Volcanic Agate

2. Spheroids (Lithophasae) in Rhyolite and Obsidian

The mineral content of magma is constantly changing as iron and magnesium combine to form olivine crystals; calcium, sodium, and potassium form feldspars and they drop out of solution, making it more and more silica laden over time.

Basalt ~ 45% Silica, Andesite ~ 55% Silica, Rhyolite ~ 65% Silica

BUT the Thunderegg Shells reach over 70% Silica!

- - - - - - - - - - - - - - - - - - - -

At 1,100^oC crystobalite needles form, making "puffballs" with very hot water trapped between the needles. The needles push out the calcium, sodium, and potassium molecules. All that movement causes friction and makes the magma hotter. It becomes too hot for crystobalite to form, but just right for Perthite feldspar to form a sphere around the puffball. The magma cools and crystobalite needles resume.

You see them all the time as "Snowflake Obsidian."






A Thunderegg Siamese Twin that formed in obsidian.


As the flow slowly cools and contracts, pressure is released and the water is finally allowed to expand into steam, forcing the needles apart.


As the gas forces the matrix apart around the thunderegg, "sutures" are formed where the sections break apart.
If the gas escapes the sutures are sucked in.




Thunderegg Cores - the outer shells removed.

4. Highly Siliceous Ash Flow Tuff

Material expelled in volcanic eruptions can be classified as:

1. Volcanic gases, a mixture made mostly of steam, carbon dioxide, and a sulfur compound - either sulfur dioxide, SO2, or hydrogen sulfide, H2S, depending on the temperature.
2. Lava, the name of magma when it emerges and flows over the surface.
3. Tephra, (Tuff) chunks of solid material of all shapes and sizes ejected and thrown through the air. Chunks smaller than 2 mm in diameter (sand-sized or smaller) are called volcanic ash.

Adding water to Rhyolite filled volcanoes can become explosive!



Here we are covering THREE separate subjects:
1. Ash FALL flat layers
2. Ash FLOW sloped layers
3. Welded Ash Flow layers



As ash fall flat layers rate -10 on a blah scale from 1 down to -10, - - -

let's look at Ash FLOW sloped layers. They are Volcanic Land Slide Debris that traveled at various degrees of hotnesss, gassiness, and wetness.

They consist of whatever was accumulated on the side of the volcano from all previous eruptions.
There are 6 Types depending on depth and age of magmas and what has collected on volcanic hillsides: 1. Welded tuff, 2. Rhyolitic tuff, 3. Trachyte tuff, 4. Andesitic tuff, 5. Basaltic tuff, 6. Ultramafic tuff

Lava, pumice, basalt, andesite, obsidian, rhyolite, agate, wind blown sand that suddenly moved away from a volcano at 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).[2] The gases can reach temperatures of about 1,000 °C (1,830 °F).

The word pyroclast is derived from the Greek πῦρ, meaning "fire", and κλαστός, meaning "broken in pieces".[3] A name for pyroclastic flows which glow red in the dark is nuée ardente (French, "burning cloud"); this was first used to describe the disastrous 1902 eruption of Mount Pelée on Martinique where over 30,000 people were killed.

Ash Flow Tuff may be fine or course grained mixtures sometimes resembling conglomerates.



But, as at Holt Canyon in southwestern Utah, they can be far more interesting.









Hot gas is expelled from the surface, glass shards begin to de vitrify, heat builds up in the core, pressure collapses space between shards, and glass shards begin to weld together.
This is reversed at the base where the cool ground underneath stops the process.











en.wikipedia.org/wiki/Pyroclastic_flow



en.wikipedia.org/wiki/Tuff

Great info. What are the number posts for? Only see the numbers, nothing else.

This is an excellent contribution. I had no idea some of these were of a volcanic heritage. Had to grab some of my halved and sliced geodes/thunder eggs and look at them. Yes, I can see where the liquid came in for some.
You have been such a blessing to me in my knowledge quest. Appreciate you very much.

Excellent information.......

