|Typical color of the thundereggs
||Unusually colorful thundereggs
"Thunderegg" is a common name given to the Lithophysae type nodules
with geometrically shaped cavities surrounded by rhyolite shells.
There are several ideas on how the thundereggs may have formed. Let's start with the most common one, the "Hot" idea.
Unlike the gas bubbles being
preserved in the solidified lava flows, the gases opened up the cavities
within the molten rock "bubbles" floating in the perlite/rhyolite flows
(think of lava lamps). The "bubbles" hardened into the balls of
needlelike H-Cristobalite, a type of quartz that can form only at higher
temperature, and feldspar crystals may be also involved. While the
"bubbles" were still very soft, and not yet solidified, dissolved gases
eventually gathered together, and forced open a cavity between the
of radiating structures within the "bubble. The fact that the lava was shrinking when cooling may had helped drive the opening of the cavities. That was how the cavities in
the thundereggs are comparable to the way the Africa and South America
continents fit each other together, and the radiating structure of the
thundereggs was the main reason for the star or crescent shaped cavities
in many thundereggs.
Eventually, the water passed through the fractures within the solidified lava rocks, altering the rocks and
leeching off the minerals in the process and some sensitive
lava rocks especially perlite, a hydrated form of volcanic glass, decomposed into clay layers. At the same time,
the mineral-rich solutions filled in the cavities of the thundereggs, resulting
in the deposition of the agate-forming solutions. while "petrifying" the
thunderegg shells with harder form of quartz.
It was said that each
band of the banded agate represents each period when water was available
kind of like tree growth rings but in reversed manner despite some unexplainable
characteristics. The impurities such as iron also were included in silica
solutions, and they acted as coloring agents that provided colors to the agates.
The interruption of the mineral rich solutions as in case of changing
from wetter environment to drier one, can cause the formation of the agates
to fail to be completed, hence, the hollow centers of the agate
nodules. The shift back to the wetter environment can resume the building
up of the agates, and the cavities can be lined or filled with the quartz
crystals if the environment favors that. It must be noted that rhyolite
and pertile lava rocks tend to be very poor in iron, an important coloring agent
for the agates, so it is rare for any agates in any thundereggs to be colored.
The best thundereggs were found in the clay beds that had
been weathered from the perlite lava beds as well as any slightly weathered
perlite lava beds. The perlite beds were associated with rhyolite and obsidian
beds as in case of those found in Oregon, Wyoming, Nevada, Washington, Arizona, California and New Mexico of USA.
The other idea ("Cold" Idea) was that the thundereggs were formed
like the Septarans in the ash/tuff/decomposed perlite beds. Eventually,
the beds either get heated or wet, the nodules would be crystallinzed in
radiating manner and when the beds start to either cool or dry, the
nodules struggle to shrink and got ruptured from within with support of
something like vapors within. The solutions later filled the cavities
with quartz, opal and agates.