Xenolith – N35 21.773 W111 31.055
Just across the bridge turn to the west and look back across the
channel. You are looking for the white chunk of rock embedded in
the black lava. It is on the opposite side of the fissure. There
are other pieces visible from this location. These pieces of rock
are called xenoliths (pronounced zee-no-liths). Xenoliths are
pieces of rock that are foreign to the surrounding volcanic
rock.
When the magma pushed its way up to the surface
it forced its way through the surrounding rock tearing off
peaces from the wall of the conduit and carrying them to the
surface. The white rocks you see are pieces of the Kaibab
Formation that is now 700 to 1000 feet below the surface. The
Kaibab is a layer of limestone that is exposed in the Grand
Canyon,
Walnut Canyon, as well as other places on the Colorado
Plateau.
The smaller pieces melted before the magma cooled, however the
larger pieces did not melt completely and were encased in the lava
when it cooled.
The upper picture is a wide angle shot with the
xenolith in the middle. It is in the far side of the fissure
beneath the one bush in the middle. The lower picture is a
soom of the same xenolith.
Cinder Cone – N35 21.753 W111 30.946
Sunset Crater Volcano began erupting between 1040-1100 AD, and
possibly during the winter of 1064 to 1065 AD. The eruption began
along a 6-mile long fissure. Showers of lava shot up into the air
powered in part by gasses that were trapped in the lava (similar to
the way a carbonated drink shoots out of the can after it is
shaken). (See
Cinder Hills Overlook Earthcache)
Very quickly the eruption became concentrated at the north end
of the fissure where the shower of lava built up the cinder cone in
front of you. The lava cooled as it traveled through the air and
formed what are called pyroclasts or cinders, the loose pieces of
volcanic rock that cover the area. These cinders are typically air
filled and relatively light for rocks of their size. The cinders
usually build up around a single central vent to create a circular
or oval cone with a crater at the summit.
Image Source:
http://pubs.usgs.gov/gip/volc/types.html
Recent studies show that the eruption of Sunset Crater was
short-lived; less than 40 years and perhaps as little as a few
months..
Erosion on the Cinder Cone – Same as Cinder Cone
Early in the history of the Monument, people were allowed to climb
to the top of the cinder cone. The cinders are not held together,
so they were easily pushed down by each visitor’s step. Soon deep
gullies had formed along the trail that people took to the top of
the crater. A significant restoration effort was completed to fill
in the gullies and the crater was closed to climbing. (If you want
to climb a crater there is a trail up Lenox Crater to the west).
Even with the restoration, you can still see the switchbacks etched
into the side of the crater. Study the side of the crater and count
the number of switchbacks you can see and e-mail me your count for
the find.
Squeeze-up – Same as Cinder Cone
After most of the gas had escaped from the magma (molten rock
under the earth), some of it found its way out from the base of
Sunset Crater Volcano. There were two lava (molten rock on the
surface) flows, the Kana-a Flow to the north and the Bonito flow to
the west. This trail is on a portion of the Bonito Flow. More of
the Bonito flow can be seen across the street at the Lenox Crater
parking area and the
Bonito Lava Flow Earthcache.
One of the features that formed in the flow is the squeeze-up or
anosma to the south of the viewing platform.
A squeeze-up forms when the top of a lava flow cools and hardens
while below the solid surface the lava remains molten and continues
to flow. When a crack forms in the hard surface cooling lava is
then “squeezed-up” through the crack. This cooling lava acts much
like clay. The vertical grooves on the side of the squeeze-up are
preserved scrape marks as the lava oozed up through the crack. As
the squeeze-up gets taller it begins to topple over from its own
weight. This one cooled to rock before it completely fell over.
Lava Tube – N35 21.818 W111 30.805
Another feature that formed in the lava flow is a lava tube.
Similar to the squeeze up, the surface of the lava flow cooled and
hardened forming an insolating cover. However, the surface didn’t
crack and the molten lava continued to flow under it. When the
eruption ended, the lava continued out the end of the tube leaving
an empty void under the solid surface. Typically, lava tubes are
sealed on both ends with rock, but in some places the surface has
collapsed creating entrances. This tube is closed for safety
concerns. (The Forest Service allows access to
Lava River Cave, another lava tube from a different eruption,
on the west side of the San Francisco Peaks)
Hornito - N35 21.832 W111 30.770
Hornitos have been described as miniature volcanoes. They form in a
lava flow when lava is forced up through an opening in the cooled
and hardened surface of a flow. Very fluid fragments of lava are
ejected from the opening and build up in a ring around it. The
resulting feature is a hornito (“little oven” in Spanish).
