Shield volcanoes

Shield volcanoes are the products of eruptions of low-viscosity materials. They have been identified on five solar system bodies. The most active volcanoes on the Earth are shields.

Volcano Classification

The shield volcano is so called because of its similarity to the ornate shields of the ancient Nordic warriors. The name is derived from the Icelandic word dyngja (shield). The typical shield volcano is circular to ellipsoidal when viewed from above. Its “ornamentation” can be superposed cinder cones, radiating lava flows, a summit or flanking crater or craters, and rift valleys. Viewed from the side, the shield can take forms ranging from inverted saucers to inverted mixing bowls. In this respect, shields differ from the familiar volcano shape represented by the symmetrical, conical Mount Fuji in Japan.

Most shields are basaltic in composition and formed primarily of lavas (more than 98 percent). Exceptions are found in shields with lavas richer in silica and of andesitic composition, such as Hayli Gub in Ethiopia, and those poorer in silica, such as the carbonatitic volcanoes of the East African Rift zone. There are also shields composed primarily of pyroclastic materials, although frequently, they are separately classified.

One of the earlier attempts to produce a systematic classification of volcanoes was made by Swiss volcanologist Alfred Rittmann in 1936. This scheme did not achieve wide recognition until the second German edition of Vulkane und ihre Tätigkeit (1960; Volcanoes and Their Activity, 1962) was translated into English. In this text, Rittmann recognized a Hawaiian and Icelandic type of shield volcano. Sir Charles Cottonin recognized a similar subdivision in his text Volcanoes as Landscape Forms (1944). In the most influential of the English geomorphology textbooks of the twentieth century, Principles of Physical Geology (1944), Arthur Holmes classified the shield volcanoes as “domes of external growth” to distinguish them from the domes of viscous lava built by the addition of material to the interior.

With the acquisition of images of volcanoes on other bodies in the solar system, a new phase began in volcano classification. These new images provided views of volcanoes produced on bodies with different gravities, atmospheres, compositions, and other important parameters. The volcanologist now needed a database to objectively compare the morphologies of the volcanoes on the different bodies. British-born American geologist James Whitford-Stark made one such attempt in the book Volcanoes of the Earth, Moon, and Mars (1975). This classification was made based on the basal diameter versus the height of the shield volcanoes and resulted in the identification of four classes of terrestrial shield volcanoes: Hawaiian, Icelandic, Galápagos, and scutulum. (The last is from the Latin scutum, or “shield.”) Lunar domes were shown to be similar to Icelandic shields, whereas Martian volcanoes had dimensions exceeding those of most terrestrial volcanoes.

A more detailed analysis was undertaken by British chemist Richard Pike and American geophysicist Gary Clow, who employed a statistical technique called principal component analysis. They first distinguished between polygenetic and monogenetic shields and then identified subgroups based on size, shape, composition, and location. The monogenetic shields, shields produced by only one eruption phase, included the large Icelandic shield, a smaller steep shield, and a low shield. The polygenetic shields included a Hawaiian type, a Galápagos type, and a locally varied type, all of which are characterized by tholeiitic basalts; an oceanic and continental type dominated by alkali-rich basalts; and a group represented by seamounts, or submarine volcanoes. The more silicic of the shields were grouped into alkalic and calcalkalic ashflow plains categories. Pike and Clow also recognized three distinct groups of Martian volcanoes and a group of lunar domes. Volcanic landforms have since been recognized on the Jovian satellite Io and Venus. Unfortunately, there is a lack of topographic data for these features, so objective comparisons cannot yet be made.

Types of Shield Volcanoes

Small low and small intermediate-slope shields are monogenetic and variously described as scutulum-type shields, lava cones, or low shields. These shields are abundant in the Snake River plain volcanic field of the western United States and are represented by the volcano Mauna Iki, which erupted between 1919 and 1920 on the flanks of Kilauea. Typically, these shields have lower slopes of about 0.5 degrees and steepen to about 5 degrees near the summit. This slope change has been attributed to a change from extensive, thin, fluid, pahoehoe flows early in the eruption to more viscous, aa flows during the waning eruption. Many have summit craters formed by collapse, and a few have spatter ramparts around the vent. In the Snake River plain, the shields are aligned along fractures and probably represent initial fissure-type eruptions that subsequently contracted to point-source eruptions.

