Vents

Vents

Vents, of course, are the locations from which lava flows and pyroclastic material are erupted. Their forms and orientations can be used to determine many characteristics of the eruption with which they were associated. There are two main endmembers in a spectrum of pyroclastic vents in Hawai'i, spatter vents and cinder cones. Their differences are due mostly to the gas content of the magma that is erupted. Additionally, there are satellitic shields formed during eruptions without fountaining and tuff cones formed during phreatomagmatic eruptions.

As a dike approaches the surface, it generates a zone of tension at the surface. This tension is usually manifested as a pair of cracks with the ground with the area in between often lower than the surrounding elevation (see below). The first phase of a Hawaiian eruption is usually characterized by breaking to the surface of a dike along one of the two fractures resulting in a line of erupting vents commonly called a "curtain of fire" (e.g. Macdonald 1972). After a few hours or few days most parts of the fissure stop erupting and activity is concentrated at one or more separate vents (e.g. Bruce & Huppert 1989). It is these vent locations that usually persist long enough (hours to weeks and sometimes years) to produce significant near-vent constructs. The change from long continuous erupting fissures to one or a few vents must be remembered when mapping eruptive fissures in remote sensing data and relating them to dike dimensions: The near-surface part of the dike is almost certainly longer than any line of near-vent constructs (see discussion in Munro 1992).

Fissures opened in the cinder-covered surface uprift from Pu'u 'O'o in July of 1985. Note that there are two parallel fractures about 50 m apart and forming a small graben. The next morning lava erupted out of the nearest fissure.

The ground surface here is covered by a ~2 m-thick layer of Pu'u 'O'o scoria, and this helped to accentuate the cracks - similar to the way that a small hole dug into sand at the beach will eventually look quite large as sand slumps into the hole.

In the case shown here the actual cracks in the rock under the scoria were only about 10-20 cm wide but so much scoria fell into them that they were wide enough to barely be jumpable.

 

 

Spatter Vents

 

Spatter refers to blobs of lava thrown a little ways into the air (by expanding gases) that is still molten when it lands.

 

Spatter Vents

Spatter ramparts and spatter cones are the vent structures formed by this type of activity. Spatter ramparts are elongate along the trace of an eruptive fissure whereas spatter cones occur as discrete mounds. They range between 1 and 5 m high, are steep-sided, and are composed of agglutinated (stuck-together) spatter. They are steep-sided because the hot spatter blobs are able to stick to each other when they land, and don't flow or roll away.

Small spatter cones forming near Pu'u 'O'o on the east rift zone of Kilauea in July of 1985. Note that the ability of molten spatter to stick together allows the spatter cones to be steep and even vertical. The pahoehoe toes in the foreground are picking up pieces of scoria that cover the ground in this area, and via a "reverse caterpillar motion" are placing these pieces (the dark spots) on top of themselves - stratigraphy is being reversed.

 

 

 

The fountaining associated with the formation of spatter ramparts is usually less than 10 m high.

 

 

A small spatter cone on Kilauea erupting in 1992. Note the fluid nature of the individual blobs making up the cone, and their ability to form a steep structure. The glowing orifice from which the spatter is erupting is ~30 cm across. The profile of Pu'u 'O'o can be seen in the background.

 

 

 

 

At the end of the eruption, lava often drains back into the fissure, forming prominent drainback features. Even if nobody actually witnessed a particular eruption, if you find spatter ramparts or cones associated with it, you can say that the fountaining that formed the cone or rampart was not very high.

 

 

Spatter vents from the 1974 eruption of Kilauea along the upper SW rift zone. Note that the last thing that happened was the drain-back of lava into the fissure (red arrows). The higher areas on the left and right are spatter ramparts and are 1-2 m high.

 

 

 

Cinder Cones

 

At the high-fountaining end of the spectrum are cinder cones. Cinder cones can be quite large in Hawai'i; those on the summit of Mauna Kea (formed during gas-rich alkalic-stage eruptions) are a few hundred meters high, whereas those on Mauna Loa and Kilauea usually range between 20 and 100 m high. Pu'u 'O'o on the E rift of Kilauea, which formed between 1983 and 1986 is unusual in that it reached a height of 255 m above the surrounding surface (Heliker & Wright 1991).

To the left is a photo of Pu'u 'O'o cinder cone, Kilauea, viewed toward the west. The prevailing right-to-left tradewind direction is obvious from the way that the plume is being blown. During the eruptions that formed Pu'u 'O'o, these same tradewinds built the cone much higher on the downwind side of the vent than the upwind side. Almost all the lava flows therefore came out of the upwind side (i.e. towards where the photo was taken).

