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earth is our home and we like to think
of it as pretty well understood but
really we're still learning a lot about
it from plate tectonics and continents
to the time the Mediterranean Sea
disappeared here are six mysteries about
our planet we're still trying to solve
first there's the question of how and
when plate tectonics started the Earth's
crust and very tip-top of the mantle are
divided into floating tectonic plates
which are constantly moving pushing
against and slipping under each other
and from this process we get phenomena
like earthquakes volcanoes and moving
continents understanding plate tectonics
is fundamental to understanding earth in
fact you could even say it's the bedrock
of geology but even after more than 50
years nobody can pin down how and when
this whole deal started part of the
reason is because there's very little
physical evidence left from the early
Earth and what is left is kind of up for
interpretation so two scientists looking
at the same sample might come to two
completely different conclusions one way
we've investigated this though is by
using mineral samples like Ohio Lights
to try to nail down a start date for
this cycle these are fragments of ocean
crust preserved in continental crust and
they could only have formed if plates
were already colliding or sliding across
each other so if we find them in rocks
of a certain age it could be evidence
plate tectonics had started by then some
of the oldest ovulates we found are 3.8
billion years old which would suggest
that plate tectonics was active in some
form early on but others suggested dates
range from almost immediately in the
Earth's life cycle over four billion
years ago to less than 1 billion years
ago so that's quite a range
meanwhile scientists are also looking
into this by creating and testing
computer models 2015 study for instance
used models to investigate mantle plumes
immense upwellings of magma from deep
within the earth and it found that one
or more ancient plumes would have had
enough force to crack the early
Earth's crust and kickstart the tectonic
cycle which would have been amazing to
watch from a spaceship not on earth
figuring out this conundrum is important
because it wouldn't just help us
understand life on our planet but in the
universe - because plate tectonics
hasn't just shaped how our planet looks
it's also done things like keep the
atmosphere stable by adding and removing
carbon dioxide that helps keep our
planet from tipping into either freezing
or boiling and because of this some
scientists think that these plates might
be necessary for life to develop and
thrive on other planets as well no
matter how things kicked off plate
tectonics started slowly rearranging the
face of the earth and that leads us to
our second mystery some 250 to 350
million years ago the continents got
squished together into one giant
landmass called Pangaea it was what
scientists called a supercontinent but
it wasn't the Earth's first and it won't
be the last
and like plate tectonics more generally
this repeat supercontinent cycle has had
a profound effect on our planet's crust
atmosphere oceans and life the breakup
of Pangaea might have triggered a global
change in climate for example as fresh
tectonic rifts released co2 into the
atmosphere but explaining why this cycle
exists and what drives it is trickier
again a lot of the physical evidence is
up for interpretation and also it's
possible that no two super continents
formed for exactly the same reason case
in point
one of these possible forces scientists
are looking at to explain supercontinent
formation is called extraversion and
it's nothing like the personality trait
in it you start with a supercontinent
then a new ocean forms in the middle and
pushes the other plates away eventually
the ocean grew so much that the plates
circle around the globe and end up
shoved into one area but as a 2008 paper
pointed out while extraversion alone
might explain some super continents it
struggles to explain every episode of
supercontinent formation including
Pangaea instead that might have been
caused by extra versions opposite
introversion in that case the cycle
starts the same with a new ocean opening
up but then the process stops and
reverses drawing the other plates
back together again like a giant drain
one test that scientists can use to try
to figure out what drove certain
episodes is by testing the age of marine
deposits from tectonic plate collision
zones this can tell you whether they
came from old oceanic crust or young
oceanic crust which would be
characteristic of extraversion and
introversion respectively another thing
that might be the result of plate
tectonics is the masini 'n salinity
crisis about 6 million years ago the
mediterranean got cut off from the ocean
as something closed the strait of
gibraltar the mediterranean naturally
loses more water to evaporation than it
gets from rivers so without new water
from the street it began to dry up and
while exactly how much it dried up is
controversial we know it left kilometer
thick salt deposits behind and had major
local effects on climate and fauna but
why did that closure happen just looking
at the rock itself is tricky because
it's hard to pinpoint the age of the
layers there aren't many fossils for
example which normally serve as
signposts so we've had to turn to other
methods like looking at volcanic rocks
in 2003 scientists comparing rocks in
the area from different time periods
noticed a shift in the ratio of elements
found in rocks which they think
indicates a change in the mantle they
proposed that around the time of the
crisis a tectonic plate shifted allowing
hot mantle material to push up lifting
the land around the Strait of Gibraltar
out of the sea and closing off the
Gateway eventually though the crisis
passed and the Mediterranean started
filling up again
although exactly how that happened is
another head scratcher and of course
this whole idea is just a hypothesis so
as always more research is needed
volcanoes regularly bring magma to the
