In 1994, a massive earthquake shook the Northridge suburb of Los Angeles, killing 57
people and injuring over 5000. The cost of damages was in excess of $20 billion. It's
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. It's 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 100 times more viscous
than water, and honey is about 100 times more viscous than that. Meanwhile, pitch has a
viscosity 2.3×10¹¹ 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 9 drips—roughly 1 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 Mainstone,
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 that you may have heard is a very viscous liquid is glass. If you look
at the stained glass windows of old churches, you'll find the bottom of the 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, and we find they still work
perfectly after hundreds of years. Plus, studies of thousand year old windows find no
real evidence of flow. So the truth is that it's 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's 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 a 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 time scales 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 Karl 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.
Hey! This episode of Veritasium was supported by Audible.com, a leading provider of
audiobooks, with over 180,000 titles in all areas of literature, including fiction,
non-fiction and periodicals. This week I wanted to recommend to you a brand new
science-fiction book published in 2015. It is by Neal Stephenson, and the book is called
'Seveneves', and it's about a massive catastrophe that threatens the Earth. I think that
if you're into science, you will really enjoy it, and you can actually download this book
for free by going to audible.com/veritasium, or you can pick any other book of
your choosing for a one month free trial.
So, I really wanna thank Audible for supporting me and I wanna thank you for watching,
and if you haven't signed up for my mailing list yet, that is something you should
probably do. You can click on this card here and go over to Veritasium.com, and that is
where I will be announcing some very cool things coming out very soon. So yeah,
check it out, if you haven't already. Bye!