Small Basaltic Volcanic Features

Brian Dempsey, West-Chatham Middle School, Georgia
Trina Knox, Hernderson Middle School, Georgia
Kay Emde
, Heritage Hall, Oklahoma
Charles Randazzo
, Southern Middle School, Pennsylvania

On Mercury (which little is known about in regards to volcanic activity), there is evidence of volcanic activity as seen by the smooth-bottomed craters thought to be infilled by basalt lava. 
         

 

Nash Lava Flow

Our Angela Lansbury look-alike standing on the basalt lava flow.
Our Angela Lansbury look-alike standing on the basalt lava flow.

We stopped at a broad, rubbly, low-slope exposure of dark rock that extended for approximately a kilometer along the road and a kilometer toward the tree line. Little vegetation occurred on the exposure. The rocks were dotted with lichens and occasional shrubs, grasses, and small trees grew in sheltered areas where soil had accumulated.

The rock was a basalt; dark in color, fine-grained, with both large and small bubbles.

Most of the lava was rough in appearance and broken along fractures. The basaltic lava mainly had a craggy a’a texture with a small area that exhibited a smooth, ropey pahoehoe texture.  In one area a channel-shaped feature was observed. It was approximately 2 to 3 meters deep and 6 meters across and extended for tens of meters before merging with the fragmented basaltic rock.

Looking into a lava channel.
Looking into a lava channel.

This flow was interpreted to be a large balsaltic flow from the Nash cinder cone, which is related to the nearby Belknap shield volcano. The presence of rough a’a lava suggests the lava may have been flowing quickly, causing the surface to crack and break into jagged blocks.   The tube-shaped feature may have been a lava channel or lava tube through which a stream of lava flowed during an eruption. The flow was relatively recent, as it has not been colonized by vegetation and the flows appeared to have undergone little weathering.

Small basaltic flows, such as the Nash flow, are the building blocks of large shield volcanos. Basalt lava has a low viscosity and low gas content; it tends to spread across the terrain in thin layers. Over time, these flows become stacked one on top of the other, eventually forming more massive volcanic features.

 

Lava Butte at Newberry Volcano Visitor Center

Lava Butte is a 500 foot high hill with steep slopes of about 32 degrees and a flatter top. The surface of the butte is composed of sand to boulder sized blocks of dark reddish fine-grained rock with an abundance of small vesicles (bubbles). A broad, rubbly, lower slope, expanse of dark, blackish fine-grained rock occurs on the side of the butte and extends down slope and onto the local landscape. Channel-like features cut through this expanse.

Lava Butte, a cinder cone on the slope of Newberry Shield Volcano.
Lava Butte, a cinder cone on the slope of Newberry Shield Volcano.

The rocks comprising the butte were identified as basalt, specifically as scoria, a basalt that cooled very fast and contains an abundance of air-filled holes. The lower slope expanse of rock also was identified as basalt, although it had a different texture. This region was similar to the a’a exposure described for Nash Lava Flow.

Lava Butte is a cinder cone that formed about 6,000 years ago. It formed as gas-rich basaltic magma erupted explosively. The bits of magma cooled quickly in the air, forming frothy scoria. As they fell to the surface they built the steep-sided cone.  Most basalt eruptions are not explosive; basalt lava commonly is a low viscosity fluid because it contains relatively little silica and little gas.  Following the butte formation, a non-explosive, low viscosity basaltic lava flow broke through the side of the butte and flowed downhill. Some of this lava traveled as rivers along the channel-like features — the lava channels and lava tubes observed in the surface. Lava Butte apparently underwent another period of explosive activity after the flow, as cinders occur on top of the flow. In total, Lava Butte and its flow contain about 1.4 cubic kilometers of lava, covering 6,117 acres).

Lava Butte is a relatively young formation based on its intact shape and small amount of vegetation growing on the butte and the lava flows around its base.  Lava Butte is an unusual cinder cone formation since, unlike most cinder cones, it is polygenetic (experienced multiple eruptions).

Lava Butte sits on the northern flank of the Newberry shield volcano and is one of many cinder cones that dot the surface of the volcano. Lava Butte’s magma ultimately came from a magma chamber that feeds Newberry Volcano. Newberry Volcano is built of stacked basaltic lava flows and cinder cones like Lava Butte.

 

Lava River Cave

We explored an underground tube-shaped structure that extended for approximately a kilometer and a half.  The tube had a flat floor and a ceiling that was 5 meters or greater in height. The cross section was oval to circular along much of the tube, but key-hole shaped in places. The walls of the tube were made of a fine-grained dark rock and were smooth. In some locations the rock appeared to have formed “drip structures” on the walls, and the walls had multiple parallel horizontal lineations on the lower levels.

Inside Lava River Cave. Lava once flowed through this enclosed tube. Past lava levels are marked by small ledges on the walls.
Inside Lava River Cave. Lava once flowed through this enclosed tube.
Past lava levels are marked by small ledges on the walls.

The tube is a lava tube, an enclosed channel through which lava flowed from the volcano or a fissure in the volcano. A continuous flow of lava can become channelized. The surface cools preferentially, and may form a roof over the flow, creating a tube. As the lava flow slows, the level in the tube drops, and lava may drip down the walls and solidify. The horizontal lineations mark different levels of the lava flow. In the section of the tube with the key-hole cross-section, lava may have flowed for a long time at a constant level, and the lava next to the wall chilled and formed benches.

 

Basalt-Ash Outcrop

One of the more complex small basaltic volcanic features observed was alongside of Route 97 on the way to Bend.

This site contained layers of fine-grained rocks of different colors. A thick black layer contained large light-colored crystals and green crystals. Other layers were lighter and contained angular fragments of different rock types. The rock layers were offset by breaks in the outcrop.

This image shows the dark and light layers, offset by faults.
This image shows the dark and light layers, offset by faults.

The rocks were identified as stacked layers of basalt and basaltic ash-flow tuff, a rare rock type on Earth. The basalt contained plagioclase feldspar and olivine crystals, indicating that the magma had cooled sufficiently for these minerals to crystallize before the lava erupted. The ash-flow contained pumice and scoria fragments. The layers were faulted with offsets of about 4 meters. 

This outcrop was confusing because it was an ash flow that mixed in other volcanic rock. The layers may have formed from an eruption(s) of gas-rich magma that moved as a frothy flow of ash and lava.

 

Planetary Connections

By recognizing volcanic features on Earth, we can identify similar features on other planets.

Cinder cones, like Lava Butte, occur on other planets. On top of Syrtis Major, a shield volcano on Mars, sits a small, steep sided dome that is about 1 kilometer tall. This shares characteristics with Lava Butte and is considered to be a cinder cone volcano. 

Cinder cone on Syrtis Major

Cinder cone on Syrtis Major
Mars Odyssey Mission THEMIS

Lava channels and tubes occur on the Moon and Mars (image below).  While a lava tube would be difficult to identify as it is beneath the surface, channels appear as long, narrow, curving features. 

A lava Channel on the Moon. Pits in the surface of Mars (left side of image) marking a lava tube under the surface.

Pits in the surface of Mars (left side of image)
marking a lava tube under the surface.
PIA07055: Lava Tube Collapse Pits

Basalt ash flows, like those observed at the Basalt Ash Outcrop, may have been observed on Mars.

Hadriaca Patera on Mars, showing lava flows from the center of a shield volcano.  These may be basaltic ash-flows.

Hadriaca Patera on Mars, showing lava flows from the center of a shield volcano. 
These may be basaltic ash-flows.
PIA00415: Hadriaca Patera


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