Flying Rocks - The Epic Journey of Volcanic Bombs

When most people think of a rock, they picture a stationary object. Rocks evoke a feeling of permanence, stability, and strength. But what many people don't realize is that the stones you see out on a hike or sitting in a museum often underwent an epic geologic journey. 

Some were transported by glaciers, others were eroded and tumbled by centuries of weathering, and some—believe it or not—actually flew through the sky...

And no, we aren't just talking about the magic of meteorites either!

Some extra special stones formed deep inside the Earth and were then violently ejected into the atmosphere. These epic rocks are called Volcanic Bombs!

What are Volcanic Bombs?

Volcanic bombs are a type of igneous rock formed from molten lava that is literally thrown from a volcano during an explosive eruption. By geologic definition, a mass of molten rock must be larger than 2.5 inches in diameter to be classified as a "bomb." 

Because this material is ejected while it is still viscous (partially molten), it flies through the air as a malleable projectile. As it twists and spins through the atmosphere, the lava cools and hardens mid-flight, creating amazing aerodynamic shapes, along with incredible internal laminations. 

The Giant Tea Kettle: Anatomy of an Eruption

To understand how a rock takes flight, we have to look underground. A volcano is much more than the mountainous cone we can see at the surface; it is a geologic structure fueled by intense heat and pressure deep inside the Earth. 

Magma chambers containing molten material at extreme temperatures feed these volcanic events. As gases expand and pressure builds up underground, the volcano begins to act like a giant, geological tea kettle. The vents and pipes that reach from the magma chamber to the Earth's surface serve as the perfect conduits. When the pressure finally reaches a breaking point, the volcano erupts—expelling not just flowing lava, but massive amounts of expanding gases, volcanic ash, and flying debris.  

Hidden Treasures: Olivine and Peridot

While the twists and ribbons on the outside of a volcanic bomb are fascinating, the real treasures are hiding on the inside. 

Some volcanic bombs contain a crystalline surprise. When sliced open, these special bombs reveal gorgeous green Olivine crystals inside! Formed deep underground at some of the hottest temperatures within our Earth's mantle, these crystal clusters are technically called mantle xenoliths.

As magma rushes up toward the surface during an eruption, it can rip away chunks of surrounding rock. The magma scoops up these pre-formed olivine clusters, encasing them in lava, and blasting them out of the volcano. When the lava cools mid-air, it forms a protective shell around the crystals.

This is a similar type of geologic environment to the famous San Carlos Reservation olivine deposits in Arizona, where gem-quality Olivine, also known as Peridot, is brought to the surface. 

More than Just Olivine: Garnet and Gabbro

Olivine isn't the only hitchhiker catching a ride to the surface inside these molten projectiles! Depending on the geology beneath the volcano, magma can rip up and encase a variety of other rocks and minerals along its journey.

Two other fascinating "stowaways" found inside volcanic bombs are Garnet and Gabbro:

Garnet: Deep red pyrope garnets frequently form under the extreme pressure of the Earth's mantle. When magma tears through these deep-seated rock layers (like garnet peridotite), it can capture beautiful, deep-red garnet crystals and blast them to the surface hidden inside a volcanic bomb.

Gabbro: Unlike single crystals, gabbro is an entire rock type. It's a dark, coarse-grained igneous rock that forms when magnesium- and iron-rich magma cools very slowly deep beneath the Earth's surface. It contains the primary minerals: plagioclase feldspar, pyroxene, amphibole (hornblende), and olivine. In some cases it may also contain mica, quartz, pyrope garnet, spinel, hematite, or rutile.

When a new eruption occurs, the fresh magma can break off chunks of this older, solid gabbro and launch it skyward. Finding a gabbro xenolith inside a volcanic bomb is like finding a rock within a rock—a physical record of two completely different volcanic events!

Conclusion

The next time you see a volcanic bomb, remember: you aren't just looking at a stone. You are holding a piece of the deep Earth that took a fiery flight through the sky!