[CommonerThe (OP)]

Back in 1967 my geology teacher said that tectonic plates were caused by the presence of life. He said it was just a theory now (in 1967) but that in the future it would be true. As I recall he made no mention of the Gaia theory which I believe was around at the time (although I've done no research on this).

His theory said that the weight of the creatures with calciferous shells or body parts which eventually made the limestone rocks we see today distorted the underlying crust until it sheared or broke, thus allowing the underlying mantle to come up and form new rock, much as we see today. Once started the process continued, giving us the continental drift we see today.

I would like to say in addition that I think this is one more example of life creating conditions that are just right for itself. On Venus the whole planetary surface undergoes a 'renewal' every 100,000 years or so - I may have got the time interval a little wrong - which doesn't allow life to propagate if it's continually being wiped out by lava flows. Life on earth 'took over' the renewal process and adapted it to its own ends.

As in the Gaia theory I don't mean that Life consciously did this any more than evolution consciously evolves. I haven't seen any references anywhere to either my teacher's theory or my little addition. Can anyone point me to references? I'm quite fond of the Gaia theory, especially its reasoning for the lack of life on Venus and Mars, but I'm no new age mystic. I don't read any Gaia stuff because of the trail of Hippies and others in it's wake.

[neilep]

Hi There,

Welcome to the forum.

I asked a question relating to tectonics a while ago and thought it might be of interest if I put the link here http://www.thenakedscientists.com/forum/topic.asp?topic_id=1346 though it's out of context with reference to your post.

Men are the same as women, just inside out !

[daveshorts]

I am not sure about life, there are some geologists out there who will correct me if I am wrong, but I think that plate tectonics has been going on for a while according to: http://pubs.usgs.gov/gip/dynamic/Pangaea.html there is evidence for 3.8 billion years, this would considerably predate big calciferous deposits, so I would doubt your teacher's theory.

I have heard that water is very important on the other hand which tends to lubricate subduction zones by forming minerals with significantly lower melting points. So indirectly by preserving liquid water life may be important.

One way to trigger the start of subduction, which would require the breaking of the crust, would be a series of large asteroid impacts breaking up and redistributing the crust. In the holes you get denser  material coming up from the mantle which when it cools would want to subduct.

[CommonerThe (OP)]

I followed your link and on the same site came up with this.
newbielink:http://pubs.usgs.gov/gip/dynamic/historical.html [nonactive].
This seems to suggest an age of about 220 million years for the first plate movements and so they could be caused by the presence of life.

[Bass]

quote:Originally posted by CommonerThe

I followed your link and on the same site came up with this.
http://pubs.usgs.gov/gip/dynamic/historical.html.
This seems to suggest an age of about 220 million years for the first plate movements and so they could be caused by the presence of life.

That web site showed the breakup of Pangea (which occured around 220-225 million years ago).  The presence of 3.0+ billion year old accreted terrains suggest plate tectonics has been active for much longer than 220 million years.
I should know[]- I feel like I've been around since "Ye Olde Ordivician" days.

Subduction causes orogeny.

[daveshorts]

I think that 220million years may well be the oldest ocean floor on the earth. The earth's magnetic field flips quite often in geological time, and this is preserved in the magnetism of the ocean floor when it hardens, so you get things like magnetic growth rings on trees which you can plot for all the ocean's floor. From these you can work backwards to where the continents were in the past. Old oceanic crust gets subducted, so the furthest back that we can say with much certainty where the continents were, is related to how old the oldest ocean crust is.

This doesn't mean that the contients weren't drifting before this, there is evidence to say they were, we just have a very sketchy idea of where they were.