Kind: captions
Language: en
[Applause]
in 1994 a massive earthquake shook the
Northridge suburb of Los Angeles killing
57 people and injuring over 5,000 the
cost of damages was in excess of 20
billion dollars its earthquakes like
this one that make us question just how
solid is the earth beneath our feet and
what does it mean to be solid anyway at
first glance pitch looks like a solid
but it's not is actually a liquid at
room temperature just a very viscous one
viscosity is a measure of resistance to
flow what we often think of as the
thickness of a liquid olive oil is
nearly a hundred times more viscous than
water and honey is about a hundred times
more viscous than that meanwhile pitch
has viscosity 230 billion times that of
water at the University of Queensland in
Australia pitch is the subject of the
world's longest-running lab experiment
and it's still going to this day back in
1927 this glob of pitch was placed into
a funnel and ever since then in nearly
90 years it has produced only nine drips
roughly one a decade and no one has ever
been in the room to see a drop fall
though in 1988 the former custodian of
the experiment John Mayne stone came
very close to observing a drip fall
except he stepped out of the room for
just a few minutes to get a cup of tea
now you can actually watch this
experiment live there's a link in the
description but since the last drop
happened in 2014 I think you'll probably
be waiting a while another substance you
may have heard is a very viscous liquid
is glass if you look at the stained
glass windows of old churches you will
find the bottom of a pane is decidedly
thicker than the top and that's because
the glass has been flowing down over
centuries actually no it hasn't you know
we've looked at old telescopes where the
optics is very sensitive to slight
shifts in the lens glass we find they
still work perfectly after hundreds of
years plus studies of thousand-year-old
windows
find no real evidence of flow the truth
is it is just very difficult to make
glass of uniform thickness and so when
the glass was originally installed
thousands of years ago they would
install the thickest part towards the
bottom the lead actually has a lower
viscosity than the glass so if the glass
had even thickened a little bit then the
lead should be a puddle on the floor by
now now glass is unusual in that it's an
amorphous solid meaning that the silica
molecules are not regularly arranged as
in a regular crystalline lattice instead
they're all in a jumble and this is
because the glass is cooled down so
quickly from the liquid state to the
solid state that the molecules don't
have time to arrange themselves in a
nice regular crystal structure but what
makes something a solid rather than a
liquid is that all of the atoms or
molecules are so strongly bonded
together chemically that they can't
slide past each other so in water or
olive oil or pitch the molecules can
slide past each other but in glass at
room temperature they can't so what
about the interior of the earth beneath
the earth's crust is the mantle which is
responsible for plate tectonics and
therefore earthquakes is it a solid or a
liquid we can obviously never observe
the mantle directly but when we do see
material come out from underground
it is red-hot rock it is lava so you
might be imagining that the mantle is
very similar made up of this
molten magma hot liquid rock and that
would make sense because in order for it
to flow it must be a liquid right
actually wrong the mantle is a solid
under all that pressure even though it's
at very high temperature it remains in
the solid state and we know the mantle
is solid because shear waves from
earthquakes can actually propagate
through the mantle these waves cannot
propagate through liquid like the molten
iron of Earth's outer core because
liquids flow in response to being
sheared or rubbed sideways and as a
result we can see the shadow of the
liquid outer core by measuring seismic
waves from an earthquake on the other
side of the world but how exactly does
this solid rock flow well the answer
lies in the fact that crystals aren't
perfect there may be a missing atom here
or there and under the high pressures in
the mantle sometimes a neighboring atom
will pop in to fill that gap now from a
human perspective it takes a very long
time for this to have a noticeable
effect but from the Earth's perspective
it happens in no time at all
the viscosity of the mantle is similar
to that of glass to several orders of
magnitude greater so it is really only
over these geological timescales
that the mantle is fluid like at all so
pitch a liquid can flow so slowly as to
seem like a solid whereas the Earth's
mantle a solid behaves like a fluid if
you just wait long enough as the famous
American geologist Grove Carl Gilbert
once said to my mind it appears that the
difficulty is only imaginary and not
real rigidity and plasticity are not
absolute terms but relative and all
solids are in fact both rigid and
plastic when great masses and great
forces are involved the distinction
loses value sometimes the rigid
definitions we create for ourselves can
introduce misconceptions or viscous
rumors like the idea that the core of
the earth is a giant ball of magma if
only we could think about liquids and
solids a little bit more fluidly
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bye