Geoscience Reference
In-Depth Information
have been evolving independently on the two land masses, with the result that the marsupi-
al line has come to dominate in Australia while placental mammals developed in Asia.
As with the evidence of magnetic reversals in ocean floor basalts that we discussed in the
last chapter, so the magnetic evidence has provided the most comprehensive picture of past
continental movements. The grains of magnetic mineral trapped like tiny compass needles
in volcanic rocks when they solidified record the direction to the North Pole at the time.
They show not only the small wiggles and big reversals in the magnetic field itself, but
also trace out over tens or hundreds of millions of years, a larger, more sweeping series of
curves: a so-called polar wandering curve. This is in effect the plot of how the continent
itself has moved relative to the magnetic pole. When you compare the curves of differ-
ent continents, sometimes you see that they move together but at other times they diverge,
tracking how the continents themselves have split, drifted apart, and come together again
in a sort of continental waltz. In fact, it's more like a clumsy barn dance, as the continents
occasionally barge into one another.
Sensitive instruments even make it possible to track the relative continental movements
today. Over short distances, such as locally across plate boundaries, surveying techniques
and in particular laser ranging can be very accurate. But that can now be done over contin-
ental scales too, via space. Some of the strangest space satellites ever launched are for laser
ranging. The satellite consists of a sphere of dense metal such as titanium with many glass
reflectors, like cats' eyes, embedded in it. These reflect light back in the same direction
from which it has come, so if you shine a powerful but narrow laser beam from the ground
and time the reflected pulse back to where it started, you can work out the distance to an ac-
curacy of centimetres. When you compare values from different continents you start to see
how they are moving from one year to the next. Astronomers can do the same sort of thing
with radio telescopes using distant cosmic sources of radio waves as their frame of referen-
ce. Now that the codes from the American military Global Positioning System (GPS) satel-
lites are no longer scrambled, geologists can get similar precision using a small hand-held
GPS receiver in the field. By the careful use of many readings, the accuracy comes down to
millimetres. The answers confirm evidence of the rate of sea floor spreading: the plates are
moving relative to one another at roughly the speed your fingernails grow, between about
3 and 10 centimetres per year.
Of course, all these plate motion measurements are relative to one another, and it is hard to
establish a background frame of reference for them all. A clue comes from Hawaii. The Big
Island of Hawaii is just the latest in a series of volcanic ocean islands that stretch away to
the northwest and continue underwater as the Emperor seamount chain. Dates of the basalt
reveal that the further northwest you go, the older the basalts are. It seems as if the chain
marks the passage of the Pacific plate across an underlying plume of hot mantle material.
