Geology questions answered: Kilauea spatter cones

Fissure 7, Kilauea, May 27-2018 (USGS file photo)

I’ve been watching the CNN live feed of the lava fountain at Kilauea on and off all day and I noticed that the black rock on the edge keeps changing shape. Why? And where is it going?

To answer this question, I have to talk a little about how eruptions like the one currently happening in Hawaii evolve over time. I’ll include diagrams, some photos from the USGS and some photos from Craters of the Moon National Monument in Idaho for comparison.

Fissure flows like those at Kilauea start as cracks caused by pressure shifts and moving magma under the surface. The cracks form a line or arc and, in more active cases, leak steam, heat and sulfur dioxide. In this USGS photo (taken May 17th, 2018), notice that the ground is warping up around the crack and the vegetation dying.

As the cracks and rifts get larger, eruptions start along the whole length of the fissure system. Lava flows begin to form and spread from the flying lava spatter. This photo is of a lava fountain, Fissure 20 (May 19th, 2018, USGS). Notice that the lava is erupting in a line, rather than from a “volcano” point.

The LONG eruptions along the fissure can’t continue forever; pretty rapidly, the eruptions start to localize and form distinct lava fountains. But, because the eruptions are continuing, the lava flows continue to form.

Lava is hot, but cools rapidly, and everything not flowing away as part of the lava flow starts forming cones. The photo of the active lava fountain is Fissure 22 on May 21st, 2018 by the USGS. With the fissures so active, lava flows are growing rapidly. This is the same day the lava flows reached the ocean.

So what does spatter actually look like up close? Let’s draw a comparison. Craters of the Moon National Monument has features EXACTLY like what are currently forming in Hawaii! These are the Spatter Cones, a series of cute (well, to a geologist they’re adorable…) spatter cones that formed in the same way that the fissure eruptions on Kilauea are currently erupting. Notice that the Spatter Cones form a line along a fissure in the same way as the Kilauea fissures.

Spatter isn’t solid lava but rather flies out and cools as individual blobs. This gives it a glued-together texture and a knobby look. Notice there is a paved trail up the side of the cone? It was installed because the spatter is very fragile and disintegrates under the pressure of visitors’ feet.

That is how cones are built along fissure eruptions, but why does the cone change shape? Well, the answer has to do with that fragile spatter. It doesn’t take much to disintegrate and re-melt a spatter cone. As the cones get large and tall, they start to drift apart in the lava flows and collapse. Or, the heat from the erupting lava melts pieces. The edges of the cone collapse inward and are re-melted, or outward to fall on top of the flow. Because lava is dense (this lava has the consistency of toothpaste), whole pieces of the spatter cones are lifted in the lava flow and are carried away. The spatter cones build and re-build themselves through the entire eruption.

Pieces of spatter cone being carried off on lava flow.

I couldn’t find any pictures of this in Hawaii right now, but this is exactly what happened at the North Crater lava flow at Craters of the Moon. These are blocks of spatter cone (still upright and layered) that were simply carried away with the lava as the eruption progressed.

Do you have a geology question? Please send it to me and I would be delighted to answer it!

Visit Craters of the Moon National Monument for more great geology!

Or, if you want more pictures of the 2018 Kilauea fissure eruption, please visit the USGS Volcano Hazards Program.

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