Large Basaltic Volcanic Features
Peggy Hussey, Mooreville Middle School, Mississippi
Tamara Porter, DeRenne Middle School. Georgia
Jacqueline Sollers, Carroll County Public Library, Maryland
Susan Wainwright, Novi Meadows School, Michigan
Basalt is a fine-grained igneous rock that is dark gray, black or brown in appearance. Basalt has a SiO2 content of ≤ 52% and contains relatively more magnesium and iron than more silicic igneous rocks. A basaltic lava flow has a low viscosity due to its low SiO2 content. Eruptions associated with basaltic lava usually are not explosive due to the low silica and gas content. Gases escape more easily when silica content is low. Because of the low viscosity, basaltic lava flows spread out, forming extensive layers. They may build up to form a large shield volcano — volcanos with low sloping sides.
Pahoehoe and a’a lava textures are typically associated with basaltic lava flows. As the lava flows, it cools on the surface and forms a crust, which breaks with the continued flow of the lava beneath it. This produces a type of lava formation called “a’a,” characterized by jagged blocks of rock. Less viscous lava produces a ropey type of formation called “pahoehoe.” Temperature can also influence lava flow textures. Slower flows tend to have pahoehoe tops; these tops get broken up into a’a in faster flows.
Columnar jointing is also frequently seen in basaltic lava flows. Columnar joints are parallel, prismatic columns in basaltic flows (and sometimes other rocks), and this pattern is a result of cooling. As the lava cools, it contracts and forms cracks, commonly in a hexagonal pattern. Once the crack develops, it continues to grow, perpendicular to the surface of the flow. The columns vary from a few inches to several feet in diameter.
Basalt may have a whitish coating (caliche) in arid areas from the deposition of calcium carbonate. In more humid areas, the basalt has a reddish color because of the breakdown of the iron minerals to make rust.
In contrast to basaltic lava flows, andesitic lava flows have a higher SiO2 content of 52–62%, which makes the lava more viscous. Andesites are dark to medium gray in appearance (although some samples have flecks of green, brown or white). Dacitic lava has a SiO2 content of 62–68% and the viscosity of the lava is higher than basaltic lava. Dacites are light to medium gray in appearance. Rhyolitic lava is high in SiO2 and has a lower iron and magnesium content than basalt lava. It is viscous and much more explosive because of the higher silica and gas pressure. These more silicic lava flows do not spread out as much as basaltic flows and form more steep-sided volcanic features such as composite volcanos. Other Web pages will explore more silicic volcanic features; this page will examine large basaltic features.
Newberry Volcano is an excellent example of a large shield volcano. Its slope measures about 6 degrees, which is typical of a shield volcano and is the result of multiple eruptions of basaltic lava that had a low viscosity, and thus, were able to spread a great distance from the magma source.
Photograph of Newberry Volcano showing the broad, low nature of this shield volcano. Newberry Caldera, Oregon Photo Archives
Newberry’s lava flows are extensive, covering an area 55 km (35 miles) long from north to south and 55 km (22 miles) wide. The volcano is about 32.1 km (20 miles) in diameter and the summit has a caldera that is 6 km (4 miles) in diameter. It is a little over half a mile (1 km) high. Paulina and East Lakes are located in the caldera.
The surface features of Newberry Volcano are complex as a result of multiple basaltic eruptions over the past 500,000 years. There are about 400 cinder cone volcanos that formed during eruptions from fissures or vents on Newberry’s flanks. The north and south flanks of Newberry, which extend the greatest distance from the summit calderas, are basalt and basaltic-andesite flows. Most flows are of block or a’a type; pahoehoe surfaces occur locally on a few lower flank flows. Lava tubes are common and some extend uncollapsed for distances of 1 mile (~ 1.5 kilometer). Prior research found the silica content of the rocks here to be about 54% — andesitic basalts — similar to rocks in the Cascade Range. However, the rocks sampled were from younger areas of the volcano; older rocks are more basaltic.
