Let me paint you a picture of our  planet, more than one billion years ago.

The land is barren and rocky, but the  shallow oceans are a tropical paradise,   with mats of greenish microbes coating the  shallow sea floors under a hazy orange-ish sky. They spread out, growing for miles, and building  on-top of each other as they reach for the sun.

Despite the biological bounty,  this time period is so far back   that we have almost nothing in the  fossil record to tell us about it. Even the geological record is piecemeal,   with the rocks having been messed  up and destroyed over the eons. But there is one band of mountains where  pieces of this paradise were preserved.

The Appalachians, running through northeastern US,   give us a unique snapshot into this ancient  world, thanks to their long history of formation. And central to that story is this  rock, called Franklin Marble. [♪ INTRO] To understand why the Appalachians are so  special, we need to get into plate tectonics. Some of the most dramatic examples  of plate tectonics on our planet   can be found at the boundaries between plates,   where two pieces of continental crust smush  into each other and build huge mountain ranges.

The classic example is the Himalayas,   formed as the Indian subcontinent  slow-motion crashes into Asia. And yes, that is present tense. It is happening right now.

The Himalayas are kind of a simple  version of this, because the Indian   subcontinent is just going to keep pushing in  one direction until it peters out and stops. But on geological timescales, tectonics is  rarely that simple, or that long-lasting. Usually, things tend to be more cyclic.

Perhaps no place exhibits this more  clearly than the Appalachian Mountains. This single mountain range has more  than a billion year-long history,   consisting of five continental collisions, and two   periods when the tectonic plates reversed  and started moving away from each other. And while different parts of the 3200  kilometer-long range contain different parts   of its history, New Jersey has some of the oldest  rocks that help us to tell this whole story...

So let’s start at the beginning, around 1.3  billion years ago, in the late Mesoproterozoic. Much like today, the continents of the  world were scattered, fragmented by the   shattering of the ancient supercontinent  Nuna some 200 million years earlier. And, also like today, those continents  were on their way to joining up once again.

The continent of Laurentia, which is the  home of most of modern-day North America,   began to collide with Amazonia, which is what  we’d now call the northern part of South America. This collision led to the assembly of  a new supercontinent, known as Rodinia. But this wasn’t, like, a nice,  tidy cosying up of continents.

Things got messy. Geologists think that the convergence  started out as a subduction zone,   with the oceanic crust of one tectonic  plate sliding underneath the other. Like we see in subduction zones today, this  would have led to melting in the mantle above the   subducted slab, creating magma that rises up and  creates a volcanic arc next to an ocean trench.

But the mechanics of subduction zones also create   another feature on the overriding  plate, called a back-arc basin. Essentially, the volcanic arc, sitting  right at the edge of a colliding plate,   changes the way the tectonic  forces act on that plate. And behind it, things switch  from pushing to pulling,   extending the crust instead of compressing it.

We can see examples of these back arc basins  today in the Sea of Japan, which was formed   by extension between Japan and eastern Asia  while the Pacific was crashing into Asia. Underwater, between Samoa and New  Zealand, there’s another back arc   basin opening up to the east of the subduction  zone between the Pacific and Australian plates. Often, back-arc extension creates small,   relatively shallow seas just  behind the continental edge,   and geologists think this is what happened at the  edge of Laurentia, about 1.3 billion years ago.

And it just so happened that this warm, sunlit sea  was the ideal place for ancient life to thrive. These days, tropical seas are  often home to busy coral reefs,   complete with schools of browsing fish  and the occasional lurking predator. But back in the Mesoproterozoic,  none of these creatures had evolved.

That did not stop these ancient tropical  oceans being a haven for life though. Organisms had started photosynthesizing  at least a billion years earlier,   and all manner of bacterial and  eukaryotic life got in on the trend. Drawing dissolved carbon dioxide  from the oceans, green algae and   cyanobacteria colonised the open water and  the sea floor, wherever the light touched.

The seabed was covered with these vast  microbial mats made of interlocking   bacterial fibers that wove themselves  together into a kind of organic fabric. And since grazing marine animals  like molluscs and worms wouldn’t   evolve for another 500 million years  or so, there was nothing to stop them   from growing like crazy, so they  were kind of all over the place. Despite their abundance, we don’t actually  have many actual fossils from this time period.