Small steep shields are typified by Mauna Ulu, produced in an eruption lasting from 1969 to 1974 and punctuated by a three-and-one-half-month quiescence in late 1971 and early 1972. The early phase was marked by lava fountains reaching up to 540 meters, whereas in the second phase, the fountains rarely exceeded 80 meters. Nearly 350 × 10⁶ cubic meters of lava contributed to the final structure. A summit lava lake characterized the eruption. Sustained overflows of this lake led to the production of tube-fed lava flows that traveled up to 12 kilometers from the vent. Short-duration overflows resulted in the growth of the shield structure. The final structure rose 121 meters above the pre-1969 base and had a basal diameter of slightly more than 1 kilometer. The smaller Icelandic and seamount shields also fall within this category.

Medium low shields are polygenetic constructs represented by calcalkalic ash-flow plains and the smaller alkalic ash-flow plains. The calcalkalic ash-flow plains are composed primarily of rhyolitic or dacitic ignimbrites, rather than lavas, surrounding a caldera ranging from 7 to about 60 kilometers in diameter. Lavas and pyroclastic fall deposits dominate toward the caldera, and the flank slopes increase from about 1 degree to about 8 degrees. These structures are the products of extremely powerful eruptions that, fortunately, have not occurred within historic times. If of volcanic origin, the Venusian coronae would probably fall within this group. Medium intermediate-slope shields consist of a wide variety of the groupings established by Pike and Clow. The steeper of the ash-flow sheet shields are included, most continental alkalic basaltic shields, and the smaller oceanic and Martian shields. Medium steep shields are represented primarily by the Icelandic shields, the larger of the seamounts, the steeper continental shields, and the small steep oceanic shields. The small-volume members can be monogenetic, whereas the large-volume members are invariably polygenetic.

The terrestrial Toba volcano, the Martian paterae, and the larger Venusian volcanoes, such as Colette and Sacajawea represent large low shields. Large intermediate-slope shields include most Hawaiian volcanoes, other terrestrial oceanic shields, and the Martian tholii. The mature Hawaiian shields typically have radiating rifts, a summit collapse caldera, and superposed small shields and cinder cones of a more evolved composition than the volcano's mass. The large steep shield group includes the Galápagos volcanoes and other oceanic shields. The Galápagos shields differ from the Hawaiian shields in being much steeper in the summit region, having concentric rifts, and having a greater portion of more alkaline basalts.

The very large low shield category currently includes only the Martian volcano, Alba Patera. A number of the Io volcanoes will likely fall into this category. Very large intermediate shields are represented by the giant Mons volcanoes on Mars (Olympus Mons) is the largest volcano in the solar system). Data suggest that lava flows dominate these very large shields.

Study of Shield Volcanoes

The morphologies of terrestrial shield volcanoes are readily obtainable from topographic maps, their compositions are obtained via standard geochemical techniques, their ages can be ascertained by radiometric dating, and their interior structures can be determined by observation of eroded structures. These quantities are less easily obtained on other bodies in the solar system. Topography can be determined by measuring shadow lengths or determining the time delay between the sending and receiving electromagnetic radiation by such instruments as laser altimeters. Since there are only rock samples from the moon (excluding an exotic group of igneous meteorites found on the Earth), the compositions of other extraterrestrial materials must be determined by analyzing their spectral characteristics. Relative ages of the outermost layers of extraterrestrial volcanoes can sometimes be determined from the density of superposed impact craters—the more craters, the older the surface. The internal structure of these volcanoes cannot be determined since none of their parent bodies has as powerful agents of erosion as those found on the Earth.

One reason for assembling morphometric data for terrestrial volcanoes is to ascertain any relationships between the volcano's size and shape and the forces that led to its construction. Then, by making corrections for the different gravities, atmospheres, and other parameters on the nonterrestrial bodies, one can infer the conditions that led to the construction of their shield volcanoes.

