[music]
Here we are in the Ka’u desert on the southwest side of the Kilauea Crater on the big island
of Hawaii.
The ground consists of old lava flows, erupted over the last 250 years, covered now by wind-blown
sediment.
How do we know these sediments are wind blown?
Take a close look at the shape of the deposits and how they sit in the landscape.
Do they cover everywhere equally?
What’s the pattern for their locations?
Notice that in general these sediments seem to have a sloping face like a sand dune and
pile up behind cliff walls and other obstacles.
Notice also that on the surface of these dunes we see ripple marks, which are left by currents
of air moving across their face.
If you were to stand here on a windy day you would see and feel the sediment grains moving
across the surface and hitting your face.
Let’s look more closely at the sediments themselves.
What are they made of?
For the most part, the sediment is composed of sand-sized particles of volcanic glass.
To form glass, we need to get molten lava to cool and solidify so quickly that it doesn’t
crystallize: no crystals or minerals form because the atoms do not have time to come
together and bond in an orderly arrangement before the material completely solidifies.
The atoms are frozen in place.
That’s what we call glass.
What is it that makes lavas cool quickly?
Coming in contact with the air.
That happens on the surface of all lava flows, so it’s typical to find a thin skin of glass
on newly formed lavas in all active volcanic regions.
Over time, however, that glass gets broken and is picked up by the wind and carried away
as sediment.
Lava also cools quickly when it’s thrown up into the air, especially if the pieces
of lava are very small.
The more gas there is in an eruption, the more the lava will splatter away from the
vent across the surrounding surface.
It’s even possible in some eruptions for the gas to move high into the atmosphere carrying
small mud-sized bits of lava with it.
These quickly solidify into glass beads, which we then call ash.
The sediment that we see in piles here in the Ka’u desert is volcanic glass that likely
came from the continual eruption of the nearby Halema’uma’u vent in the Kilauea crater.
We are only a few miles from that vent.
The golden color of the glass comes from the composition of the magmas that feed this volcano:
typically about 50% silica and high in iron and magnesium.
One of the most spectacular sediments in the Ka’u desert can be found trapped alongside
thick edges of lava or in small depressions.
Notice this golden-colored hair-like material?
It’s called Pele’s hair and is also made of volcanic glass.
But this glass is very thin and looks as though it would be soft and flexible.
However, it is hard and will break quite easily.
How did it get this shape?
When lavas spattered out of the nearby vent, high winds or high amounts of escaping gas
blew through the lava separating it into thin streams that solidified as they spread.
The same process is used by glass blowers to create unique shapes of glass – melt
a silica-rich material like beach sand, and then as it cools down, blow air through it
to elongate it, stretch it out, or blow it into a balloon.
They can also use tools to shape the molten material before it cools.
Of course the air will cool the molten material so quickly that glass blowers have to work
quickly and reheat it if they want to keep shaping it.
How does Pele’s hair form so thin?
A similar process happens when cotton candy is made.
Sugar is melted by a heat coil in the center of a cotton candy machine and then the liquid
melted sugar is blown outwards while the device spins creating thin threads of liquid that
freeze into non crystalline sugar or glass.
So the sediment covering the old lava flows in the Ka’u desert is made of glass, in
two different forms: Pele’s Hair and sand-sized ash grains.
This material is lightweight and easily picked up by the wind and moved across the surface
before being dropped and collecting in piles or dunes.
The shapes made by the broken-up tops of the old lava flows provide a number of places
for the wind-blown ash and Pele’s hair to collect, and that makes hiking the Ka’u
desert a delightful adventure through a landscape strewn with sparkling golden sand and glass
strands and even frequently rainbows.
[music]