark, dense, greenish rocks such as
peridotite, rich in the mineral olivine, a magnesium/iron silicate. Similar rock is sometimes
found in the deep cores of mountain ranges which have been thrust up from great depths.
Slow flow
The magnificent medieval stained-glass windows of Canterbury Cathedral can tell us
something about the nature of the Earth's mantle. The windows are composed of many
small panes of coloured glass in a leaded frame. If you look at the sunlight filtered through
the panes, you will notice that some of them are darker at the bottom than at the top. This
is because the glass flows. Technically, it's a super-cooled fluid. Over the centuries, gravity
has made the panes slowly sag so that the glass is thicker at the bottom. Yet, to the touch,
or, heaven forbid, a hammer blow, the glass still behaves as a solid. A key to understanding
the Earth's mantle is the realization that the silicate rocks there can flow in the same sort
of way, even though they do not melt. In fact, the individual mineral grains are constantly
re-forming, giving rise to the motion known as creep. The effect is that the mantle is very,
very viscous, like extremely thick, sticky treacle.
