I have read that the oldest oceanic plate that we see today is from the Cretaceous period. If so, how do scientists reconstruct plate movements and continental structure pre Cretaceous? And aren't all fossils before Cretaceous lost?

Why are we able to deduct geology information pre Cretaceous? And what is the oldest period we are able to gain information from?

  • 1
    I'm glad you want to participate, but you have to tell us the source of your assumption that "the oldest oceanic plate that we see today is from Cretaceous period", which isn't true. Some oceanic crust dates back to the Permian, and some ophiolites are as old as 3.8 billion years old. Finally, geological history is reconstructed from rocks in continental crust, and these show a full history with even some rocks from the late Hadean, 4 billion years ago. – Spencer Sep 9 at 19:24
  • Older oceanic crust than Cretaceous is destroyed by subduction. I think this is the missunderstanding of poster. There are continental rocks with +180 my but not oceanic – Universal_learner Sep 9 at 19:30
  • @Spencer yes ophiolites are an exception and there migth be some others. Olders at 'common' oceanic crust are Jurassic in fact and I think we agree what the poster missunderstood is he was reading about oldest oceanic crust and there are very very more old rocks on earth's total crust – Universal_learner Sep 9 at 19:52
  • @Universal_Learner The rocks under the eastern Mediterranean are of Permian age. There are also very old ophiolites, which are arguably oceanic crust. Anyway that's a side issue; the title asserts that "all plates that are currently left" are that young. – Spencer Sep 9 at 19:55
  • I think I might be mistaken then, I went back to the post I found and see that the author of the post did indeed mentioned pacific ocean plate instead of all tetonic plate. So is the Permian period the oldest plate we can see currently, if so is there techniques to deduct what happen geologically before the oldest plate we can excavate ? – Jal Sep 9 at 22:55
up vote 8 down vote accepted

You forgot about continents

First of all, it's necessary to point out the false premise in your question: It is not true that "all plates that are currently left are younger than Cretaceous". There is a near-continuous record of rocks stretching all the way back to the Hadean Eon 4 billion years ago. And there are older crystals called zircons that are found as inclusions in younger rocks.

The thing is, these rocks are on land, part of continental crust. Your question seems to be unaware of the existence of continental crust. Of course, since you're not currently sitting at the bottom of the ocean, you're on top of continental crust right now. Most of the big tectonic plates are part oceanic and part continental.

Continents have been built up little by little over geological time. Their rocks are a mixed-up jigsaw puzzle of various types and ages. It's the geologist's job to figure out how those pieces fit together.

Here's just a taste, and it doesn't even show the complexity of the puzzle in North America by a long shot:

basement rocks (courtesy of United States Geological Survey, via Wikimedia Commons)

It is true that the oceanic plates are mostly destroyed by subduction, but the oceanic crust has to go beneath something, and that's often a continent. Besides, subduction helps make continents grow:

  • not all of the rock is subducted: some seafloor sediments are scraped off and piled against continental margins

  • The top layers of rock that are subducted melt, and the melted rock bubbles its way up through the overlying crust, producing plutons and volcanic arcs. Sometimes these are on continents (such as the Cascades and Andes) so their rocks directly add mass to them.

  • When the volcanoes are out in the ocean, the subduction process pulls them into continents and they get cemented to the edge. Sometimes they form little mini- continents (such as Japan and New Zealand).

The oldest oceanic crust currently at the seafloor is in the eastern Mediterranean. In an article in Nature Geoscience Roi Granot of Ben-Gurion University of the Negev suggests that the [Herodotus Basin] is a remnant of ancient Tethys Ocean, having formed up to 340 Ma during the Early Carboniferous period. Most oceanic crust is 200 million years old (Jurassic) or younger. They're an important part of putting the puzzle together, but as I stated at the beginning of my answer, the geological record is much older, because of continental rocks.

Finally, there is really old oceanic crust in the form of ophiolites, which are bits of oceanic crust that have been incorporated into continents. Since they're no longer part of the oceanic conveyor belt, some can be quite old. The oldest is about 3.8 billion years old.

Your Answer

 

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.