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Comment by kijin | Hacker News Reader

What does it mean for a mountain range to be older than the continent it's on?

The article doesn't seem to go deep enough into the logistics of this apparent contradiction. Did the mountain range appear in the middle of the ocean and the continent gradually formed around it? Or was the mountain range simply part of an ancient continent that isn't immediately recognizable as Africa, but parts of which eventually became parts of Africa?

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The first one - "one of the first landmasses to rise out of the ocean"

Here's an action replay. If you follow the red blob I think that must be the mountains. I recommend setting the speed to 2x https://www.youtube.com/watch?v=ovT90wYrVk4

There was a lot of moving going on. The purple bit seems to have ended up as the Hamersley Range in Australia. (http://www.oldest.org/nature/mountain-ranges/)

I am sure there is a lot of great science, data, and tons of work by a lot of people behind this video. But I am having a hard time shaking the thought "How could they possibly know that".

Just mind blowing that it could recreate all that movement. Imperfect as to the details, sure.

Thank you for sharing.

I was wondering how they figured it too. There must be some guess work. It seems to work a bit like doing a jigsaw puzzle where if say the shapes and fossil species match you can figure they were next to each other at the time of the fossils. https://www.geolsoc.org.uk/Plate-Tectonics/Chap1-Pioneers-of...

This is what science does. It's human attempts to explain the natural world we observe and to make suggestions as to how it got that way and how it works. Scientists don't claim that they have the final answer or that ancient events had to have happened they way they explain. They are just claiming an explanation that is consistent with the facts and observations they have.

shapes, rock formations, mineral content, magnetic striping. There are lots of clues.

A geologist I know very well has remarked to me before that before a certain point, the geological records aren't good enough to know. The problem is that continents get pushed under each other, and their contents are recycled through the earth in complex (fluid dynamics) ways. So, we can be pretty confident up to a certain point (for some reason, 2 billion years feels right in my brain, but I might be making that up - I think it's correlated with the oldest rocks in Australia?), but after that point, the records are so muddled that it's pretty hard to come up with a confident picture. So saying "this is what the earth looked like 4 billion years ago" is an imprecise endeavor

The continents rarely get pushed under each other, but the ocean floor does commonly get pushed under the continents and back down into the Earth. The Himalayas are the exception where two continents have hit and are still moving toward each other with a lot of crust getting smashed and pushed up much higher than usual. But even in this case the continental crust is not going back into the earth. It is much less dense than the rock below it and thus will "float" on top.

Reconstruction of where the plates were in the past is mostly determined by paleomagnetism (subject area of my PhD). When lava erupts and cools, it gets magnetized in the direction of the earth's magnetic field. This can be measured directly in the rocks themselves or seen as magnetic anomalies in the oceans. Since the magnetic field flips directions every 1/2 million years or so (a somewhat random process) the rocks around the ocean ridges that are spreading and erupting continuously all get magnetized in one direction and then another. This produces magnetic anomaly stripes that have been measured all over the world.

Using these ocean stripes, which can be treated like slices in time, it is quite straight forward to reconstruct what Pangea, the most recent supercontinent, looked like and where it was located when it began to break up about 175 million years ago. Almost all of the current ocean floor was created after this breakup, so before that reconstructions become much more difficult.

This a very simplified explanation. The older the rock, the less likely the original magnetization was preserved. Also, the continents have been growing slowly over the 4.5 billion history of the earth. The earth was almost all ocean at the start. The oldest rocks known are about 3.2 billion years old, so before that almost nothing is known about the magnetic field of earth (some older zircons survive in these rocks, but they are unlikely to preserve a record of the magnetic field). The wikipedia entry on plate tectonics is a decent place to start to read more about the process.

If the Earth's magnetic field flips somewhat randomly every 1/2 million years, and there is randomness in that timing, how can it be a reliable measurement mechanism at 50 million years? 500 million years? 2 billion years?

From the sea-floor anomalies, we are pretty certain that the field is mostly a dipole (magnet like) during last 160 million years. The actual magnetic field direction at any one place at a specific time will vary from the ideal dipole field by up to around 30 degrees, but over thousands of years the average field direction is that of a dipole(mostly). So, to get a good idea of the true direction of the magnetic field at a given place on a continent in the past, one would want a decent number of measurements of different ages (but not enough time to pass so that tectonic motions are significant, say less than a few million years). The more measurements the more certain one is on the accuracy of the direction and a lot of Fischer statistics are used in this determination to understand how certain one knows the true direction.

The further back we go the more unreliable and less certain we are that this is a reliable mechanism. The rocks get changed by chemical reactions or heat. The solid inner core grows, so the solid core size will change the dynamics of the field greatly. The whole mantle seems to have moved very quickly all at once durning some times, but so this was so long ago there are few rocks that have been studied that this strange possibility is still highly debated.

If someone tells you about the magnetic field of the Earth at 2 billion years, they are just stating a speculative theory. The last 160 million years, we have a pretty good idea.

If you want to learn in depth about this science, I recommend you check out a text book by Butler (1992) https://www.geo.arizona.edu/Paleomag/

which is good intro textbook or Tauxe (2018) https://earthref.org/MagIC/books/Tauxe/Essentials/ Which is more up-to-date and more in-depth.

Or even chat online if you like. email is in my profile.

Just to put that video in perspective:

Multi-cellular life appears at about 2:30

Plants start colonizing land and the Appalachian mountains form around 4:15

Fish show up around 4:10

Animals start colonizing land at 4:20

Dinosaur show up around 4:40

Mammals show up somewhere between 4:43 and 4:46*

Crocodiles show up at 4:54

Dinosaurs go extinct at 4:55

Humans show up, invent language, paint a cave in Europe, litter stone tools all over North America, domesticate animals, invent the wheel, learn agriculture, cross the oceans by flying, invent computers, drop atomic bombs, go to the moon, send glorified RC cars to drive around other planets, invent the internet and shitpost on hackernews at 5:03

*turns out it's kind of hard to tell mammals from mammal-like reptiles when all you have is fossils, hence the wide range

Thanks for this. I was imagining how cool the presentation would have been with overlays of beginning of life & species. You've done it - and ended it with good humour! Thanks.

The mountains predate a recognizable African continent by about 3 billion years. But the article also mentions that they were one of the first pieces of land to rise out of the ocean, predating any continent. So both your guesses are kind of correct.

The crust formed piecewise over time as the earth naturally separated into denser mantle material (containing more metal, broadly) and lighter crust rocks (more silicon). The little bits would float about until they bumped into each other and stuck. The surface rocks in this region happen to be a remnant of one such blob. They're at the surface now by dumb luck.

I think this is a good question. Perhaps the author means older than the current position of the continents? The Appalachian mountains, though not as old as the Makhonjwa, would also fall into that category.

> What does it mean for a mountain range to be older than the continent it's on? The article doesn't seem to go deep enough into the logistics of this apparent contradiction.

That’s one way that terraformation works. It’s quite normal for something like a volcano (or a bunch of volcanoes / mountain range) to form, and then for land to develop around it. This is how many islands develop, and why many islands also have a mountain.

In the future the islands of Japan could become one big continent with a mountain range backbone.

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