While exploring downtown San Antonio, I came across an interesting sculpture that would make a great spot for an earthcache!
The sculpture is made from 80 peices of Mexican volcanic stone, otherwise known as basalt. The west coast of Mexico is part of the Pacific Ring of Fire and has an extensive volcanic belt where basalt is commonly found.
Basalt is an igneous rock, meaning it was formed from the solidification of liquid magma. There are two types of basaltic rock: intrusive and extrusive. Intrusive basaltic rocks form usually inside or part of a volcano. This can happen either inside the caldera or part of the volcanic system such as dikes, lava tubes, etc. Because intrusive rocks form underground they take time to cool, allowing macroscopic crystals to develop. Extrusive rocks, on the other hand, cool rapidly so there isn't much time for crystal growth to develop before the rock hardens.
There are three ways basalt can form. One is formed under water at either divergent plate boundaries or at mid-ocean ridges (MOR). Another is formed at volcanic hot spots like Hawaii where lava pours into the ocean. And lastly, one is formed in lava tubes or dikes where it never reaches the surface. If basalt solidified on the surface, it's classified as an extrusive rock. If it solidified underground, then it's classified an intrusive rock since it had to intrude rock to form.
Lava can also be formed by flowing or erupting out of a volcano on a continent such as the Cascades in the Pacific Northwest. Granted the Cascades and other volcanic features associated with subduction (where one tectonic plate dives beneath another) produce lavas that are more rich in silica than basalt. The high amount of silica content in these types of volcanoes produce "sticky" or viscous lava which allow it to form tall peaks and steep slopes we typically associate with volcanoes which are only a few miles across. Basalt forms from "runny" lava because of the low silica content and they form shield volcanoes, which is what we think of on the Big Island of Hawaii, with very low angle slopes but can spread out over a very large area to hundreds of miles across. Note in the diagrams below, the as you increase the size of the volcano, you decrease the viscosity and silica content of the lava.
How do we tell the difference if it's formed on land or under water? If it formed in the ocean or on land, then by nature it's an extrusive rock. By looking at the rock, we can classify them by observing the size of the bubbles known as vesicles. If it formed under water, the basalt would have to cool extremely rapidly due to the cool water temperature. The rapid coolness of basalt flow makes it vesicular. Vesicular rocks contain vesicles or tube like structures that contain pockets of air. They were formed when bubbles inside the liquid magma, from boiling seawater, were captured as the rock hardened. Land formed basalt looks quite different. It too cooled rapidly, but not as fast as basalt formed underwater so the size of vesicles are not as dramatic as for basalt that formed underwater.
Note that the air temperature around a river of molten rock is near the same temperature of the rock. It's also why you see wavy patterns in the air on streets on hot days. This is because the hotter things are, by nature, are less dense than cooler things. This difference in density refracts light from the sun and you see a wave in mid air because that elements in our atmosphere are a good conductor of heat hence why global warming is such a big issue.
So, basalt that formed on land does cool fast, but because it is not cooling as fast as an undersea lava, it can't create large bubbles or vesicles. And because of this, the size of mineral grains are microscopic. We can tell if a basaltic rock formed underground, such as a dike or lava tube, by looking for no bubbles or holes and the size of its mineral crystals. This is because the rock around the liquid magma acted like an insulator, making the magma cool more slowly, allowing time for the gasses present to dissipate away while allowing the crystals to grow over a longer period of time. Below is a chart showing the different textures (or grain sizes) of volcanic rocks. As coarseness increases, so does the length of time it took to cool that rock.
Works Cited
https://www.sandatlas.org/basalt/
https://geologyscience.com/rocks/basalt/
https://www.youtube.com/watch?v=RliAW03RKxU
https://study.com/academy/lesson/intrusive-igneous-rock-definition-examples.html
https://sciencenotes.org/viscosity-definition-and-examples/
http://www.explorevolcanoes.com/Volcanic%20rock%20types.html
https://openpress.usask.ca/physicalgeology/chapter/11-3-types-of-volcanoes/
http://geophile.net/Lessons/volcanoes/ND_volcanoes_03.html
https://en.wikipedia.org/wiki/Vesicular_texture
http://www.luckysci.com/2014/08/types-of-volcanic-rocks-and-deposits/
https://www.mantleplumes.org/Mexico2.html
https://events.getcreativesanantonio.com/public-art/stargazer-citlali/
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Stargazer" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the sculpture in the background. This photo MUST be uploaded to your "found it" log. Please do not send any photos through the message center as Groundspeak compresses images. Each log MUST have a unique and different photo. No two logs may contain the same photo.
3. (a) Describe the size of the basalt, (b) range of colors, and (c) range of physical textures.
4. Using the "Volcanic Rock Textures" chart, describe the mineral textures that you see.
5. Are these rocks extrusive or intrusive? Explain using your own observations AND provide evidence by using the answers to your previous questions.