Although it may seem to
us that Earth has finally arrived at its ultimate state, this is an illusion
based on the egotism of humanity and our inability to comprehend the full dimension
of geological time. To put it plainly, humanity has not been an important factor
in Earth history. Compared with the age of the planet (about 4.5 billion years),
the age of the species Homo sapiens (about 150,000 years) is but the
blink of an eye. In quantitative terms, humans have been around for only 0.003
percent of Earth's history, or less than eighteen minutes over the course of a
year.
We may understand
this intellectually but not intuitively. Because planetary change takes place so
slowly, we don't notice it and tend to think it's not happening. Nevertheless, most
of the processes that have defined Earth's history, such as plate tectonics,
seem to function as they always have. The lithospheric plates, for instance, continue
to move at roughly the same rate they have over the last three billion years.
At just centimeters per year, this movement is difficult to perceive, but it's
nonetheless significant.
According
to plate tectonics, the continental plates have in the past, and will again,
come together to form supercontinents. For example, about 225 million years
ago, all of the extant continents joined together to form a single landmass known
as Pangaea, fitting together like the pieces of an enormous jigsaw puzzle. This
supercontinent, one of many that have formed over the course of Earth's history,
took tens of millions of years to come together through multiple collisions and
tens of millions of more years to break apart through a process called rifting.
Geologists believe that unitary supercontinents form about every 250 million years,
which is roughly the same amount of time that it takes our solar system to complete
a single orbit around the center of the galaxy. Some geologists have suspected
a connection, but none has yet been demonstrated.
Could some
past cataclysmic events on this planet have resulted from Earth's passing
through a particularly dangerous region of space? That is certainly a possibility,
and if so, such events will likely happen again-but not quickly. (The Bedside
Baccalaureate, edited by David Rubel)