surface reshaping and creating new land
most of the time this is only important
in small areas or for short periods of
time like sometimes a powerful eruption
might destroy an island or temporarily
affect the world climate but more
massive eruptions known as flood basalts
can occur they can cover more than a
million square kilometers with magma
thousands of metres thick and some of
them like the Siberian and Deccan Traps
have been implicated in mass extinctions
so
truly we want to know why they happened
one conventionally sited cause might be
mantle plumes those giant upwellings of
material from near the Earth's core
another might be delamination we're part
of the lithosphere
that's the crust an upper mantle falls
away from the tectonic plate it was
attached to but we don't have definite
answers about how or why these things
might happen after all their events that
happened deep within the earth millions
of years ago but there are ways
scientists can try to decipher why flood
basalts occurred in 2006 for instance a
paper studied the age and makeup of
xenoliths which are rocks stuck inside
other igneous rocks they were looking at
xenoliths that would have formed in the
mantle just underneath the crust and
they were looking in a layer of rock
that would have been deposited soon
after a flood basalt event what they
were working off of is that if
delamination was actually the cause of
the flood the loss of that chunk of
lithosphere should also mean the loss of
those xenoliths sitting just inside or
below it so the scientists could look at
the age of any xenoliths they do find
and if there was delamination there
should be no mantle xenoliths older than
that event looking at these rocks it
does appear that delamination happened
at some point under China for instance
but not for other events like ones in
South Africa so the science continues
the Earth's axis of rotation naturally
drifts and wobbles over time in the 20th
century for instance it seems to have
moved about 10 centimeters per year in
part because of a rebound effect from
the last ice age we can map this polar
wander via satellite and understanding
it is important for climate modeling but
the Earth's wobble might not always
happen at such a steady and stately pace
in fact there's evidence there have been
major shifts around 174 to 157 million
years ago for example East Asia went
through a drastic change from wet
seasonal to extremely arid and this was
devastating for animals living there
there have been some explanations for
this before but in 2019 a paper
suggested this shift was due to polar
wander if that's the case it would mean
the earth essentially twisted sending
the area that
now China 2,800 kilometers south like
today's minor wobbles these major twists
would be tied to some kind of
redistribution of Earth's mass from
things like impacts or glaciers the
excess mass gets flung so to speak
toward the equator along with the
Earth's crust and mantle
it's just what physics says will happen
to a spinning sphere if it becomes
unbalanced that said it's really hard to
pin down exactly where and when these
shifts happen one technique we can use
this paleomagnetism when magma cools it
takes on different magnetic signatures
based on where it is in the Earth's
magnetic field so if we see a series of
rocks undergoing a quick drastic change
in their signatures it might be assigned
the area they were in moved very quickly
that said the rocks can be an imperfect
record since later bumps and squeezes
can change that signature which means
again it's hard to say anything
definitive alternatively for more recent
episodes it's been suggested that we can
use volcanic hotspots like the one
underneath Hawaii because the crust is
only floating on top of the mantle it's
technically possible that the crust
shifted while the mantle stayed about
the same and because those hot spots are
features in the mantle rather than the
crust we can use them to track how the
crust has moved finally one of the very
well-established facts in geology is
that the Earth's core is very hot what
the question is how hot those specifics
would help illuminate a ton about how
the earth works like how the magnetic
field is generated how the planet formed
and even how we could predict
earthquakes figuring out that
temperature is tricky though because we
can't just drill down there with
thermometers so we have to get creative
one thing scientists are doing is
studying how iron acts under high
pressures and temperatures in diamond
anvil cell experiments in these
experiments scientists measure what
happens when small amounts of a sample
are heated and squished between two
diamond tipped pushers the idea is to
put iron in one of these anvils then
squeeze and heat it until it's the same
density as the Earth's core a density we
already know thanks to other
measurements when you reach that rate
density you can check what the iron's
temperature is and that should give us
the temperature of the core using this
method a 2013 experiment revised the
estimate
for the melting temperature of iron at
the inner core boundary from about 48
hundred degrees Celsius to about 59
hundred degrees which is about as hot as
the surface of the Sun it doesn't look
like there's been any super huge shake
ups in that number since then so maybe
this is a mystery we're putting to rest
dad said that's the thing about
mysteries it only takes one new study to
open up the door again earth is our home
but it's a home built by forces and
processes many of which are invisible
long gone or mind-bogglingly complex
often all three so our planet is not
just set dressing it itself is one of
the great mysteries of science - thanks
for watching this episode of scishow if
you liked learning about geology you'll
probably really enjoy our special
three-part series about the geology of
Olympic National Park in Washington we
actually went to Washington to film it
which was a whole new thing for us and
we learned a lot as a result you can
watch the first episode after this
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