Lava Butte is located on the northwest flank of Newberry and formed as a result of an eruption from a side vent about 7,000 years ago. Lava Butte’s cone is 500-feet high and its slope measures 32 degrees. The top of Lava Butte is mostly cinders and represents the younger eruptions. As the lava was ejected, it built layer upon layer around this vent, forming Lava Butte. The eruption is estimated to have occurred over a period of 4 years. Ash flows, pumice falls, mudflows and other pyroclastic deposits are found on the west and east flanks of Newberry. The youngest period of volcanism associated with Newberry Volcano is Big Obsidian Flow which occurred approximately 1300 years ago.
Similar to Newberry, we saw other shield volcanos from a distance on the trip, including Belknap, and the lower portions of Mount Washington, Mount Thielson, and Three Fingered Jack.
Horse Ridge Summit
Horse Ridge Summit is located in the Western area of the High Lava Plains in the Brothers Fault Zone near Harney Basin.
The area where we stopped (pictured above) was exposed when overflows from a nearby lake cut down through the rock layers, exposing the loosely consolidated lava flows. This area is a classic example of flood basalt. A flood basalt forms when basaltic lava pours out quietly from long fissures — instead of central vents — and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. The original lava flow at this site had such a low viscosity that it spread like syrup — fast and far. Horse Ridge Summit area was not formed from a single basalt lava flow but from numerous flows approximately 14–16 million years ago. The oldest rocks found at Horse Ridge Summit are all aphyric (meaning it does not contain phenocrysts — large crystals) and phyric (meaning it does contain phenocrysts) basalt, overlapped by sedimentary rocks.
Connection to the Solar System
Volcanos exist on planets in our Solar System. The planet Venus has a basaltic surface composition with hundreds of shield volcanos. The shield volcanos were created by basaltic lava flows and have the same low slope as the basaltic shield volcanos on Earth.
Sapas Mons is one example of a large shield volcano located on Venus. It measures 400 km across the base and 1.5 km (4,921 feet) in height. The lava flows of Spas Mons continue for hundreds of kilometers. We saw this type of lava flow at Newberry Volcano.
Sapas Mons, a shield volcano on Venus. Magellan image courtesy of JPL/NASA
PIA00203: False Color Image of Volcano Sapas Mons
On Mars, the largest volcano is Olympus Mons, which measures 640 x 840 km across (397 x 521 miles)! The slope of Olympus Mons is 5O, suggesting it is a basaltic shield volcano. In the field, we measured the slope of Newberry volcano and found it to be 6O — not that much different than the slope of Olympus Mons. The lava associated with a shield volcano is basalt — low viscosity lava that spreads quickly.
Arsia Mons, Pavonis Mons, and Ascraeus Mons are other large shield volcanos on Mars. Why are the volcanos located on Mars much larger than those on Earth? To put it simply, Mars currently does not have plate tectonics and Earth does. The volcanos on Mars do not move on a plate. Instead, they are stationary and situated over their magma source. This allows extremely large volcanos to form as eruption after eruption occurs in the same location.
Syrtis Major is one of the darkest areas on Mars and probably basalt in origin. It resembles large flood basalts on Earth in color and size.
Jupiter’s Moon, Io, has many, many volcanos that are wide and almost flat like shield volcanoes. The surface of Io is covered with craters and calderas which are similar to those found on Earth but much larger. Io is about the same size as our Moon. The volcanos on Io are large and active. One is almost 90 miles high. Pillan Patera and Amirani were observed producing 620 km of lava over a period of five months. Io’s volcanism differs from Earth in that it is the gravitational pull of Jupiter and its moons on Io that creates heat through friction, leading to volcanism.
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.
Earth’s Moon showing dark basins (mare) where basaltic lava has flowed from the Moon’s interior and covered the basin floors.
Moon Becomes Geologically Inactive