Question

we've learned about the asteroid impact that occurred at the end of the Cretaceous period of Earth history, which led to the extinction of the dinosaurs. It created a crater called the Chicxulub crater, and temporarily altered the Earth's atmosphere, climate, and oceans so much that it caused a mass extinction.

(You might also find it helpful to look at this geologic time scale because it shows when the Cretaceous, Tertiary, Permian, and Triassic portions of the geologic time scale took place.)

Starting with the Alvarez team in 1979, people began to realize there could be significant cosmic influences on the Earth and its many life forms.

The Cretaceous-Tertiary extinction*, when the dinosaurs died out, was one of the worst in Earth history, with approximately 75% of species going extinct.

(*In recent years, geologists have stopped using the term "Tertiary", and nowadays they call it the "Cretaceous-Paleogene" extinction... but that's just a minor technical point from our point of view. You might run into that term, though, so there you go, FYI.)

What the resources in Part 1 didn't tell you, however, was that the end-Cretaceous extinction wasn't the worst one in Earth history! :-O The "Age of Dinosaurs" (also called the Mesozoic Era) of Earth history didn't just end with a mass extinction, it began with one - one which was significantly worse than the one at the Mesozoic's end.

In other words, the dinosaurs diversified and spread across a world where life had nearly died out. Whoa. This event is called the Permo-Triassic extinction.

I'm not going to ask you to write about what caused the P-Tr extinction, because we simply don't know. It happened 250 million years ago, and a lot of the rocks from that time have been destroyed by erosion or covered by other rocks... or have perhaps suffered at least one other possible fate... hmm.... (looks upward and rubs his chin thoughtfully)... I wonder what at least one other fate for those rocks could be?..

I should be clear about one thing, though: We don't actually know that an asteroid impact caused the P-Tr extinction. That might not be the cause! It's only one possibility at the present time, and some other possible causes are still serious "contenders".

Your task:

Write a short-answer response to this question:

I) F an asteroid impact caused the Permo-Triassic extinction, how could could the crater be totally impossible to find?

2) There's something that could have happened to the crater that would mean we would never ever find it.

3) There's a particular possibility that I have in mind here, and your job is to figure out what it is and write a short description of it.

To Help:

Hints and tips:

• What if the impact occurred far out in the middle of an ocean basin? Could this have anything to do with what I'm getting at? (I don't mean `how would that help the asteroid cause destruction' - that's not what we care about here. I mean 'how would an impact in the middle of an ocean basin make the crater so un-findable?')
• What does plate tectonics have to do with this? Can plate tectonics be a destructive process?
• You may want to look at section 8.2 in the textbook again.
• You may want to review the lecture video in which I went over my version of the "Parts of the Plate-Tectonic System" drawing.
• This map of the age of the seafloor (Links to an external site.) may be useful. The Permo-Triassic extinction, whatever its cause, happened about 250 million years ago. (That's also called 250Ma or 250 m.y. You can find more versions of the seafloor-age map here (Links to an external site.).)

It's okay to do web research to help figure out the answer, kind of like if you were responding to a post in a Discussion thread. Just list your sources, like you'd do in a Discussion post.

Please list below if you use any information from those webistes I will grealty appreciate it.

Answer 1

There are several ways for an asteroid impact crater occurring 250 million years ago to not be found:

i) The crater has been completely eroded. The odds of this happening are quite low, since impacts as old as 3 billion years are still found on the surface of the Earth.

ii) The crater has been deposited over. This is quite possible, since a crater forms a depression,and sediments can easily be deposited in sediments causing the crater to be completely buried.

iii) This explanation is the one being hinted at in the question. If a meteoric impact occurred in the ocean, the impact crater would be in the oceanic crust. Plate tectonics, through seafloor spreading would ensure that the oceanic crust 250 million years ago would be subducted. The oldest oceanic crust today is around 65 to 70 million years. This means that the oceanic crust 250 million years ago is subducted and is not preserved anymore. Thus, any meteroic impact that took place at this time in the oceanic basin will never be found, since the rock containing the impact crater has now been melted in the mantle.