Monday Geology Picture(s): Vesicular Basalt Boulder, Mauritius

Grand Gaube sign on a large boulder of vesicular basalt, northern Mauritius.

Continuing with some pictures from my trip to Mauritius back in March, for this week’s “Monday Geology Picture” post I am sharing some pictures of a large boulder of vesicular basalt. This particular boulder is located in the town of Grand Gaube in northern Mauritius and is used as a decorative stone on which a plaque has been mounted.

Vesicular basalt is a dark-colored volcanic rock that contains many small holes, more properly known as vesicles. A vesicle is a small cavity in a volcanic rock that was formed by the expansion of a bubble of gas that was trapped inside the lava. Sometimes, vesicles can become filled in with secondary minerals, such as calcite, quartz, or zeolites. When vesicles are filled with such minerals, they are called amygdales. The vesicular basalt boulder featured in this post contains some amygdales. Some of the vesicles in the boulder are filled with a clear to white mineral, possibly calcite.

The boulder featured in this post also has a notable reddish-brown weathering rind. A fresh (and beautiful!) surface of the basalt boulder was exposed when the boulder was cut to make a flat surface for displaying the plaque.

Here are a few more pictures of the vesicular (or amygdaloidal) basalt boulder:

Another view of the vesicular basalt boulder.

A closer view of the vesicular basalt. — A closer view of the vesicular basalt boulder.

Another close-up view of the vesicular basalt. — Another close-up view of the vesicular basalt boulder.

The boulder almost certainly originates from a local source since basalt is abundant on the young volcanic island of Mauritius. Basaltic rocks can be found all over the town of Grand Gaube. For example, basaltic rocks are commonly seen on Grand Gaube’s beautiful beaches. I shared one picture of a basalt-covered beach in Grand Gaube last week.

Below is another picture of basalt on the beach in Grande Gaube; this picture was taken at the resort where my husband and I stayed:

A beautiful basaltic beach, Grand Gaube, Mauritius.

Stay tuned for more pictures from Mauritius!

Geology Word of the Week: A is for Amygdale

amygdaloidal basalt — Amygdaloidal basalt collected during dredging along the Ninetyeast Ridge in the Indian Ocean in summer 2007.

def. Amygdale (also spelled Amygule):
Vesicles, generally in extrusive igneous rocks, that are filled with secondary minerals, most commonly quartz, calcite, chlorite, and zeolite.

When a rock contains amygdales, geologists often describe it as an amygdaloidal rock. For example, basalt is often described as amygdaloidal basalt.

Here are a few previous, related geology words of the week that may be of interest:

V is for Vesicle (and Vug)

Z is for Zeolite

M is for Magma

And here are a couple more pictures of amygdaloidal rocks:

amygdaloidal basalt2 — Amygdaloidal basalt from the Isle of Skye, Scotland. Picture courtesy of Matt Kuchtam.

DSC06418 (1) — A zeolite amygdale in a rock collected from Quartzville, Oregon. Picture courtesy of Dana Hunter.

***Thanks to my twitter follower Laura Hamilton for suggesting this week’s word. Thanks to Matt Kuchtam, Dana Hunter, and Lockwood De Witt for providing pictures.***

Geology Word of the Week: V is for Vesicle (and Vug)

Vesicles in basalt, image courtesy of Ron Schott of the Geology Home Companion Blog.

def. Vesicle:
A small cavity in a volcanic rock that was formed by the expansion of a bubble of gas that was trapped inside the lava.

Vesicles are commonly found in volcanic rocks– that is, in rocks that solidified from lava. As you might remember from M is for Magma, lava is what you call molten rock when it is extruded onto Earth’s surface. Molten rock– magma and lava– often contains dissolved gases such as water and carbon dioxide. When lava erupts, these gases expand and often are able to escape to the atmosphere as the lava cools and solidifies to form a volcanic rock. Sometimes, particularly in very small, interior vesicles, the gases do not escape and geologists are able to study these gas (or sometimes fluid) inclusions to learn more about the gases and fluids that were in the lava when it erupted. When there are many vesicles in a volcanic rock, this indicates that the lava from which the rock formed was very rich in gases.

Since vesicles are open cavities, they sometimes become filled in with secondary alteration minerals such as calcite, quartz, or zeolite. When vesicles are filled with a secondary alteration mineral, they are called amygdules (or amygdales, if you’re British). Since calcite, quartz, and other secondary minerals are generally light in color compared to the brown or black volcanic rock, amygdules can make a volcanic rock look spotted– like a reverse Dalmatian rock.


Spotted rock (brown basalt with white amygdules), collected from the Ninetyeast Ridge, Indian Ocean, Summer 2007.

Spotted dog, Johannesburg, South Africa, December 2009.

When you describe a volcanic rock, you can say that it is “vesciular” if it contains vesicles and that is it “amygdaloidal” if it contains amygdules.

The word vesicle generally applies only to cavities formed in volcanic rocks by gases expanding in lava. The word “vug”– another v word– is another term used to describe cavities in rocks. Vugs can be formed in a variety of ways– such as through faulting, folding, or collapse of a rock– and are often partially filled with secondary minerals.

Since I’m currently traveling and don’t have all my geo-pictures at hand, I’ve asked some of my geologist friends and geobloggers to send me some pictures of vesicles, amygdules, and vugs. Ron Schott already kindly gave me the vesicle picture at the top of this post. I am going out now for a few hours, but this evening I’ll hopefully be able to add some more pictures to this post. Feel free to send your own vesicle, amygdule, and vug pictures to georneysblog (at) gmail (dot) com, and I’ll post them here. Please include what– if anything– you’d like me to use as credit for the picture. Thanks!

Update:

From Callan Bentley over at Mountain Beltway, here are some posts with vesicle pictures:
Rock varieties of Hawai’i
Hol(e)y basalt, Batman!