Hi All,
Been lurking for a couple of days, but following the discussion. Roger's 3-sentence
version of plate tectonics is basically accurate, but really doesn't answer the question
that Michael put.
To answer that question we have to go back about 4.5 billion years. Here I will state
some supported hypotheses and tell a bit of the story.
At about that time the earth was being formed from gravitationally captured material.
most of this material was left over from a prior local solar system which had been destroyed
by it's novating sun. Gradually the material and gasses began to reaccumulate into
the future sun and it's planets. Gravity began pulling heavier material to the center through
the semi-molton exterior. As this process was occurring a Mars-sized planetoid and the
proto-earth collided - splatt. The collision did not split the earth but did cause a large
portion of the mantle material to be cast into space. After several thousand years in
orbit, this material reaccumlated forming the moon.
Back on earth the scars had not quite healed and differentiation contined (that is
gravitational settling continued). Gradually the lighter portions of the mantle settled
like froth on the surface of a lava pond. As these cooled and hardened they became
the proto-continents. But no continents were forming in the scarred area where the
moon had come from because most of the mantle material had been removed and
differentiation was at a different stage.
The cooling proto-continents (shield areas), because of their lighter (less dense)
nature were large floating blocks on gradually cooling material which was heavier
(more dense than they were). Gradually mantle material re-mixed, but the scar
left by the moon's departure never disappeared entirely. This scar became the central
Pacific basin. Despite billions of years of continental drift this area has remained
essentially without continental masses. Why?
Continents are too light to sink!! On the other hand ocean floor material which is
made from heavier mantle material with iron and magnesium in abundance (as opposed
to continents which have lots of Aluminum) may be able to sink provided it is heavier
than the material below it. How does this happen?
As new sea floor is created along the mid-ocean ridges from violent volcanic eruptions,
the older sea floor is pushed aside towards the continents. As this gets older, it gets
colder and more dense, gradually forming depressions in the ocean floor. Given enough
time, when this material is pushed as far as the continental mass, it may be heavier
than the material below it. It keeps being pushed, but can't go up over the continent,
and can be compressed, so to only remaining option is it must go under the continent
(that is it subducts). Because it is heavier than the material under the continent, it
begins to sink on it's own accord. This process of sinking pulls the rest of the slab
of oceanic material with it, opening up the mid-ocean rifts and continuing the process
of ocean floor creation at the rifts and sinking and destruction of ocean floor at the
subduction zones.
As the ocean floor sinks, it encounters much hotter material (which is lighter and
less dense than it is). The material is hotter due to radioactive decay of radiogenic
elements and because of heat still left over from the formation of the earth. This begins
to melt the ocean floor and sediment material. Since much of the sediment is light
compared with the surrounding melting sea floor (sediment comes from light continents),
it tends to rise to the surface where possible. This forms many of the volcanos at
ocean-continent boundaries.
Why is the Pacific so prone to subduction, then. The continents move back and forth,
but the Pacific (or at least an ocean resembling the Pacific) is always there. Continents
cannot plow through the ocean floor which was formed about 4.5 billion years ago and has
been reformed continuously ever since. Movement of continents can only occur as
they are shoved around by old ocean floor making way for new. The Pacific and the
Ring of fire, then is an ancient feature constantly replenished by new material rising from
and sinking into the mantle.