Versatile Vesicular Volcanics 🌏 EarthCache

At GZ, you will have the opportunity to relax on some seats...these are the subject of the earthcache!

The Questions:

1.) Examine the rock at GZ. Describe what you see, include colour, general grain texture.

2.) What conditions do you need to form vesicular textures in Basalt? Describe the texture created by the vesicles and what is their general shape and size?

3.) Suggest a couple of reasons why this rock was chosen for this particular function.

Then post a photo of you relaxing at the location with your log, (please do not show a close up of the subject of the questions in your photo). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. No need to strain your brain, the questions are not that hard, just have a go! 

The Lesson:

Look at the chunks of rock you see here, these chairs have been cut from basalt. Generally, basalt is an aphanitic (fine-grained) igneous (formed from magma) rock. The viscosity of basaltic magma is relatively low, similar to the viscosity of tomato sauce and it tends to cool quickly which explains the dense rock with small crystal size and fine grained nature. Due to its make up basalt is tough and erosion resistant. This basalt however, has a very interesting honeycombed texture….but this texture isn’t the result of weathering….it's something else entirely.

The next time you hold an unopened bottle of soft drink, take a moment to make some observations. Can you see any bubbles? If so, are there a lot, or very few?

The reason you can’t see a lot of bubbles is because the contents of the bottle is under pressure. You can verify this by giving the bottle a squeeze. It feels hard because of the high pressure inside. When carbon dioxide is added to a sealed bottle or can containing water, the pressure in the container increases, and the carbon dioxide dissolves into the liquid essentially disappearing. As long as the bottle remains unopened, the liquid and gas are blended into a homogeneous mixture, forced together under pressure.

But what happens the moment you open the bottle? There is a sudden release of pressure and the carbon dioxide is no longer forced to be in the mix with the liquid. It instead makes its way out of the liquid in the form of thousands of little bubbles. If you disturb the liquid (ie, shake the bottle) prior to opening it, the small act of relieving pressure may even result in an explosion, (I think we've all experienced a soft drink explosion!)

Let’s take this analogy to volcanoes. As long as magma is inside a volcano, the pressure of being underground keeps gases dissolved within the liquid. The drop in pressure that magma experiences as it flows from underground to the Earth's surface allows water and gases in it to form bubbles. As soon as the volcano begins to erupt, the dissolved gases are free to expand and make their way out of the melt. If you have ever observed a glass of soft drink, you know the bubbles make their way up to the surface to escape. The very same occurs in lava, as it hardens and crystallizes into solid rocks, the gas dissolved within it tries to make its way up and out in the form of gas bubbles. Gas bubbles are much less dense than liquid so they “float” to the surface. Sometimes the gas doesn’t quite make it all the way out if the lava cools quickly. The rock solidifies around the gas bubbles and traps them inside, preserving them as holes in the rock. The resulting holes are called vesicles.

Vesicular texture of a volcanic rock is characterised by the rock being pitted with many cavities (the vesicles) at its surface and inside. Vesicles can vary in shape (spherical, elongated, elliptical, or irregular) and size (from microscopic to several centimeters). The abundance of vesicles can range from sparsely scattered to making up a significant portion of the rock's volume. These rocks are porous in nature. Other types of igneous vesicular rocks are scoria and pumice.

Photo: Earthbound Chief holds a piece of vesicular basalt with Olivine

Vesicular textures in rock are significant because their presence provides geologists with insights into the cooling history of the rock and the conditions under which it formed. Analyzing vesicular rocks allows geologists to infer details about volcanic activity, such as eruption dynamics, the conditions present in the magma chamber, the gas content of the lava and the speed of cooling. The presence of vesicles indicates that the lava had a high gas content, so bubbles were trapped in the magma during its solidification, indicating the rock cooled rapidly and likely in a volatile-rich environment during magma crystallization. This rapid cooling prevents gas from escaping, leading to the formation of the vesicles.

Resources:

https://joidesresolution.org/vesicular-basalt/

https://sites.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/7VesicularAmygdaloidal.html