Hey folks, I'm Dr. Chapman. Let's learn
about Petrified Forest National Park,
the Painted Desert, and mineralization.
Petrified Forest National Park is
located about 200 kilometers southeast
of the Grand Canyon in eastern Arizona.
The petrified wood and other fossils in
the park are found in the Chinle
Formation that was deposited around 220
million years ago
during the Triassic period. The Chinle is
one of the more colorful and distinctive
rock units on the Colorado Plateau
and the region where it outcrops is
called the Painted Desert.
The variety of colors in the Chinle
Formation are a result of numerous
volcanic ash layers
and paleosols. Paleosols are ancient
soils and the movement of water through
the soils with organic
and inorganic acids helps to mobilize
and redistribute various elements that
give both modern and ancient soils their
colorful banded appearance.
During deposition of the Chinle there
were large fluctuations in the height of
the water table.
 Sometimes the soils and sediments were
waterlogged and the region was
swamp-like and other times the region
was very arid and dry. Geoscientists have
theorized that this may be evidence for
a monsoonal paleoclimate.
A monsoonal climate pattern is one with
yearly changes alternating between a
rainy and wet season and a dry season.
When the Chinle
sediments were waterlogged and covered
by water, it prevented
oxygen from getting into the sediments
and oxidizing elements like iron and
manganese.
The blue and green hues and the Chinle
Formation are caused by the
presence of reduced
or non-oxidized iron and manganese
compounds.
The pink, orange, red, and purple colors
are related to
oxidized versions of the same elements.
The giant petrified logs in the Chinle
Formation
are all jumbled up and concentrated in
specific
beds or rock layers.  Geologists have
determined that the beds were deposited
during large floods
in which the trees were uprooted,
carried along in these raging rivers, and
then
dumped out onto a delta or a flood plain.
A flood plain is the flat-lying area
surrounding a river channel
that receives sediment during floods. The
flood deposits rapidly buried and
encased the trees in mud.
This prevented the trees and other
organic matter from decomposing.
In addition to
trees, 
paleontologists have found fossil rushes,
ferns, cycads - 
that all suggest the floodplains were
warm, 
swampy marshes.  Plate reconstructions
suggest that the area was located much
closer to the equator during the
Triassic, at tropical latitudes.  A modern analog
may be the swampy floodplain surrounding
the Brahmaputra river in Bangladesh.
During the summer monsoon season, massive floods carry
trees and sediment down from the
Himalaya and during flood years as much
as 75 % of Bangladesh may be underwater.
In the dry season, when the water recedes, thick deposits of mud and sand and
organic material like trees are
left behind.  Geologists believe that the
logs that ended up in Petrified Forest
National Park
were transported by a major river
flowing towards the northwest,
with its headwaters in Texas.  However, much of the sediment
in the Chinle Formation was derived
from the Triassic continental
volcanic arc, which was located to the
southwest.
The continental arc is the source of the
volcanic ash layers in the Chinle.
Silica leeched or dissolved from
volcanic ash is what silicified or
fossilized the petrified wood and other
fossils in the park.
The warm wet conditions  - with acid
leeched from decaying plant matter -
helped facilitate silica dissolution.
As groundwater circulated through the
buried logs, the silica either
precipitated
or became concentrated into a gel inside
of the inter-cellular cavities
and in between the cell walls of the
wood. Over time
the silica-rich gel dehydrated and
formed a non-crystalline version of the
mineral quartz.
This process is called
permineralization
and is the first step towards
petrification.
In permineralized fossils you can still
see the relic tissue and the cellular
structure
that has been filled in by silica.  During
petrification,
the cellulose and cell walls are
completely replaced by silica
and most of the fine cellular detail is
lost.
Most of the wood in Petrified Forest
National Park is petrified,
but many dinosaur fossils have also been
discovered in the park
and these fossils are mainly
permineralized.
One of the most famous dinosaurs
discovered in the park is Chindesaurus,
which is one of the oldest dinosaurs in
North America.  For comparison,
T-Rex evolved over 100 million years
after Chindesaurus went extinct.
Petrified wood is one of those materials
that seems to break the rules of geology.
It has the exact same composition as
quartz - silicon and oxygen,
but is not considered a mineral because
it doesn't have a crystalline structure.
To be classified as a mineral a
substance has to meet
5 criteria. First, it has to be
naturally occurring. So man-made
materials like steel
are not minerals. Second, it has to be
solid at room temperature.
Ice meets all the criteria of a mineral,
but it is water at room temperature so
some geologists don't consider it a
mineral.
Third, it must have a dominant chemical
composition and chemical formula.
For example, quartz is dominantly SiO2.
Minerals are never 100 %
pure though and it is normal to have
some contamination
or elements substituting for one another
within the mineral.
Fourth, minerals are inorganic so
materials like
wood or shells are not minerals, although
that criteria is increasingly debated.
Fifth and finally, minerals must have an
ordered atomic structure that forms a
systematic and repeating pattern -
called a crystalline structure. This is
why the words "mineral" and "crystal" are
synonymous to geologists.
It's also why petrified wood is not a
mineral. If you look at the
silica material that makes up petrified
wood with a powerful microscope,
you can sometimes make out very tiny
crystals so it is called
cryptocrystalline quartz or
microcrystalline quartz.
The most generic name for
cryptocrystalline or microcrystalline
quartz is chert.  If the silica had absolutely no
crystalline ordering to it  - it would be
called amorphous.
Traditional glass, which is made up from
melting and fusing quartz
sand, is amorphous. Microcrystalline
quartz can take on many
shapes and colors depending on the
conditions it formed in
and which trace elements or compounds
are present in it.
The variety of colors in petrified wood
are related to these
impurities.  Trace amounts of chromium,
cobalt, and copper
cause green and blue hues in petrified
wood.
Iron and manganese cause yellow, orange,
red, and brown colors.
Very pure microcrystalline quartz is
white or clear.
There are other names for microcrystalline quartz depending on its
color, shape, and structure including chalcedony,
agate, onyx, jaspe,r and flint.  All of these
could be said to make up petrified wood
and it's why Petrified Forest National
Park is a real gem.
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