Scoria Cones and Tuff Rings
One of the most common subaerial volcanic landforms on Earth is the scoria or cinder cone. Scoria cones are usually formed during single eruptions of basaltic or basaltic-andesitic magmas. Construction of a scoria cone commonly follows the opening of a narrow fissure, a short period of lava fountaining, and (sometimes) a lava flow (Foshag and Gonzalez, 1955; Budnikov et al ., 1975). Soon after the eruption begins, effusive activity is concentrated at one or more points along the fissure. Strombolian or Vulcanian eruptions, consisting of the explosive ejection of bombs, blocks, and ash, continue intermittently for weeks to years. During this period, one or more cones develop through a process of ballistic
Fig. 6.1
(a). Schematic cross-section of the lithosphere beneath kilauea Volcano. The magma ascent
funnel is shown as a region of extensional fractures and magma batches migrating from the
asthenosphere. Variations in stress orientation from the lithosphere and volcanic edifice are shown.
(Adapted from Ryan, 1987a, b.)
(b) Simplified block diagram of the east rift zone of Kilauea Volcano. In one interpretation,
stresses that cause rifting are related to a gravitational collapse of the island's flanks, which allows
intrusion and further widening by dike complexes (A). Other interpretations (B) show the rise of
magma toward the summit of the shield; this movement, cutting across the listric faults, may have
only a minor effect on dike distribution.
(Adapted from Decker, 1987.)
deposition and subsequent slumping when scoria deposits that form the cone exceed the angle of repose, as is shown in Fig. 6.2(a) (McGetchin et al ., 1974). The cone consists of unconsolidated and sometimes welded scoria, blocks, and bombs that make up thick beds dipping outward from the vent at the angle of repose [Fig. 6.2 (b)]. Craters in scoria cones are occasionally modified after they are filled with lavas; in some cases, when leaks develop in the crater walls, dikes and sills penetrate the surrounding scoria deposits (Gutmann, 1979). Lava flows can also erupt from the cone flanks or overflow the crater rim.
Considerable heat is released during the eruptions that produce a cinder cone and its associated lavas. Scandone (1979) estimated that during the 8-year-long eruption of Parícutin in Mexico, 1309 × 106 m3 of tephra and 700 × 106 m3 of lava were erupted; the associated thermal energy was 2.75 × 1018 J (0.66 × 1018 calories). This is a substantial amount of heat, but it was deposited above the ground surface and therefore was lost through radiation and convection of heated rain water in the cone.
If rising magma intersects an aquifer or shallow surface water, the resulting volcanic structure will be a tuff cone or tuff ring rather than a cinder cone (Heiken, 1971). Tuff rings are broad, low rings composed of well-bedded, fine-grained tuffs in plane beds and cross-bedded surge deposits [Fig. 6.2 (c)]; poorly exposed tuff ring deposits are often mistaken for fine-grained sedimentary rocks (Fisher and Schmincke, 1984). During the formation of tuff rings by phreatomagmatic or hydrovolcanic eruptions, pyroclastic debris are deposited at relatively cool (~30°C) temperatures because most of the heat is lost in the steam that drives these explosive eruptions.
Most cinder cone fields were constructed by many small-volume eruptions from widely spaced vents, and they do not provide the thermal mass required for a high-temperature geothermal resource. In such areas,
Fig. 6.2
(a). In this diagram of the four major stages in the development of a scoria cone, the numbers refer to
deposits from corresponding eruption stages; only one-half of the cone is shown. During stage 1,
a low-rimmed pyroclastic ring is composed of scoria-fall beds and ballistically emplaced blocks and
bombs. During stage 2, the ring reaches the angle of repose for unconsolidated clastic material;
slumping and avalanching of scoria begins; and the outer slopes of the cone are covered with talus.
In stage 3, the original rim of the cone is destroyed by inward migration of the talus pile. During
stage 4, the talus apron reaches the ballistic limit of the ejecta. The many sizes and shapes of scoria
cones depend on the stage reached as the eruption ended. It is also possible for the crater to fill with
a lava lake and for lava to then spill out of the crater and stabilize the cone slopes.
(Adapted from McGetchin et al ., 1974.)
Fig. 6.2
(b) Cross section of the Rothenberg scoria cone in Germany. Scoria cones are the Earth's
most common volcanic landform above sea level. (Adapted from Houghton and Schmincke, 1989.)
(c) Cross section of a tuff ring formed by the phreatomagmatic eruption of a basaltic magma.
1 = country rock; 2 = explosion breccia; and 3 = bedded tuffs of the tuff ring. Tm = maximum
thickness of the tuff ring at crater rim; Wr = rim-to-rim diameter; We = crater diameter
(excavation width); qm = maximum outward dip of tuff beds; Lr = tephra runout distance; and
De = excavation depth. Tuffs characteristic of tuff rings and tuff cones are generally fine
grained; most of the ash and lapilli were deposited by pyroclastic (base) surges and not by
fallout. Magma volumes and compositions for scoria cones and tuff rings can be identical.
The differences are caused by the interaction of magma and water, which results in the much
more violent phreatomagmatic eruptions that form tuff rings.
(Adapted from Wohletz and Sheridan, 1983.)
the magmas have risen to the surface from the mantle without forming shallow crustal magma bodies. Crater Flat, in south-central Nevada, has 15 small basaltic centers that were erupted during three phases over a period of 3.7 m.y. (Vaniman and Crowe, 1981). The volumes are small (0.3 to 1.5 km3 for each center), as is the cone density (spacing)—10-3 to 10-4 /km2 . The San Francisco volcanic field is larger: 5000 km2 is covered by a few silicic volcanoes and hundreds of scoria cones. The magma sources for the scoria cones and associated lavas there are deep (15 to 40 km), and volumes of individual eruptions are small (Moore et al ., 1976).