- Jul 24
- 6 min read
Updated: Aug 4
If you have walked across a young volcanic landscape in Hawaiʻi, you may have noticed a golden or brown fragment that resembles a delicate honeycomb. It can feel so light that it barely registers in your hand.
That unusual volcanic material is reticulite.
Reticulite is not separate from Tephra. It is a rare and extremely bubble-rich type of tephra produced by very high lava fountains.
Tephra is the broad term for material ejected from a volcanic vent, while reticulite describes one specific form of fragile volcanic glass.
Reticulite vs. Tephra: The Quick Answer
The simplest distinction is:
Term | What it means |
Tephra | Any solid material thrown from a volcanic vent, including ash, lapilli, blocks, bombs, pumice, scoria, Pele’s hair and reticulite |
Reticulite | A rare, highly vesicular type of tephra made from an open network of thin volcanic-glass walls |
Tephra describes where the material came from and how it was deposited. Reticulite describes a particular material with a recognizable structure.
What Is Tephra?
Tephra is the collective term for fragments of rock, magma and volcanic glass ejected into the air during an eruption.
It can be as fine as dust or large enough to damage buildings and vehicles. The word does not describe one specific rock type. Instead, volcanologists primarily classify tephra according to particle size.
How Tephra Is Classified by Size?
Tephra type | Particle diameter |
Volcanic ash | Less than 2 mm |
Lapilli | 2 to 64 mm |
Volcanic bombs and blocks | More than 64 mm |
Bombs are ejected while still molten or partly molten. Blocks are solid fragments when they leave the vent. USGS uses these size divisions when describing material produced by Kīlauea’s lava fountains.
How Tephra Forms?
As magma rises, pressure decreases and dissolved gases begin forming bubbles. Those bubbles expand as the magma approaches the surface.
When gas-rich magma reaches a vent, the expanding gas can break the magma into fragments and throw them into the air.
The fragments cool during flight or after landing.
At basaltic volcanoes such as Kīlauea, lava fountains can produce several forms of tephra, including:
Scoria
Pumice
Pele’s hair
Pele’s tears
Spatter
Reticulite
The type of material produced depends on the magma, gas content, eruption intensity, bubble structure and cooling rate.
What Is Reticulite?
Reticulite is an exceptionally frothy form of basaltic volcanic glass. Its structure can contain approximately 95 to 98 percent open space created by bubbles, leaving only a delicate framework of thin glass walls.
Its name comes from reticulum, a Latin word meaning a small net. That description fits the open, interconnected structure visible under magnification.
Reticulite is often golden-brown, tan, gray or nearly black. When viewed closely, it resembles a brittle sponge or honeycomb made from glass.
USGS describes it as an extreme form of pumice in which the bubble walls have burst and become interconnected.
How Reticulite Forms?
Reticulite requires an unusual combination of conditions:
Gas-rich basaltic magma rises rapidly.
The lower pressure allows gas bubbles to expand throughout the magma.
A very high lava fountain develops.
At Kīlauea, reticulite is strongly associated with fountains roughly 300 metres, or 1,000 feet, high or greater.
Bubbles expand until their walls rupture.
Instead of remaining as sealed cells, the bubbles connect and create an open mesh.
The material cools rapidly in flight.
The thin glass framework freezes before the foam can collapse.
Ordinary fountains may produce scoria or pumice.
The highest and most energetic fountains can produce the exceptionally fragile structure found in reticulite.
A recent example of this type of high fountaining can be seen in Kīlauea Episode 50, when the north vent produced a fountain approximately 1,000 feet high.
Why Reticulite Sinks Despite Being So Light?
This is one of reticulite’s most surprising characteristics.
Pumice often floats because many of its bubbles remain sealed, trapping air inside.
Reticulite has open and interconnected bubbles.
Water enters those spaces immediately, causing the material to sink despite its extremely low density.
Reticulite, Pumice and Scoria Compared
Reticulite, pumice and scoria are all vesicular volcanic materials, but their internal structures are different.
Here’s a quick comparison of reticulite, pumice and scoria based on their composition, bubble structure, density, appearance and eruption setting.