Bacteria and algae are made up of squishy,  carbon-based compounds without any mineral   skeletons or shells, so they tend  to rot away as soon as they die. But what we do have are stromatolites. Stromatolites are kind of a bacteria-sediment  lasagna, built up over thousands of years.

You start with simple photosynthesising  cyanobacterial mats, spread out over the   seafloor to catch as much sunlight as possible. But over time, those mats would  become covered in sediment. The sticky organic fabric  would trap and bind particles,   and even chemically precipitate  minerals like calcium carbonate.

All of that mineral stuff would end up  blocking the sunlight to the photosynthesizers,   so the microbes would migrate upwards to  recolonise the new surface of the sediment. The process then repeats, with more  minerals being trapped and precipitated,   and the photosynthesizers creeping up  back into the sun, over and over again. The result, after thousands of years of growth,  is a domed, layered, mineralised structure that   may also contain some of the carbon of  the original bacteria, trapped within it.

These layered biosedimentary  structures aren't very common   any more, but they were a major  feature of the Proterozoic seas,   and were the microbes' answer to  living on a changeable sea floor. And unlike a squishy microbe on its own, a  stromatolite is built to stand the test of time. So, in the back arc basins next  to the colliding continents of   Laurentia and Amazonia, stromatolites  grew amongst the carbonate sediments.

Over time, they were buried completely  and became incorporated into deep   limestone deposits that may have  been about 2 kilometers thick. And since this was still very  much an active tectonic setting,   the limestone rock would occasionally  be injected with fluids from the nearby   volcanic arcs, depositing metals like  zinc amongst the calcium carbonate. But this tropical, volcanic, microbial  Eden wasn’t going to last forever.

Laurentia and Amazonia kept on grinding towards  each other, and eventually the volcanic islands   and back arc basin were crushed together  as the edges of the continents touched. This collision took about 200 million years in  total, and by the end of it around 1 billion years   ago, Laurentia and Amazonia were melded together  to form the core of the new supercontinent   Rodinia, with a towering range called the  Grenville mountains marking the dividing line. The back-arc limestones were buried inside these  mountains up to 20 kilometers below the surface,   where the intense heat and pressure transformed  it from sedimentary rock into metamorphic marble.

This was the birth of the Franklin  marble, sometimes known as Franklin ore,   considering the deposits of zinc it contains. There’s a lot more that has happened  to these rocks over the millennia,   so here’s a real quick rundown of, like,  800 million years of plate tectonics. Another 200 million years or so later,   tectonic forces shifted and the  Rodinian supercontinent shattered.

Laurentia was sent off on its own  again, with the remnants of the   Grenville mountains and the Franklin Marble  at their core, still clinging to its edge. Then, around 450 million years ago, the ocean  basin was swallowed underneath the supercontinent,   smushing another volcanic arc onto  the side of the Grenville mountains,   burying the Franklin marble and  chemically altering its composition. It would take yet another mountain  building event in the early Permian,   about 300 million years ago, for these  ancient layers to see the light of day.

At this time, the African continent collided with  Laurentia as the assembly of Pangea was completed. The impact pushed these Grenville mountains and   the Franklin marble all the  way back up to the surface. And thus ended the game of bumper cars  that gave us the Appalachian Mountains.

Pangea is long gone, as is Nuna,   and all the other supercontinents you  may not have heard of before today. But the plates are still moving, and there’s  bound to be more collisions in the future. The places where we find Franklin marble are  among the oldest parts of the Appalachian Mountain   range, alongside other places like the Blue Ridge  Mountains and the Adirondacks in New Jersey.

And the region where these pretty rocks  come from is valued for its fertile soil,   the zinc deposits, and the lime  that can be used for building. Studying these rocks has allowed  geologists to piece together a lot   of this super-ancient geologic history,  revealing the unexpected existence of an   extensional back-arc basin sandwiched  between two colliding continents. And for the rest of us, Franklin  marble serves as a handy reminder   of a billion years of American continental  history, bringing together sedimentary,   igneous, and metamorphic processes, shaped by the  tiniest microbes and the biggest supercontinents.

Not bad for this unassuming little rock! Rocks Box subscribers are getting their own piece  of Appalachia’s history in this month’s box. Every month, we send our Rocks  Box club a hand-picked mineral   and the story of what makes it so cool.

If you want to learn more  and maybe join the waitlist,   head over to scishow.rocks or  click the link in the description. And thank you, as always, for watching! [♪ OUTRO]