Hey, Thoughty2 here.
If you’ve ever found yourself flipping furniture as you search frantically for your car keys,
you’ll know how easy it is to misplace something. Maybe you’ve lost your phone,
your wallet, or the will to live.
But of all these, the biggest mystery is how we lost a billion years of history. This is
far more serious than when you found your keys in the bread bin, because there’s a
colossal blank spot in our planet's past and nobody’s quite sure where we put it.
To put things into perspective, a billion years is almost a quarter of the earth’s
existence. And yet, there’s a billion-year gap in the fossil record that geologists call
“The Great Unconformity”. Let’s dig a little deeper, shall we?
Speaking of which, if you ran outside right now and started to dig, you’d uncover older layers
of rock the deeper you got. First, you’d find an Alanis Morisette tape from the 90s, maybe a disco
ball from the 70s, but eventually, you'd excavate some Roman coins and other assorted artefacts.
So why is it that both human and natural remains sink deeper into the earth, as time passes?
Well, the earth’s surface is constantly being littered with fine particles like
sand and dust - this sediment accumulates in an agonisingly slow process called deposition. Then,
in another process called cementation, these mineral particles gradually bind together until,
a few million years later, and sedimentary rock is formed.
Then another layer of rock forms on top of that, then another, and another, and so on,
until the ground beneath your feet is like a trifle - a rock trifle, with millions of layers
made over millions of years - and not a single one of them contains custard, what a tragedy.
But during all this sedimentation and cementation animals are doing their usual party trick of
dying. All the soft, squidgy bits get eaten or decompose, but sometimes the harder parts like,
bones, shells and husk get covered in dust - slowly succumbing to the sedimentation - thus
becoming encased in rock. Over time, water penetrates this rocky tomb, which dissolves
the biological remains within. Eventually, minerals fill the void and form a fossil,
which is of course just a rock replica of the creature that once resided there.
Importantly, though, that fossil is frozen in time,
enshrined in the sedimentary rock from the era in which the animal lived.
Much of our understanding of the history of the planet and life itself, has come through
studying the sequence of sedimentary rock layers and the treasures they’ve trapped through time.
Often, the different layers, called strata, are clear and distinct, with each revealing
something unique about the age it was formed in. The stupendously fun pastime of studying
strata is called Stratigraphy, and one of its guiding principles is the Law of Superposition,
which states that “if undisturbed, the lower down the rock layer, the older the rock”. Not
exactly rock-et science, but it does mean that generally, when you’re looking at a
cross-section of geologic formations, you have a fairly neat picture of the passage of time.
But unfortunately history isn’t so predictable. While new layers of sedimentary rock are being
formed in one location, in another conditions can change and sedimentation stops. Wind or
water erosion can move particles elsewhere, effectively deleting part of the geologic record.
After a while, sedimentation might resume, but these temporary interruptions will forever leave
a gap in the rock record, and sometimes that gap - called a hiatus - can be millions of years long.
It’s like hitting pause, rewind and then record again while in a very long conversation – in the
final recording there will be missing parts that are irrecoverable, and if these parts
are integral to the meaning of the conversation, then the whole thing will no longer make sense.
When these hiatuses happen, it creates a noticeable difference in the angle,
colour and texture of the new rock that sits on top - as you can be seen here, and here.
Geologists call this feature an unconformity. It was Danish man Niels Steensen - considered
the father of Stratigraphy - who first sketched an unconformity in 1669. But it
was a Scottish farmer called James Hutton who discovered the first significant unconformity.
Whilst walking near Siccar Point in Scotland, in 1787, he noticed the striking difference
between the lower and upper rock formations and he coined a name for it, “Hutton’s Unconformity”.
If that sounds a little rude to you, but you’re not really sure why, join the club.
The greyish lower layer of rock is made up mainly of deep-sea deposits from around 425
million years ago. But the reddish upper layer was deposited by rivers and streams
about 345 million years ago. That’s a hiatus of about 80 million years.