Reticulite:
Composition: Basaltic
Bubble structure: Open, interconnected mesh
Vesicularity: Extremely high, sometimes 95 to 98%
Relative density: Extremely low
Floats on water: Usually no
Appearance: Golden-brown to black honeycomb
Typical setting: Very high Hawaiian lava fountains
Pumice:
Composition: Often silicic, although composition varies
Bubble structure: Mostly enclosed bubbles
Vesicularity: High
Relative density: Low
Floats on water: Often yes
Appearance: Cream, gray or tan volcanic froth
Typical setting: Many explosive eruptions
Scoria:
Composition: Commonly basaltic to andesitic
Bubble structure: Larger bubbles with thicker walls
Vesicularity: Moderate to high
Relative density: Higher than reticulite and pumice
Floats on water: Usually no
Appearance: Dark red, brown or black
Typical setting: Basaltic fountains and cinder cones
Reticulite is therefore not simply “lighter scoria.” Its open bubble network records a different style of expansion and cooling.
Where Kīlauea Has Produced Reticulite?
Kīlauea has produced tephra repeatedly throughout its history, but well-preserved reticulite is linked to particularly high lava fountains.
The Keanakākoʻi Unit B Deposit:
A distinctive layer known as Unit B formed near Kīlauea’s summit around 500 years ago. The deposit contains material made of more than 95 percent bubbles.
Its presence helps geologists reconstruct the depth and shape of Kīlauea’s former caldera, as well as the intensity of the lava fountains that produced it.
The 1959 Kīlauea Iki Eruption:
The 1959 eruption of Kīlauea Iki produced 17 episodes of lava fountaining.
During episode 15, the fountain reached approximately 580 metres, or 1,900 feet, the highest measured in Hawaiʻi during the twentieth century. The eruption built Puʻupuaʻi and carried tephra as far as 16 kilometres downwind.
These exceptional fountains created ideal conditions for producing and dispersing reticulite.
The 1969 Mauna Ulu Eruption:
The Mauna Ulu eruption began in 1969 and continued until 1974.
Its early months included 12 fountaining events. Several exceeded 300 metres, and one reached approximately 540 metres, or 1,770 feet. Fallout from these fountains built tephra deposits downwind of the vent and included reticulite.
The Early Puʻu ʻŌʻō Eruption:
The early episodes of the Puʻu ʻŌʻō eruption between 1983 and 1986 also produced high fountains and reticulite.
Much of the light material travelled downwind before falling, which explains why reticulite may be found well beyond the immediate vent area.
How to Recognize Reticulite?
Look for several characteristics together:
A golden, brown, gray or black colour
An open honeycomb or net-like structure
Extremely thin glass walls
Very little weight relative to its size
A brittle texture that breaks easily
A location associated with tephra deposits from high fountains
Colour alone is not enough. Weathered pumice, scoria and other volcanic fragments can sometimes appear similar from a distance.
The open mesh is reticulite’s most distinctive feature.
Can You Collect Reticulite?
Do not remove reticulite, Pele’s hair, lava, rocks, sand or other natural material from Hawaiʻi Volcanoes National Park.
National Park Service regulations prohibit visitors from possessing or removing these geological resources. Reticulite is also extremely fragile, so handling it can destroy the structure that makes it scientifically valuable. Photograph it, observe it and leave it where it formed.
What Reticulite Reveals About an Eruption?
Reticulite is more than an unusual volcanic curiosity.
Its bubble structure can reveal:
How gas-rich the magma was
How rapidly the magma rose
How high the lava fountain reached
How bubbles expanded and connected
How quickly the material cooled
How far eruption products travelled downwind
A preserved reticulite deposit can therefore help scientists reconstruct an eruption that occurred centuries ago.
At Kīlauea, these deposits have contributed to research into past caldera conditions, lava-fountain behaviour and the evolution of gas-rich basaltic magma.
Frequently Asked Questions
Is reticulite a type of tephra?
Yes. Tephra is the broad category for volcanic material ejected into the air. Reticulite is one specific and unusually bubble-rich type of tephra.
Does reticulite float?
Usually not. Its bubbles are open and connected, allowing water to enter the structure. Pumice is more likely to float because many of its bubbles remain sealed.
Is reticulite the same as pumice?
No. Reticulite is sometimes described as an extreme form of pumice, but its open, interconnected bubble network distinguishes it from ordinary pumice.
Can visitors take reticulite from the park?
No. Removing rocks, lava, sand, Pele’s hair, reticulite or other natural material from Hawaiʻi Volcanoes National Park is prohibited.
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