 

As their name suggests, cinder cones consist of cinders, more properly called scoria. Scoria is very vesicular, low density basalt. Lava fountains are driven by expanding gas bubbles; the bubbles are trying to expand in all directions but the only way to relieve the pressure is up out the vent so fountains are usually directed relatively vertically. The Pu'u 'O'o fountains were at times up to 350 m high, and those during the early stages of the Mauna Ulu eruption were up to 500 m high. Because the pyroclasts are thrown so high, they cool before they land and don't stick together. Cinder cones are therefore composed of loose pyroclastics at the angle of repose (~33ยบ).

 

Right is an image of fountaining at Pu'u 'O'o, Kilauea (July 1985). These particular fountains were ~200 m high, and were sending short fast flows in many directions.

 

In plan view, cinder cones tend to be roughly circular. They are usually formed later in an eruption when activity has localized to one or more discrete vents. If the precise location of the vent changes during an eruption, the cone loses its simple circular shape, and becomes more complex. Roadcuts through most cinder cones expose very complicated crosscutting relationships relating to the different locations of the fountain centers.

 

Photos of cinder cones on Mauna Kea (arrows), viewed from the summit of another cinder cone. The reddish color is common to cinder cones and occurs both during and soon after the associated eruption due to the combined efforts of moisture and oxidizing gases. The light blue line marks the Mauna Kea-Mauna Loa boundary. Note that one of the cones (yellow arrow) has been surrounded by (younger) Mauna Loa lavas.

 

Cinder cones can also be distinctly asymmetric if there was a persistent wind blowing during the eruption and/or they form at the heads of major lava flows. In this second instance they are horseshoe-shaped (see below), with the lava flow issuing out of the open end because during the eruption any pyroclasts that landed on the flow were rafted away.

A Mauna Kea cinder cone viewed from the air. A lava flow field (white outline) has issued from the base of the cone, giving the cone an asymmetric form. The flow spread almost all the way around the cone (white arrows). Magenta lines mark the rims of older cinder cones nearby.

 

Between the two extremes of spatter ramparts and cinder cones are all gradations. Some pyroclastic constructs consist of alternating layers of agglutinate and cinder, indicating that the vigor of the fountaining varied during the eruption. The early part of an eruption, often called the "curtain of fire", produces mostly spatter ramparts and spatter cones. As the activity becomes localized at one or more points along the fissure, this concentration of activity usually leads to higher fountaining. Cinder cones are built at these points, often at the same time that spatter ramparts are forming at the (less active) ends of the fissure. During the Mauna Loa eruption of 1984, there was a distinct gradation from vigorous fountaining at the main vents, progressing to lower and lower fountaining both up and downrift (Lockwood et al. 1987). At the farthest uprift end of the fissure, only gas was being emitted from a spatter cone that had been active earlier in the eruption.

 

 

Satellitic Shields

 

Approximately 50% of Hawaiian eruptions have no pyroclastic activity associated with them at all. Instead, lava is quietly erupted onto the surface.

 

This lava flows away in all directions forming a miniature shield volcano. These vents are called "satellitic" or "parasitic" shields, and produce tube-fed flows. Satellitic shields have diameters of 1-2 km, and can be ~100 m high with slopes of only a few degrees.

 

 

 

 

On the right is an image of Mauna Iki satellitic shield on the SW rift zone of Kilauea. Note the gentle slopes (similar to Kilauea as a whole). The distance from left to right on the horizon is about 5 km

 

While a satellitic shield eruption is going on, a lava pond usually exists at the summit of the shield. Overflows from the pond build the shield.

 

 

 

Kupa'ianaha lava shield and pond in September 1986 (~2 months after it formed). Notice the shape of the pond, with a large near-circular part and an elongate extension to the left. The main lava tube was fed from the end of the extension. When this photo was taken the pond was also overflowing to the right. A break-out low on the far side of Kupa'ianaha was feeding another surface flow that in this photo had almost reached the contact with 'a'a flows from Pu'u 'O'o (3 km uprift; at left).

 

There have been 4 major satellitic shields formed on since the arrival of Westerners (Mauna Iki 1919-1920, Mauna Ulu 1969-1974, Kupa'ianaha 1986-1992, and the presently-active vent 1992-who knows?). Including these, 16 satellitic shields have been mapped on Kilauea (Holcomb 1987).