Lava Flows EarthCache
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Warning: the lava flows are on private
property and there are no trespassing signs posted all along the
road.
The coordinates will lead you to a pull out on the side of the road
next to a green electrical box a couple of miles from an Air Force
Installation. You can plainly see the lava flows from this pullout,
there is no need to trespass to log this Earthcache.
Lava is molten rock expelled by a volcano during an eruption. When
first expelled from a volcano it is a liquid at temperatures from
1,300 °F to 2,200 °F. Lava is about 100,000 times more viscous
(i.e., thicker) than water but it can flow great distances before
cooling and hardening.
A lava flow is an outpouring of lava which is created during a
non-explosive eruption. When lava stops moving it solidifies to
form igneous rock. Explosive eruptions produce a mixture of
volcanic ash and other fragments called tephra. Tephra is air-fall
material produced by a volcanic eruption regardless of composition
or fragment size.
The viscosity of lava is important because it determines how the
lava will behave.
Highly viscous lava shows the following behaviors:
a. tends to flow slowly, clog, and form semi-solid blocks which
resist flow,
b. tends to entrap gas, which form bubbles within the rock as they
rise to the surface, and
c. correlates with explosive eruptions and is associated with tuff
and pyroclastic flows .
Highly viscous lavas do not usually flow as liquid, and usually
form explosive fragmental ash or tephra deposits. However, a
degassed viscous lava or one which erupts somewhat hotter than
usual may form a lava flow.
Lava with low viscosity shows the following behaviors:
a. tends to flow easily, forming puddles, channels, and rivers of
molten rock,
b. tends to easily release bubbling gases as they are formed,
c. rarely pyroclastic and are usually quiet, and
d. volcanoes tend to form broad shields rather than steep
cones
There are three forms of low-viscosity lava flows: aa, pahoehoe,
and pillow lava.
Aa (pronounced ah-ah) is a Hawaiian word meaning "stony,” "burn,"
or "blaze" is one of three basic types of lava flows. Aa is
basaltic lava characterized by a rough or rubbly surface composed
of broken lava blocks called clinker. The loose, broken, and sharp,
spiny surface of an aa flow makes hiking difficult and slow. The
clinkery surface actually covers a massive dense core, which is the
most active part of the flow. As pasty lava in the core travels
downslope, the clinkers are carried along at the surface. At the
leading edge of an aa flow these cooled fragments tumble down the
steep front and are often buried by the advancing flow. This
produces a layer of lava fragments both at the bottom and top of an
aa flow. Accretionary lava balls as large as 10 feet are common on
aa flows. Aa is usually of higher viscosity than pahoehoe. Pahoehoe
can turn into aa if it becomes turbulent from meeting impediments
or steep slopes. The sharp, angled texture makes aa a strong radar
reflector, and can easily be seen from an orbiting satellite. Aa
lavas typically erupt at temperatures of 1000 C to 1100 °C.
Pahoehoe (pronounced pha hoe hoe) comes from Hawaiian, meaning
"smooth, or unbroken." Pahoehoe is basaltic lava that has a smooth,
billowy, undulating, or ropy surface. These surface features are
due to the movement of very fluid lava under a congealing surface
crust. A pahoehoe flow typically advances as a series of small
lobes and toes that continually break out from a cooled crust. It
also forms lava tubes where the minimal heat loss maintains low
viscosity. The surface texture of pahoehoe flows varies widely,
displaying all kinds of bizarre shapes often referred to as lava
sculpture. With increasing distance from the source, pahoehoe flows
may change into aa flows in response to heat loss and consequent
increase in viscosity. The rounded texture makes pahoehoe a very
poor radar reflector and is difficult to see from an orbiting
satellite. Pahoehoe lavas typically have a temperature of 1100 to
1200 °C.
Pillow lava is the rock type typically formed when lava emerges
from an underwater volcanic vent or a lava flow enters the ocean.
Pillow lava can also form when lava is erupted beneath thick
glacial ice. The viscous lava gains a solid crust on contact with
the water, and this crust cracks and oozes additional large blobs
or pillows as more lava emerges from the advancing flow. Since the
majority of Earth's surface is covered by water, and most volcanoes
are situated near or under it, pillow lava is very common.
Other cachers are of the opinion that the lava comes from the
super volcano located in Yellowstone. I sent pictures of the lava
flows to a friend that lives in Tucson who loves the field of
geology and his opinion is that the lava there is extremely ancient
lava flow represented by the erosion and the fact that the most of
the lava is hidden under deposited top soil or it is a “false lava
flow,” i.e. it is ordinary rock that just looks like lava. Either
way the deposits are interesting to visit.
To log this Earthcache do three things:
1) In your log entry, when you log this cache, write your opinon on
the lava field, is it: aa, pahoehoe, pillow, or just ordinary rocks
that happen to look like lava.
2) In your log entry, when you log this cache, tell us where you
think this lava comes from: the Yellowstone super volcano, a very
ancient local lava flow, or from someplace else?
3) I think everyone enjoys seeing pictures of geocachers in the
logs, so either place a picture of yourself in your log entry with
the lava fields in the background, or, if you don’t have a camera
and look like a cromagnon like me, then speculate on how old the
flows actually are.
Additional Hints
(No hints available.)