While walking around downtown San Diego during Comic-Con, I came across a unique fireplace (volcano in minature) which features vesicular basalt that you are able to pick up and examine closley.
Vesicular basalt tells a complex volcanic story. It forms when basaltic lava — low in silica and relatively fluid — traps gases (mostly water vapor, CO₂, and sulfur gases) as it cools and solidifies.
As lava erupts and decompresses, gases dissolved in the molten rock come out of solution, forming bubbles. If the lava cools quickly enough before these gases escape, the bubbles become frozen in place, creating vesicles — holes or cavities — throughout the rock.
🧱 Types of Vesicular Basalt (by Texture)
Not all vesicular basalt looks the same. Texture varies based on factors like gas content, cooling rate, and viscosity of the lava. Here are common types:
1. Uniformly Vesicular Basalt
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Vesicles are evenly distributed throughout the rock.
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Indicates steady degassing and relatively consistent cooling.
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Texture: Sponge-like or foamy.
2. Scoriaceous Basalt (Scoria)
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Extremely vesicular — can contain over 50% void space.
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Vesicles are large, irregular, and often connected.
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Often dark red or black; may appear jagged.
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Forms during fire fountaining or explosive eruptions.
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Very lightweight, often used in landscaping.
3. Banding or Zonation in Vesicular Basalt
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Vesicles appear in bands or layers.
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May indicate pulses of gas escape or lava flowing in layers.
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Often found at the top or bottom of lava flows.
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Indicates that different parts of the flow cooled at different rates.
4. Amygdaloidal Basalt
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Vesicles are partially or completely filled by secondary minerals.
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Filling occurs when mineral-rich fluids percolate through the rock after it cools.
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The result is a solid, colorful interior — called amygdules.
💎 Common Vesicle-Filling Minerals (Amygdules)
Over time, hydrothermal fluids or groundwater may deposit minerals inside the vesicles. These secondary minerals crystallize and often stand out in contrast to the dark basalt. Some common vesicle-filling minerals include:
1. Calcite (CaCO₃)
2. Zeolites (e.g., stilbite, natrolite)
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Pale colors (white, pink, peach).
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Radial or fibrous crystal textures.
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Form in low-temperature hydrothermal systems.
3. Chalcedony (cryptocrystalline quartz)
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Waxy luster, gray to blueish.
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Fills vesicles with smooth, concentric layers.
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Very fine-grained quartz.
4. Prehnite and Epidote
5. Celadonite or Chlorite
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Green minerals often seen in older basalts.
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Give vesicles a dull green to olive tint.
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Related to volcanic glass alteration.
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. "Seaport Village 'Volcano' " 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) holding your favorite vesicular basalt rock(s) with the fireplace (volcano) 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. Describe the (a) range of colors, (b) average textures, (c) average shapes (i.e. rounded or angular and jagged) of the basalt.
4. What type of Vesicular Basalt (from the list in the cache description) is this? How can you tell? Hint: Tap on the rock (gently): Amygdaloidal basalt sounds denser, while scoria sounds more hollow.
5. Look for distribution patterns: (a) Are the vesicles within the basalt clustered, layered, or evenly spread? (b) What may this suggest about the cooling rate?
6. Check for vesicle shape: (a) Are they rounded, elongated, or flattened (indicating flow direction)? (b) What may this suggest about the deformation rate? Hint: think about how stress could stretch or squeeze a rock as it cools.
7. Identify vesicle fillings: (a) Do you see bright or pale colors? If so, (b) identify the two minerals that are most prevalent that have partially filled in the vesicles. If not, explain why this might be the case.