As with the cinder cone itself, this feature is much more
fragile than it looks. Years of visitors touching and climbing the
walls have broken up the walls and reduced the size of the splatter
cone.
Until the trail guide is corrected, this is not a fumarole.
Fumaroles are vents that release volcanic gases, including water
vapor.
Aa Basalt Flow – N35 21.725 W111
30.985
Igneous rocks are formed from the cooling of molten (liquid) rock.
Igneous rocks can be divided up into four types based on the amount
of silica they contain. Silica is common compound found in the
earth’s crust. For each of the four types of igneous rocks, there
is a volcanic and plutonic name for the rock. Plutonic rocks form
underground and cool slowly forming interlocking mineral crystals.
Volcanic rocks form on the surface and cool quickly forming very
few if any visible mineral crystals. See
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html for a
set of graphics. The lava at Sunset Crater Volcano is called basalt
and contains the least percentage of silica compared to other
volcanic rocks. As basalt cools it tends to form either a surface
of jagged rocks or a smooth ropey surface. A lava that cooled with
a surface of jagged rocks is called by its Hawaiian name, Aa
(ah-ah). At Sunset Crater Volcano you will find mostly Aa.
The smooth ropey lava has been given another Hawaiian name
Pahoehoe (pa-hoy-hoy).
While there is no chemical difference between the two types of
basalt, a combination of temperature, viscosity (how well a liquid
flows), and gas content appears to play a part. Hot, low viscosity
lava with little gas tends to form pahoehoe while a cooler, high
viscosity, gaseous lava tends to form Aa. As you might have guessed
from the other volcanic features, the lava at Sunset Crater tended
to have a lot of gas in it.
When flowing, cooled blocks of lava sometimes are rafted down
the lava flow. When these blocks knock against each other they make
a clinking sound, giving them the name “clinkers.” Even now, the
blocks will make a clinking sound if dropped on each other.
Recolonization – N35 21.745 W111 31.008 and -N35 21.801
W111 31.113
One of the first colonizers on a fresh lava flow are lichen. These
symbiotic organisims are made up of fungus and either algae or
cyanobateria or both and grow right on the rock. Very slowly these
organisms break down the rock into soil. As you might imagine, it
takes a very long time.
Plants also begin to grow in the lava flow, but as you can see,
some areas have more plant life than others. One of the critical
factors is the availability of water. Any water that does fall
flows quickly through the many cracks and holes in the basalt. Even
the slightest advantage to retain water helps plants gain a
foothold in the lava. As a result, north-facing slopes, which
receive less sunlight tend to have more vegetation. Have a look at
the crater slopes. The south slopes are almost barren while the
north slopes have quite a few trees growing on them. Much of the
vegetation growing on the lava flow is in soil that has blown
in.
Logging requirements:
Send me a note with :
- The text "GCZ5GD Sunset Crater Lava Flow Trail" on the first
line
- The number of people in your group.
- -E-mail the number of switchbacks you can still see on the side
of the crater and give your ideas on why they are still visable,
and
- -E-mail the number of xenoliths you can see from the xenolith
coordinates
-
The above information was compiled from the
following sources:
- National Park Service, 2005,
Lava Flow Trail, Trail Guide, Sunset Crater Volcano National
Monument.
- USGS, 2005, Geology of Sunset Crater National
Monument, How about going on a volcanic field trip?
http://wrgis.wr.usgs.gov/parks/sunset/sunsetft.html
- Hanson, Sarah L. 2003, Roadside Geology:
Wupatki and Sunset Crater Volcano National Monuments, Arizona
Geological Survey, Down-to-Earth 15
- National Park Service, Sunset Crater Volcano
National Monument, Geologic Resource Evaluation.
- Holm, Richard F, and Richard B. Moore, 1987,
Holocene scoria cone and lava flows at Sunset Crater, northern
Arizona, in Geological Society of America Centennial Field Guide –
Rocky Mountain Section, 1987.
Placement approved by the
Sunset Crater National Monument