Despite the wide range in the volumes of shield volcanoes, all appear to be constructed of low-viscosity material, whether basaltic lava or silicic ignimbrite. A significant difference exists in the volume eruption rates between the lava and ignimbrite shields. The lava shields are formed primarily by an eruption style called Hawaiian, which is characterized by volumetric eruption rates of about 100 cubic meters per second for short durations but less than 1 cubic meter over periods of a few years for the smaller structures, to about 1 million years for the larger Hawaiian shields. The ignimbrite shields are products of a plinian or ultraplinian eruption style. They are thought to have had volumetric eruption rates of 1 million cubic meters per second for eruptions lasting less than one day.

A feature that distinguishes the majority of terrestrial shield volcanoes from cone volcanoes is that the shields are commonly not found at plate margins; they are predominantly intraplate volcanoes. It has been suggested that many shields are associated with hot spots or areas of anomalously hot mantle. The lack of terrestrial continental shields with heights over 2 kilometers implies that the Earth's oceans act as buttresses to prevent the collapse of the oceanic shields. The great heights of the Martian Mons volcanoes are probably, in part, a function of the lower gravity on that planet.

Significance

Shield volcanoes include the most active volcano on Earth (Kilauea), the tallest volcano in the solar system (Olympus Mons on Mars), and perhaps some of the highest eruption columns (more than 200 kilometers high on Io). They have undergone some of the most violent eruptions ever on Earth, such as the eruption at Toba Caldera, Sumatra. On Earth, shields are present on the continents and ocean floor, occurring as oceanic islands. Mauna Loa is the largest mountain on Earth, rising some 10 kilometers above its base from the sea floor to its summit. Small shields resembling those of Iceland are found on the moon. Gigantic shields have been identified on Mars. Active shield volcanoes are present on Io, and numerous shieldlike structures occur on Venus.

Chains of terrestrial shield volcanoes extending across a plate and formed by a single, fixed hot spot have been employed to determine the direction of motion of the plate, and the ages of the various volcanoes along the chain have been used to infer the plate's rate of motion. At the large and very large end of the shield volcano scale, inferences can be drawn as to the nature of the planet's interior, such as the internal heat transfer as a function of time and the strength of the outer layers of the planet required to support such huge masses. The heights of shield volcanoes have been employed to determine lithosphere thicknesses, and the horizontal separations between volcanoes have been used to infer the depths in the interior at which magma originated.

The high eruption frequency of terrestrial shield volcanoes, such as those of Hawaii, is important to those living in the paths of lava flows, which may destroy their homes. The enormous scale of eruptions associated with forming the ash-flow shields has fortunately not been witnessed in historic times. Still, such eruptions have been relatively frequent throughout geologic history and may recur in the future. On a more positive note, shield volcanoes can provide tourist revenue, provide new, rich arable land, and endow geothermal energy.

Scientists continue to study shield volcanoes in the twenty-first century, especially within the context of planetary bodies within the solar system. A 2024 study revealed the presence of a massive shield volcano in the Noctis Labyrinthus region of Mars. This discovery of this incredibly tall volcano using data from the Mars Reconnaissance Orbiter provided new and valuable information about Mars. Scientists also plan to study the volcanic activity from shield volcanoes on Venus, which is believed to be far more frequent and widespread than previously accepted. 

Principal Terms

coronae: ring structures on Venus consisting of alternating concentric ridges and valleys higher than the external terrain; possibly volcanic in origin

hot spots: areas of anomalously hot mantle

ignimbrite: rock formed by widespread deposition and consolidation of block, pumice, and ash flows

monogenetic: pertaining to an eruption in which a single vent is used only once

paterae: inverted, saucer-shaped features considered to be of volcanic origin

polygenetic: pertaining to volcanism from several physically distinct vents or repeated eruptions from a single vent punctuated by long periods of inactivity

pyroclastic materials: broken rock formed by volcanic explosion or aerial expulsion from a volcanic vent

tholus: an inverted, bowl-shaped feature considered to be of volcanic origin

Bibliography

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