Oldest Rocks on Earth

[dougettinger]

How does anyone possibly know the oldest rocks on Earth ? If the continents were created in a rapid growth early in Earth's history and the surfaces were continually reshaped by plate tectonics, how could any rock be older than the last known, major continental movements that occurred 750 to 180 mya ? The claim is that the oceanic crusts are mostly less than 100 million years old. So how can any rock on Earth be as old as 4.404 (+ or - 8 billion years) as claimed ?

Doug Ettinger
Pittsburgh, PA

[rstevenson]

I've walked on rocks that a visiting geologist said were around 3.5 B years old. That's not to say they've been there, above ground and above water for all that time, just that a vein of rock currently visible is that old. What it's been doing for all those years would make an interesting tale I'm sure.

They're out on the point in the middle of this Google Maps picture.

Rob

[dougettinger]

I've walked on rocks that a visiting geologist said were around 3.5 B years old. That's not to say they've been there, above ground and above water for all that time, just that a vein of rock currently visible is that old. What it's been doing for all those years would make an interesting tale I'm sure.

What you are suggesting is that some of the original oceanic crust when the Earth first differentiated survived and was pushed upward above present sea level at one of the tectonic plate boundaries. This rock then remained undisturbed on a continent that has drifted to many parts of the globe in the intervening billions of years. Is this a proper assumption ?

Doug Ettinger
Pittsburgh, PA

[rstevenson]

Not being any kind of geologist myself, I simply don't know its likely history. A quick glance at a geology map of the region shows that area to be very mixed up (a technical term I use when I don't know what I'm talking about). The rock that was pointed out to me is a fairly narrow band (about 3' wide and about 20' long where it is exposed) of alternating dark and light gray stripes, turned nearly on edge and in the midst of rock layers which are themselves about 420 to 450 million years old (according to the map). So I assume the old material has been eroded out and then surrounded by newer deposits -- perhaps more than once -- and is coincidentally at the surface to be found today. I doubt if it's been at the surface for very long (in geological terms), since it would erode away, as would any rock. The fact that it's visible now means it has led a very sheltered life.

Rob

[bystander]

Wikipedia: Oldest dated rocks
National Geographic (25 Sept 2008): Oldest Rocks on Earth Discovered?
MSNBC Live Science (25 Sept 2008): Oldest rocks on Earth found
Science Daily News (26 Sept 2008): Oldest Known Rocks On Earth Discovered

[Chris Peterson]

How does anyone possibly know the oldest rocks on Earth ? If the continents were created in a rapid growth early in Earth's history and the surfaces were continually reshaped by plate tectonics, how could any rock be older than the last known, major continental movements that occurred 750 to 180 mya ? The claim is that the oceanic crusts are mostly less than 100 million years old. So how can any rock on Earth be as old as 4.404 (+ or - 8 billion years) as claimed ?

The fact that the crust is continually being reshaped doesn't mean that every cubic meter of it has been entirely recycled. Rocks older than about 2.5 billion years are rare, but some material has survived- mainly the cores of ancient mountains far from plate boundaries, and exposed by erosion.

(Very old rocks are normally dated by radiometric examination of zircons, a method also used for some meteorites.)

[dougettinger]

The fact that the crust is continually being reshaped doesn't mean that every cubic meter of it has been entirely recycled. Rocks older than about 2.5 billion years are rare, but some material has survived- mainly the cores of ancient mountains far from plate boundaries, and exposed by erosion.

Thanks for all the responses. It still puzzles me that the oldest rocks range between 2.5 to 3.8 billion years with the oldest rock found until 1999 being 4.031 billion years. The rocks remaining in both the oceanic and continental crusts due the continual reshaping are mostly in the range of 10 to 100 millions of years. Why is there a tremendous gap in the ages of rocks found on Earth?

There are also arguments about the recent discovery of the oldest rocks found in Canada to range from 3.8 to 4.28 billions years. This is a significant range. What are the arguments about ? In most accounts the geologists seem to nail down the ages based on the mineral zircon by + or - 10 to 100 million years.

The oldest super-continent is determined to be about 750 million old before it drifted apart and then recombined to form the next supercontinent. What happen to this continent(s) between the ages to 2.5 billion years and 750 million years ? Did the first supercontinent which I believe resided mostly in the southern polar regions stay in this dormant position all those years ?

Doug Ettinger
Pittsburgh, PA

[rstevenson]

... Why is there a tremendous gap in the ages of rocks found on Earth? ...

I don''t see that big a gap. I've personally walked on rocks approximately 1 B years old, as well as others in the range of 2.7 B years old -- both in and around the Canadian Shield -- as well as that first one I mentioned, the 3.5 B year old bit. Within an hours drive of home I can put my hands on sandstone about 275 M years old, and then drive another hour to reach a Unesco World Heritage site where fossils are being found in rock about 320 M years old. In short, there's a wide range of ages of rocks all around us.

The oldest super-continent is determined to be about 750 million old before it drifted apart and then recombined to form the next supercontinent. What happen to this continent(s) between the ages to 2.5 billion years and 750 million years ? Did the first supercontinent which I believe resided mostly in the southern polar regions stay in this dormant position all those years ?

I think you're referring to Rodinia. That's only one of many super-continents that have formed and broken up during the course of the Earth's life, and it's definitely not the oldest. Wikipedia has a page listing them all and linking to more info about each.

Rob

[dougettinger]

Thanks for your responses. You have answered a number of questions for me. The 1 B year old rock do fill in the gap of time that I speak of. Also, your Wikipedia reference must have been updated since I saw it last time. It now lists supercontinents going back more than 3 billion years. The supercontinent and Wilson cycles were very revealing.

Doug Ettinger
Pittsburgh, PA