Well done.... again! I hope you finish the thread. Looking forward to it. You've done a good job of visually explaining the processes.  

What am I referring to? Rocks like these.




Notice something odd about that surface? a closer look.



SiO² does not take that form, but pyrite does.

That surface was the top of an ASH FLOW TUFF!



Tommy remember digging under that bush?


Again, how did it get there? as brief as I can, it started 22-23 million years ago during the Laramide orogeny.

As North America moved west and the Pacific seafloor worked its way east beneath the continent, one of a series of volcanoes began erupting and building a cone in the Bull Valley west of Pine Valley.

Then there was a collapse of the east wall of the volcano around 22-23 million years ago, creating the Rencher Ash Flow Tuff that flowed toward the Pine Valley area.

Next the Pine Valley syncline started as a Laramide downwarp which was further depressed during the intrusion of the 3,000 foot tall mushroom shaped Pine Valley laccolith - The largest laccolith in the U.S., and possibly in the world!.

That caused a Gravity Slide Breccia to the west and south of Cove mountain.

The growing laccolith bathed the are with hot water for millions of years!
Minerals were dissolved and others precipitated and our agate was born.

Interesting! I have a pal, an old timer who has a number of the thunder egg cores like the center one in the bottom most photo of #2. He got them from Dugway and as we were looking them over we noticed that each one has a male and female dimple on opposite sides. You can see the male dimple on the top of yours in your photo. My bud theorizes that the opposite dimples are a result from polarity forces during formation. I have no clue. What do you think?

The Iron Axis laccolith swarm consists of a series of early Miocene calc alkaline hypabyssal laccoliths and associated volcanic rocks (Figures 1a and 1b). The intrusions occur as isolated bodies, but spatially coincide with a major fault zone at depth [Mackin, 1960; Blank and Mackin, 1967; Blank et al., 1992; Rowley et al., 1995; Rowley, 1998]. The intrusions are distributed along a northeast trending zone that follows the trend of the Sevier orogenic front (Figures 1b and 1c). More than a dozen exposed intrusions have been mapped in the magmatic province and others are inferred from structures in the cover rock and aeromagnetic signatures or are known from drilling and seismic data. During the emplacement of the Iron Axis intrusions, the area lacked any tectonic activity (i.e., regional strain) facilitating forcible emplacement into Mesozoic sedimentary rocks driven only by magmatic pressure at depths ranging mostly between 2.5 and 0.25 km from the surface [Hacker, 1998; Hacker et al., 2002]. Several of the intrusions deformed their roofs by upward folding and faulting, typical of the classic style of laccolithic emplacement [Gilbert, 1877; Corry, 1988; Jackson and Pollard, 1988]. The Pine Valley, Bull Valley Big Mountain, Pinto Peak, Stoddard Mountain, and Iron Mountain intrusions were partially unroofed by catastrophic gravity sliding, leading to explosive eruptions forming ash flow tuffs and lava flows that buried the gravity slide masses [Mackin, 1960; Hacker, 1998].

Conclusions

[30] The Pinto Peak intrusion provides insight into the evolution of a relatively small intrusive—extrusive volcanic system. The system is highly unusual in that the country rocks show no deformation associated with the emplacement of magma to shallow depths that led to a gravity slide off the roof of the intrusion leading to an explosive eruption. Rock magnetic and remanence studies indicate that the AMS fabrics are carried by multidomain titanomagnetite grains that vary in composition spatially across the intrusion. The intrusion is a porphyritic andesite and the silicate and oxide minerals do not typically show planar or linear fabrics in the field or in thin section. The results from the AMS study map out a sequence of events from steep magma ascent from depth to a shallow crustal level as defined by magmatic fabrics in the southern part of the intrusion. Following eruption triggered by the gravity slide, the magma drained away from the vent area with subhorizontal magma flow as defined by the magnetic fabrics located in the northern part of the intrusion. The deceptively simple outcrop pattern of the Pinto Peak intrusion hides a more complex magma flow and emplacement history as evidenced by the results of this study.