How Volcanoes Work



One way to classify lavas is by their alkali content, reflected in their weight percent of Na2O + K2O. In contrast to the common lava types (basalt, andesite, dacite, and rhyolite), there exists less common lavas that define mildly alkaline trends (e.g., with increasing silica content: alkali basalt, trachybasalt, trachyandesite, trachyte, and comendite), and strongly alkaline trends (e.g., with increasing silica content: tephrite, phonotephrite, tephriphonolite, and phonolite). Although these lavas can occur in a variety of tectonic settings, they are typically found in either (1) continental or oceanic intraplate settings, where there is often a lack of significant tectonic control, (2) continental rift zones, and (3) the back-arc setting of subduction zones.




Alkali-rich phonolite lavas extruding from basaltic scoria cones on the Harrat Kishb lava field, western Saudi Arabia. Note -- Phonolite lavas derive their name from the high-pitched bell-tone created when striking the lava with a hammer.

Alkali-rich lavas are often charateristic of the waning stages of volcanism, as demonstrated, for example, in the late-stage parasitic cones and flows found on Hawaiian shield volcanoes. Differentiated trachytic and phonolitic lavas typically erupt as low-volume flows with high aspect ratios, as demonstrated above in the phonolite coulées of western Saudi Arabia. However, extensive sheetflows of similar lavas have been recognized in continental rift zones, as exemplified in the voluminous phonolitic lavas of the Ethiopian rift system.


Carbonatites are perhaps the most unusual of all lavas. They are defined, when crystalline, by having more than 50% carbonate (CO3-bearing) minerals, and typically they are composed of less than 10% SiO2. There are only 330 known carbonatite localities on Earth, most of which are shallow intrusive bodies of calcite-rich igneous rock in the form of volcanic necks, dikes, and cone-sheets. These generally occur in association with larger intrusions of alkali-rich silicate igneous rocks. Extrusive carbonatites are particularly rare, and appear to be restricted to a few continental rift zones, such as the Rhine valley and the East African rift system.

Most carbonatite lavas have low eruption temperatures, between 500 and 600 degrees Centigrade (compared with >1100 degrees Centigrate for basaltic lavas). They typically have low viscosities due to their lack of silica polymerization. Thus, carbonatite flows are generally only a few centimeters thick, with surface textures that vary from a'a to pahoehoe. Although they often resemble flowing lobes of black mud, they are hot enough to display glowing, deep-red colors when seen at night. Some active carbonatite flows are enriched in alkalies (Na and K) - these are called natrocarbonatites. Soon after their eruption, dark natrocarbonatite flows will cool and rapidly turn white due to reaction with atmospheric water.



A 5-cm thick carbonite a'a flow advancing from lava lake. Courtesy of Marco Fulle -

Pahoehoe carbonite flow bounded by blocky lava levee. Note older lava, turned white by hydration. Courtesy of Celia Nyamweru. 


The East African Rift System contains a vast array of igneous rocks, dominated by voluminous floods of basaltic lava. Its highest peaks, Mt. Kilamanjoro and Mt. Kenya, are active volcanoes located along the eastern rift of Lake Victoria. Lying between these impressive peaks is the only know active carbonatite volcano in the world - Oldoinyo Lengai. Rising over 2200 meters from the valley floor, the bulk of this volcanic cone is composed phonolitic tephra. However, its upper portion is dominated by natrocarbonatite lava flows. Historic eruptions of natrocarbonatite have filled much of the summit crater, shown here courtesy of Marco Fulle - Note the tall conical hornitos protruding from the crater floor.

Many of these hornitos are quite large, as demonstrated in the image left (Courtesy of Burra Gadiye). The dark natrocarbonatite flows shown here erupted in 1998. Continued ativity from on Oldoinyo Lengai from 1995 to 2001 is demonstrated in the following images of erupting hornitos.




December 1995 - Spectacular bubbling of natrocarbonatite spatter from a small hornito on Oldoinyo Lengai. Courtesy of Peirre Vetsch

July 2001 - With very low light, carbonatite lavas can display reddish colors, as shown in this erupting hornito. Courtesy of Marco Fulle -

March 1999 - Squirting of natrocarbonatite spatter from openings in the wall of a hornito on Oldoinyo Lengai. Courtesy of Frank Pothe.


A history of controversy has surrouned the origin of carbonatites, and whether or not calcite carbonatites are primary or secondary lavas. Some scientists believe that the Ca-carbonatites are generated by fractional melting of crustal carbonate rocks. However, others believe that the Ca-carbonatites are not primary magmas at all, but rather derived from the alteration of Na-rich natrocarbonatite lavas. Virtually, all natrocarbonatites on Earth are found in historic eruptions, and none in ancient rocks. Many have attributed this to the very high solubility of sodium carbonate. The argument being that all carbonatites were originally natrocarbonatites that have had sodium removed by hydrothermal solutions and rainwater.

The natrocarbonatites themselves appear to have clear isotopic signatures indicating their origin from a mantle source. The very high Na2O and very low MgO compositions of the Oldoinyo Lengai natrocarbonatites suggest that they are highly differentiated magmas. The common association of carbonatites with alkali-rich silicate rocks (e.g., nephelinite) suggests further that liquid immiscibility between silicate and carbonate magmas may have played a role in their origin.


Named from their type locality along the Komati River in South Africa, komatiites are ultramafic volcanic rocks, having very low silica contents (~40-45%) and very high MgO contents (~18%). These lavas are exceptional not only for their compositions, but also for their very old, restricted ages. These lavas have no modern analogs. The youngest komatiites (from Gorgona Island, Columbia) have been dated at about 90 million years; however, all other komatiites are about three billion years old or older. These ancient lava flows erupted at a time when the Earth's internal heat was much greater than today, thus generating exceptionally hot, fluid lavas with calculated eruption temperatures in excess of 1,600 degrees C (2,900 degrees F). In comparison, typical basaltic lavas erupting today have eruption temperatures of about 1,100 degrees C.

A spectauclar identifying trait of komatiites is their spinifex texture, which resembles a lacey mesh of acicular (needle-like) olivine crystals, typically surrounded by lighter-colored, interstitial minerals such as plagioclase, tremolite, and/or chlorite. The elongated nature of the komatiite olivine crystals is quite distinct when compared with the equant to tabular olivine crystals seen in most basaltic rocks.



Thin section of spinifex texture from a komatiite sampled from the Kaapvaal region of South Africa. Courtesy of Tim Grove.

Spinifex texture under cross-polarized light. Note acicular olivine crystals with third-order blue interference colors. Courtesy of Allen Glazner. 

Since we have never observed a komatiite eruption, we have had to deduce the fluid flow character and eruption style of these lavas from the properties and textures of the ancient rocks. Their high eruption temperatures, for example, are calculated from their olivine-rich compositions. The charateristic spinifex textures on the otherhand, indicate that these lavas cooled very rapidly. Combined, the high eruption temperatures and the low silica contents indicate that komatiites erupted as very fluid lavas, having exceptionally low viscosities and low aspect ratios. It is believed that these hot, fluid lavas would have been turbulent, and therefore capable of a significant amount of both mechanical and thermal erosion. Indeed, many scientists believe that the sinuous rilles exposed on the moon's surface are erosive valleys produced by komatiite lava flows, contemporaneous with the ancient eruption of similar lavas on Earth.