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Igneous fossils.


ynot

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Hey hi Y'all,

In a recent thread there was some discussion as to whether an ash fall from a volcanic eruption produced a volcanic or sedimentary rock.

Also a lahar (volcanic mudflow) was referred to as being a sedimentary event.

 

I would like to get the opinions of the members here on TFF.

Does a volcanic eruption produce sedimentary rock?

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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I haven't read any opinions on this but having a sedimentary rock composed of ejected volcanic material seems OK. It's only the particles that are igneous.

Tarquin

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3 minutes ago, TqB said:

I haven't read any opinions on this but having a sedimentary rock composed of ejected volcanic material seems OK. It's only the particles that are igneous.

 

That seems reasonable to me. Looking forward to what others think as well

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I suppose if you think of the term volcanic rock, it's a more general term than igneous rock that is more specific to deposits formed by cooling lava/magma.

 

A volcanic rock could be an ash fall (pyroclastic) deposit. When the deposit is reworked or deposited into or via transport by water it can also be called a sedimentary rock.

 

i would call a lahar a volcaniclastic sedimentary rock as it is a volcanic deposit transported by water. It is not a primary deposit.

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Certainly if it lands in a lake and becomes cemented in layers then definitely a sedimentary rock.  We have found insect fossils in such stuff here in Arizona.  In hawaii, I found a log impression in the black lava flowing out of a recent outpouring.  I suppose this is a fossil, but never have found any tertiary lavas with bark impressions.

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Arizona Chris

Paleo Web Site:  http://schursastrophotography.com/fossiladventures.html

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12 hours ago, TqB said:

Volcaniclastic sedimentation:

 

http://volcanology.geol.ucsb.edu/facies.htm

My take from this is...

If the rock is deposited by direct action of the eruption it is a volcanic deposit and if it is redeposited by weathering or  processes unrelated to the eruption it is a sedimentary deposit.

 

So the eruptions that buried Ashfall Fossil preserve in Nebraska or Santorine and Pompei created igneous fossil deposits

Also many of the petrified wood sites of the western parts of the United State are created as the direct result of eruptions so would be fossils in igneous rock.

 

 

 

9 hours ago, Doctor Mud said:

I suppose if you think of the term volcanic rock, it's a more general term than igneous rock that is more specific to deposits formed by cooling lava/magma.

This statement is backwards.

There are 2 types of igneous rock-- Intrusive (granitic type rocks.) and extrusive (volcanic type rocks.).

The difference is where the molten rock cooled, underground (magma) or above ground (lava).

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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Maybe I'm behind the curve of the debate (been out of the loop until just now and have not read that thread mentioned at the top), but I would prefer to think of it as: Sedimentary rocks are formed by deposition from above (thru the air or water), igneous rocks come up from inside the earth. So a volcanic eruption that deposits a blanket of ash on the landscape would result in a sedimentary rock - as TqB pointed out, it's only the particles that are igneous. I guess most of the particles that comprise sedimentary rocks are broken igneous particles. It doesn't matter to me how quickly or slowly it went from being volcanic to sedimentary.

 

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8 hours ago, ynot said:

My take from this is...

If the rock is deposited by direct action of the eruption it is a volcanic deposit and if it is redeposited by weathering or  processes unrelated to the eruption it is a sedimentary deposit.

 

So the eruptions that buried Ashfall Fossil preserve in Nebraska or Santorine and Pompei created igneous fossil deposits

Also many of the petrified wood sites of the western parts of the United State are created as the direct result of eruptions so would be fossils in igneous rock.

 

 

 

This statement is backwards.

There are 2 types of igneous rock-- Intrusive (granitic type rocks.) and extrusive (volcanic type rocks.).

The difference is where the molten rock cooled, underground (magma) or above ground (lava).

You are right YNot,

 

Rereading my post it is muddled :blink:

 

Lets try that again.....

 

I meant to point out the difference between igneous rocks formed from magma/lava (which don't often produce fossils) and pyroclastic deposits which can.

 

my interpretation is that the term volcaniclastic is best used as a general term to refer to pyroclastic deposits as their deposition can involve both igneous and sedimentary processes. Aeolian (wind) as well in some cases.

 

Primary deposits - terrestrial , can be called igneous or pyroclastic deposits.

 

I would also  be comfortable using the term sedimentary to refer to ash fall deposits in water bodies such as the ocean or lakes.

 

 

 

 

 

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Here are three paleobotanical examples I've personally encountered of fossils preserved in volcanic rocks--that is to say, the ancient plant material was enveloped by consolidating igneous constituents unrelated to primary or secondary (re-deposited tuff, for example) sedimentary depositional processes.

 

Top to bottom:

 

Top: A Ficus sp. (fig) leaf preserved in a chunk of rhyolite tuff from the Lower Miocene Valley Springs Formation, western foothills of California's Sierra Nevada. It's approximately 23 to 19 million years old. The locality is a rather rich one, producing in addition to fine Ficus foliage some superior oaks and willows, in addition to several exotic hardwood species now native to the eastern United States.

