Our Dynamic Earth
The configuration of the continents on our globe has been steadily
changing over geological time. What is familiar today is the product of
thousands of millions of years of relative movements at rates of typically
only a few centimetres per year. These movements still continue today,
causing earthquakes and volcanoes. This animation demonstrates how the
Indian Ocean evolved by dispersing the 'supercontinent' of Gondwana that
existed as a single entity from about 500 to 200 millions years ago (500-200
Ma) - before the age of the dinosaurs began.
Hidden topography of the sea bed
Images of sea-floor topography obtained by satellite altimetry were
published early in 1997 (Smith and Sandwell, 1997; www.baltica.ucsd.edu)
and revealed more clearly than ever before the geometry of the crust that
was created as tectonic plates were pushed apart from the mid-ocean ridges.
Offsets of plate boundaries at these ridges are known as transforms.
Transforms appear almost everywhere as curvi-linear features on the sea-floor.
These features are largely self-sustaining as they grow and, created over
many millions of years, they now record the relative movements of the
separating fragments, often over thousands of kilometres.
Animation of Earth history
This animation is one result of a determined effort to use this new
information to better constrain the history of continental
dispersal and create a more accurate geological map of reassembled Gondwana.
In making the animation, the growing termini of transforms at the mid-ocean
ridges were (justifiably, we feel) assumed to have been always coincident
and colinear. This has been used as a condition as we try to recreate
the geography of the steadily more distant past. Ocean floor, once created,
has not been destroyed in the study area, providing another important
constraint on possible solutions to the puzzle of the past configuration
of the continental plates.
A dispersion time-scale
The timing of dispersion is recorded in part by the repeated reversals
of the earth's magnetic field that have led to stripes of anomalous
magnetisation, perpendicular to the transforms, as the sea floor was created.
These can be recorded by shipborne magnetometers on oceanographic voyages.
However there is a long period (~83 to 118 Ma) when important changes
in continental configuration occurred without any such magnetic reversals
The rate of progress of fragments has been assumed to be constant unless
there is evidence to the contrary. This simply extends the principle of
economy of hypothesis to the time dimension.