Mt. Kilauea is the world's most active volcano.  It sits atop the Hawaii Hotspot in the central Pacific Ocean, and has been erupting continuously since 1983.  Kilauea lavas are basaltic in composition, but the physical appearance of Kilauea's output varies tremendously.  Many of Kilauea's ongoing basaltic eruptions have occurred along its East Rift Zone, a fracture system extending ~eastward from the vent area.




Active basalt lava flow at the Hawaii Hotspot.

(photo from the United States Geological Survey)


The rock shown below shows the cross-section interior of a lava toe (see above for examples in the field).  Lava toes are bulbous, elongated features formed at the distal ends of low-viscosity basaltic lava flows.  This sample was part of a lava flow covering a road in the town of Kalapana (outside the national park), at the southeastern margin of Hawaii.  The lava flow came from the Kupaianaha eruptive center along Kilauea's East Rift Zone in the early 1990s.  Note that the interior of the lava toe is a dark gray-colored vesicular basalt.  Note also the black-colored rim, which cooled more quickly than the interior.  This black rim is mafic obsidian (a.k.a. basalt glass; a.k.a. tachylite).  Mafic obsidian has the same physical characteristics expected for the common felsic variety of obsidian - it has a glassy texture, with conchoidal fracture, sharp broken edges, and a jet black color.


Lava toe cross-section (~7.5 cm across at its widest), showing core of vesicular basalt and rim of mafic obsidian (basalt glass; tachylite).  From an early 1990s pahoehoe lava flow erupted from Kupaianaha, East Rift Zone, Kilauea Volcano, southeastern Hawaii.





Basalt lava flows in Hawaii have two types of surface morphologies: 1) aa lava and 2) pahoehoe lava.  The Hawaiian names for these lava flow surfaces have become technical names in volcanology.  Pahoehoe lava (pronounced "pa-hoy-hoy") has a smooth or ropy top surface.  Pahoehoe surface morphologies are not restricted to lava flows at the Hawaii Hotspot.  It's commonly seen in many basaltic lava fields on Earth.


Pahoehoe lava from Mauna Iki Crater, Kilauea Volcano, Hawaii Hotspot.  (CMNH 12388, Cleveland Museum of Natural History, Cleveland, Ohio, USA)





Sideromelane & Tachylite (above & below; above: field of view ~9.3 cm across; below: field of view ~3.2 cm across) - exterior surface of a lava tube ceiling sample.  The surface has abundant, consolidated basalt glass filaments, formed by stretching.  In cross-section view (see below), the rock is seen to be a highly vesiculated basaltic glass.  The rich golden brown-colored basaltic glass is called sideromelane.  The blackish-colored basaltic glass is called tachylite.

This rock formed the thin ceiling of a lava tube that was active during the 1969 to 1974 eruption of the Mauna Ulu Vent.  The Mauna Ulu Lava Field is located along Kilauea’s East Rift Zone.  This lava tube ceiling sample ranges from 1 to 3.5 cm thick.



Tachylite (above & below; above: field of view ~8.6 cm; below: field of view ~4.2 cm) - interior surface of lava tube ceiling sample.  Unlike the external surface, the inside roof of the lava tube doesn't display stretched textures.  The heat inside the lava tube was sufficient to partially re-melt already solidified lava.  This surface is relatively smooth and lustrous, somewhat like chocolate cake batter.



Vesiculated tachylite (field of view ~4.0 cm across) - the lithology of this lava tube ceiling sample is best revealed on the broken edges.  This cross-section view shows the the rock cooled very quickly - the material itself is glassy, with very few to no crystals, but with abundant preserved gas bubbles (vesicles).  The lithology is a vesiculated tachylite, in other words, a black, frothy-textured, basalt glass.





Vesicular porphyritic olivine basalt (above & below; above: ~6.5 cm across; below: ~4.0 cm across) - these two gorgeous pieces of lava are from the 13 January to 19 February 1960 Kapoho Eruption at the easternmost point of Hawaii.  The former town of Kapoho was located in the Kapoho Graben (Puna Rift Zone) segment of Mt. Kilauea's East Rift Zone.  Kapoho was famously burned, buried, and destroyed by aa lava flows erupting continuously for a little over a month in early 1960, despite the hasty construction of barriers.  The lava flows overtopped the barriers and even “burrowed under” the barriers.  See the full Kapoho Eruption story here, with field pics.

These rocks are vesicular porphyritic olivine basalts (more technically, they can be called vesicular tholeiitic picrite basalts).  They have abundant subrounded to stretched vesicles and common large greenish forsterite olivine phenocrysts.  The basalt matrix is glassy-looking, very finely crystalline plagioclase feldspar & pyroxene.  The large sample shown above has an obvious black-and-white speckled xenolith of gabbro from the lower oceanic crust.

These particular samples are from the post-30 January 1960 phase of the Kapoho Eruption.  Starting the 31st of January, it was noted that large olivine phenocrysts were becoming common in the newly erupted lavas.




Reticulite (1.8 cm across) - here's a sample of reticulite (a.k.a. thread-lace scoria).  It is greenish-brown, highly vesiculated basalt glass (sideromelane).  Nearly all of the cells walls of the gas bubbles have burst, resulting in a complex three-dimensional honeycomb structure.

Reticulite forms during high lava fountaining episodes.  The honeycomb network is the result of extreme vesiculation from vigorous gas escape as lava cooled ver quickly within the lava fountain.  Reticulite is very lightweight for its size (low density), and is readily carried downwind.

Hawaiian reticulite is an incredibly fragile material, having a dark, drab olive green color.  Reticulite's glass threads typically form hexagonal & pentagonal geometries.  Despite its low density, reticulite does not float in water - its porosity is too high.







Achneliths (Pele's Tears) (from left to right, specimens are 9 mm tall, 10.5 mm tall, 9.5 mm tall, and 18 mm tall)

Pele's tears are small, raindrop-shaped blebs of quickly solidified basalt lava.  The technical term for such structures is achneliths.  These Hawaiian achneliths represent rapidly chilled basalt lava spray blown by winds during a high lava fountain eruption.  The broken tips of the three smaller specimens reveal that they are composed of black, finely-vesiculated basalt glass (tachylite).  In the field, Pele's Tears are many times seen attached to the distal ends of extremely long threads of mafic glass (Pele’s Hair - see below).

Locality & eruption info.: unrecorded locality on Hawaii, but probably derived from a Mt. Kilauea eruption; unrecorded eruption date, but probably a mid- to late-20th century high fountain eruption.





Pele’s Hair (above & below; above: field of view ~2.8 cm across; below: field of view ~3 cm across) - Pele’s hair refers to extremely long threads of brownish- to blackish-colored basalt glass (sideromelane & tachylite).  It forms as very fluid basalt lava spray is stretched while airborne.  Achneliths (Pele’s tears - the small, black, glassy structures) are often found attached to one end of individual threads.  Multiple threads can be found attached to some achnelith masses.

Provenance: vintage 1930s tourist specimen; unrecorded eruption date; unrecorded collection locality. 


Achnelith (Pele's tear) with two attached threads of Pele's Hair.  Longest dimension of specimen is 9 mm.



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