 

Middle: A diminutive evergreen live oak leaf (Quercus sp..)--it's roughly a half inch long (12.7mm)--captured by a pyroclastic ash deposit about 10 million years ago in the Middle to Upper Miocene Mint Canyon Formation, Southern California. An interesting sidelight here is that famed paleobotanist Daniel I. Axelrod was collecting from the most prolific fossil leaf-bearing locality in the Mint Canyon Formation (a site that has been swallowed up by real estate developments, by the way) on the morning of December 7, 1941. Upon returning to civilization later that day, Axelrod heard the Pearl Harbor news and soon thereafter (at the age of 30) enlisted in the Army.

 

Bottom: An upright redwood trunk (over six feet tall) silicified in its original growth position in the upper Lower to lower Middle Eocene Challis Volcanics at Malm Gulch (a Bureau of Land Management-protected area), Custer County, Idaho. Two species of redwood have been identified from the Challis Volcanics exposed at Malm Gulch--Metasequoia occidentalis and Sequoia affinis. The volcanic eruptions that preserved the Malm Gulch petrified forest occurred 50 to 48 million years ago.

 

hG5Fi4i.jpg

 

vK5T9yH.jpg

 

bqbBOpK.jpg

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I think this debate highlights the fact that the traditional divisions of sedimentary, metamorphic, and igneous rock are convenient in that they work 98% of the time, but they are not absolute.  Just as a reasonable argument could be made for volcanic ash and tuff from either the igneous or the sedimentary rock perspective, some marbles (metamorphosed limestone) or slates (metamorphosed shale) still contain recognizable (though distorted) fossils.  Are such rocks sedimentary or metamorphic?  Seems a bit arbitrary.  Rather we should be a bit more careful about saying fossils are never found in igneous or metamorphic rock, and recognize that such occurrences, while relatively rare, are possible.

 

Don

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Yes. I know of an example here on the Island of marine snails preserved in lava that apparently came down to the beach back in the Eocene as it does today in Hawaii. I don't know where exactly the site is (those in the know don't tell) but I've seen a piece of it. Interesting stuff.

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5 hours ago, FossilDAWG said:

I think this debate highlights the fact that the traditional divisions of sedimentary, metamorphic, and igneous rock are convenient in that they work 98% of the time, but they are not absolute.  Just as a reasonable argument could be made for volcanic ash and tuff from either the igneous or the sedimentary rock perspective, some marbles (metamorphosed limestone) or slates (metamorphosed shale) still contain recognizable (though distorted) fossils.  Are such rocks sedimentary or metamorphic?  Seems a bit arbitrary.  Rather we should be a bit more careful about saying fossils are never found in igneous or metamorphic rock, and recognize that such occurrences, while relatively rare, are possible.

 

Don

I agree Don that our categories and rules and laws we apply to nature sometimes have grey areas and exceptions. 

 

I suppose in terms of rock types we are attempting to classify them in terms of the processes that led to their formation. The presence or absence of fossils may or may not help with this classification. 

 

In terms of metamorphic rocks fossils can survive the heat and pressure that accompanies metamorphosis, but the original rock has changed - metamorphosed. There are distinct phases of heat and pressure (facies) where these conditions cause the formation of new minerals and metamorphic texture.

 

https://en.m.wikipedia.org/wiki/Metamorphic_facies

 

So for the marble example I would say it is a metamorphic rock, but the conditions that have altered the limestone to marble weren't enough to obliterate fossils.

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27 minutes ago, jewelonly said:

This is a very interesting and informative thread. :popcorn:

 

Leah

Thanks to @ynot for providing us with this brain teaser.

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It is the type of discussion that I thrive on!

 

23 minutes ago, Doctor Mud said:

Thanks to @ynot for providing us with this brain teaser.

 

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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if you know about stuff like C(onodont)Al(lteration) I(ndex) ,chemofossils,palynology,and vitrinite reflectance(and other thermal maturity indicators).the ambiguous (and interesting) relation between temperature and fossilis is not altogether surprising.

For instance graptolite periderm,acritarchs(carbonized at metamorphic temperatures up to 250 deg Celsius),pollen(sporopollenin!)  are pretty darn (heat)resistant biological materials

This might be a repost,BTW:

 

Galvez-metamorphlagerstatnewzealgeobiology-2012.pdf

rhieqps.jpg

beavhc.jpg

 

 

 

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This title alone tells something

The graphitization of plant pollen or algal cysts is not surprizing.

rhieqps.jpg

 

 

 

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That's what makes paleontology and geology so interesting: the processes of change,and trying to decipher them.

 

 

 

 

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Yes I was surprised until recently that carbon inclusions known as phytoclasts (pretty much little particles of coal) can be preserved in the dominant metamorphic rock type in New Zealand - The Torlesse low grade metamorphosed sandstone/mudstone. 

It seems that these phytoclasts can be used in much the same way as conodonts to give an idea of metamorphic facies.

I got interested in these as they can erode out if the bedrock and may form an important source of carbon in rivers draining the mountains in New Zealand.

 

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