It was on the back of this discovery that Hutton helped develop some major
theories about the movement of tectonic plates, and the forces that shape the
face of the earth. Some geologists call Siccar Point a geologic shrine,
which tells you how incredibly strange both this formation and geologists actually are.
However, the most astonishing unconformity was found in 1869, when John Wesley Powell
was observing a strange mound. Whilst traversing the Grand Canyon he noticed
vertical rock formations butting up against horizontal formations. Channelling the supreme
creativity of his geologist predecessors he named it ‘Powell’s Unconformity’.
But since then it’s been more commonly referred to as Powell’s Great Unconformity, and there are two
striking reasons why it’s deserving of a ‘Great’. Firstly, the unconformity actually spans an entire
continent, stretching across Laurentia, which is the name for the ancient heart of North America.
Secondly, many other unconformities had been unearthed around the world by this point, but
they mostly represented small gaps, on a planetary scale at least - maybe tens of million years.
But the smallest time gap between the two rock layers in the Great Unconformity is
a modest 250 million years, and the biggest is over 1.2 billion years. That’s right, that
missing piece of the prehistoric pie sandwiched between these two unassuming rock formations
represents up to 1.2 billion years of missing history - and we have no idea where it went.
But there’s much more to this rocky weirdness. Since Powell’s grand discovery
a few more formations have been found all over the world that have the same time period plucked
from their record. Together we collectively refer to these as “the Great Unconformity”;
it’s a mystery that spans the entire planet and over a billion years of missing knowledge.
Take the Grand Canyon example, here the 525 million-year-old Cambrian-era Tapeats Sandstone
sits snugly on top of the 1.7-billion-year-old Vishnu Basement Rocks, dating back to the
Precambrian-era - like I said, a time gap of 1.2 billion years - weird right? Scuttle over
to the Ozark Plateau in Missouri, where we see a similar sight - 500-million-year-old sandstone
sits suspiciously atop 1.4-billion-year-old granite - representing a 900-million-year
hiatus. For the sake of all that is igneous, what the metamorphic heck is going on here?
Well, some have suggested a theory to explain the Great Unconformity. And
it’s called the “snowball earth” theory - it refers to a time during the Cryogenian
period around 650 million years ago when the earth´s surface was thought to be covered by
glaciers and ice sheets. Glaciers are capable of causing erosion on a massive scale as they
grind across landscapes, and it’s been estimated that they collectively removed
two to three miles from the earth’s surface – basically the earth had its face exfoliated.
Do you remember me saying that the younger rocks that sit atop the Great Unconformity
all around the world date back to a period called the ‘Cambrian Period’?
Well, what’s really interesting here is that the Cambrian period was when the biggest explosion of
life in our planet’s history took place. During a tiny timeline of only 11 million years, between
approximately 541 and 530 million years ago, the original ancestors of almost all the lifeforms
we currently know today sprang into existence; for example, this is when fish first appeared.
There was life before this event, but this short window was when mother nature downed a Redbull,
snorted some cocaine and hit the big evolutionary boost button.
So how does this relate to the ‘Snowball Earth’ theory? Well,
when all these ice flows started to hit the ocean, they deposited gargantuan quantities
of nutrient-rich minerals into the water, where - the theory goes - it was responsible
for the aforementioned explosion of life. Over 1 billion years of sedimentary rock was carved
from the face of the planet by glaciers, then plunged into evolution’s breeding ground, the
ocean - where the many minerals contained within acted as a catalyst for life as we know it today.
So in the end it looks like we did lose a billion years of history down the back of the sofa;
only the sofa in question turns out to be the bottom of the ocean. But Snowball Earth remains
a theory, one that was only posed as recently as 1998 by Dr. Paul Hoffman. And if you ask me,
two decades doesn’t seem like enough time to definitively explain a billion years of
missing history. Maybe someday new techniques or technology will rock the geology world and enable
us to fill in more of the missing gaps in our planet’s very long, very empty history book - but
until then the Great Unconformity will remain a great mystery.
Thanks for watching.