Lava tube on the way
down the Chain of Craters Road[1]
Lava tubes are natural conduits
through which lava travels beneath
the surface of a lava flow, expelled
by a volcano during an eruption.
They can be actively draining
lava from a source, or can be
extinct, meaning the lava flow
has ceased and the rock has cooled
and left a long, cave-like channel.[3]
Tube
is approx 15ft diameter. Kona
Coast, Hawaii [2]
Lava tubes are formed
when a flow of relatively fluid lava
cools on the upper surface sufficiently
to form a crust. Beneath this crust,
which by dint of being made of rock
is an excellent insulator, the lava
can continue to flow as a liquid.
When this flow occurs over a prolonged
period of time the lava conduit can
form a tunnel-like aperture or lava
tube, which can conduct molten rock
many kilometers from the vent without
cooling appreciably. Often these lava
tubes drain out once the supply of
fresh lava has stopped, leaving a
considerable length of open tunnel
within the lava flow.
Lava tubes are known
from the modern day eruptions of Kilauea,
and significant, extensive and open
lava tubes of Tertiary age are known
from North Queensland, Australia,
some extending for 15 kilometers.[4]
Lava tubes are formed
when an active low-viscosity lava
flow develops a continuous and hard
crust, which thickens and forms a
roof above the still-flowing lava
stream. Tubes form in one of two ways:
by the crusting over of lava channels,
and from pahoehoe flows where the
lava is moving under the surface.
Lava usually leaves
the point of eruption in channels.
These channels tend to stay very hot
as their surroundings cool. This means
they slowly develop walls around them
as the surrounding lava cools and/or
as the channel melts its way deeper.
These channels can get deep enough
to crust over, forming an insulating
tube that keeps the lava molten and
serves as a conduit for the flowing
lava. These types of lava tubes tend
to be closer to the lava eruption
point.
Further away from the
eruption point, lava can flow in an
unchanneled, fanlike manner as it
leaves its source, which is usually
another lava tube leading back to
the eruption point. Called pahoehoe
flows, these areas of surface-moving
lava cool, forming either a smooth
or rough, ropy surface. The lava continues
to flow this way until it begins to
block its source. At this point, the
subsurface lava is still hot enough
to break out at a point, and from
this point the lava begins as a new
"source". Lava flows from
the previous source to this breakout
point as the surrounding lava of the
pahoehoe flow cools. This forms an
underground channel that becomes a
lava tube.[3]
A broad lava-flow field
often consists of a main lava tube
and a series of smaller tubes that
supply lava to the front of one or
more separate flows. When the supply
of lava stops at the end of an eruption
or lava is diverted elsewhere, lava
in the tube system drains downslope
and leaves partially empty cave-like
conduits beneath the ground.
Such drained tubes commonly
exhibit step marks on their walls
that mark the various depths at which
the lava flowed, known as flow ledges
or flow lines depending on how prominently
they protrude from the walls. Lava
tubes generally have pahoehoe floors,
although this may often be covered
in breakdown from the ceiling. A variety
of speleothems may be found in lava
tubes[4] including a variety of stalactite
forms generally known as lavacicles,
which can be of the splash, shark
tooth, or tubular variety. Lavacicles
are the most common of lava tube speleothems.
Drip stalagmites may form under tubular
lava stalactites, and the latter may
grade into a form known as a tubular
lava helictite. A runner is a bead
of lava that extrudes from a small
opening and then runs down a wall.
Lava tubes may also contain mineral
deposits that most commonly take the
form of crusts or small crystals,
and less commonly, as stalactites
and stalagmites.
Lava tubes can be up
to 14-15 metres wide, though are often
narrower, and run anywhere from 1-15
m below the surface. Lava tubes can
also be extremely long; one tube from
the Mauna Loa 1859 flow enters the
ocean about 50 km (over 30 miles)
from its eruption point, and the Cueva
del Viento - Sobrado system on Teide,
Tenerife island, is over 18 km long,
due to extensive braided maze areas
at the upper zones of the system.
A lava tube system in
Kiama, Australia, consists of over
20 lava tubes, many of which are breakouts
of a main lava tube. The largest of
these lava tubes is 22 m in diameter
and has columnar jointing due to the
large cooling surface. Other tubes
have concentric and radial jointing
features. The tubes are infilled due
to the low slope angle of emplacement.[3]
Written and produced
by Donald B. MacGowan; videography
by Frank Burgess and Donald MacGowan;
Narration by Frank Burgess; Original
Music by Donnie MacGowan
Nahuku, the Thurston Lava Tube, gives
the visitor an opportunity for a close-at-hand
inspection of the inner plumbing of
a volcano. It also makes for an interesting
and unique way to escape the noonday
heat or afternoon shower briefly.
Lava tubes form when the outer crust
of a flowing river of lava begins
to cool and crust over, but the lava
continues to flow beneath it; when
the flow has completely drained away,
the lava tube is left behind.
Thurston lava tube is a remarkably
large, well-preserved and accessible
example of a lava tube-type cave.
An easy, 0.3 mile trail (about a 15
minute hike) winds through lush fern
forest alive with singing bird and
buzzing insects, down into a collapse
crater entering the lava tube and
slipping about 300 feet through the
well-lighted, floored cave, popping
up through a skylight in the tube
and returning to the parking lot.
A very easy walk and certainly a "must
see" for any visitor to the park.
The World Wonders .Com-visit 1,000 world wonders at www.theworldwonders.com
