Migration of Magma and Dike Formation
Information on the movement, shape, and size of magma bodies below fissure swarms is based on both geophysical measurements during recent eruptions and structural/volcanologic studies of historic eruptions. Magma overpressures result in the beginnings of extension, which in turn lead to magma rise and eruption. Sigvaldason (1987) inferred that during the 1975–1981 Krafla eruptions, magma moved through "holding chambers" at depths of 30, 25, 8, and 4 km. After the eruptions, refilling of these intermediate chambers took ~3 weeks. From the high-level chambers—at depths of 3 to 7 km—repeated lateral magma injections into fissure swarms north and south of the Krafla central volcano initiated a rifting event. [This rifting episode was activated by the subsequent release of tensional stress that accumulated over the plate boundary during the previous 250 years (Tryggvason, 1984)]. The fissure swarm was extended by an 80- to 90-km-long section during this period; the average widening for a fissure during the accumulated 20 discrete events was 5 to 6 m (Tryggvason, 1984). Each extensional event was accompanied by subsidence near the center of the Krafla caldera, which demonstrates the link between the fissure swarms and the high-level chamber below the central volcano.
Tryggvason (1984) determined that the accumulated area of fissure widening during the 1975–1981 Krafla event was ~377,000 m2 . Based on observed ground deformation, he suggested that most of the magma was injected into vertical fissures rather than into sills. Using gravity and elevation measurements, he determined that the volume of magma leaving the reservoir was ~1.75 times that of caldera subsidence. The dike volume (V) is equated with that of magma leaving the reservoir, minus the volume of material erupted. The estimated dike height (h) = ~1.75 V/A, where A = area of horizontal extension by the dike. For the best recorded events at Krafla, calculated dike heights are 2.4 to 2.8 km. The total volume of magma that flowed out of the reservoir into fissure swarms during this episode at Krafla is estimated at 1.08 km3 , of which 1.03 km3 remains in the dikes to become a renewed heat source for the associated geothermal fields. The measured volume of lavas erupted is 0.2 km3 , which is four times greater than the volume predicted. Tryggvason concludes that perhaps the volume estimates of magma leaving the caldera are too low.
Gudmundsson (1986) used his work on the Reykjanes Peninsula of Iceland to develop a method for estimating the volume of magma reservoirs below fissure swarms, as is depicted in Fig. 6.15; his method requires the measurements and assumptions listed here.
· The maximum length for a magma reservoir is taken to be equal to the length of vents in the fissure swarm.
· The width of the reservoir is estimated to be 1.72 times the width of the vents in the fissure swarm; this ratio is based on observations of older dike swarms that are exposed in outcrop.
· A reservoir is taken as an ellipsoid, the volume of which is calculated as V = 4/3 p ah bh ch and the area of which is calculated as A = p ah bh ,
where ah , bh , and ch = the half-width, half-length, and half-thickness, respectively, of the ellipsoid. The half-thickness of the reservoir is calculated by ch = 0.75 V/A, where V = total volume of the magma (both erupted and in dikes).
· The average volume of individual fissure lava flows on the peninsula is 0.11 km3 . Walker (1959) estimated the average volume of a corresponding feeder dike by using the average length of the volcanic fissures (2.2 km), a crustal thickness of 8 km, and an average dike width of 4 m. The estimated volume of a feeder dike here is 0.07 km3 and an average value for ch is 1.5 km (2ch = 3 km). It is likely that only the uppermost 3 km of the reservoir participates in an eruption.
The volume of feeder dikes (~0.07 km3 ) is small, but as a result of intrusions and eruptions every 10 years, the active fissure swarms contain excellent heat sources. Bodvarsson (1976) calculated that in Iceland, heat reaches the surface by conduction (~50%), as erupted magma (~30%), and as thermal waters (~20%).
South of Krafla caldera is Askja-Öskuvatn caldera; Sigurdsson and Sparks' (1978) documentation of the 1874–1875 eruption provides another view of fissuring and dike injection along the fissure swarms. The Askja central volcano straddles a 75-km-long fissure swarm. Magnitude 6 or 7 earthquakes in 1872 marked a new phase of rifting, and by the fall of 1874 the fissure swarm was rifted along a 70-km segment. A graben 1 to 2 km wide was formed, bounded by normal faults with throws of 40 to 60 m south of the central volcano and 10 m north of it. In early January 1975, a major injection of magma into a high-level reservoir was followed by phreatomagmatic eruptions of rhyolitic ash. Caldera inflation was relieved by periodic injections of magma out into the fissure swarms. At the surface, the central fissure is flanked by en echelon spatter ramparts. The Sveinagja lava field covers 30 km2 ; it is
Fig. 6.15
(a) Fractures measured in the western part of the
Thingvellir swarm in Iceland. (b) The width and
throw of three fractures from the Thingvellir
swarm were measured along
the strike of the fissures.
(Adapted from Gudmundsson, 1987.)
located 40 to 70 km north of the Askja-Öskjuvatn caldera and consists of 0.3 km3 of mostly tholeiitic aa and pahoehoe lavas. The estimated volume of intruded magma was 1.5 km3 , which could be accounted for by a single 100-km-long, 5-km-deep, 3-m-wide dike. Fissure widths range from 2.5 to 4 m. In the northern part of the lava field, activity was centralized at an offset in the rift and formed a line of cinder cones.
Brown et al . (1987) used gravity surveys to document the presence of a 20-mGal, north-south-trending anomaly that may correspond with a dense dike swarm below the Öskuvatn-Askja caldera. They also noted that caldera fill is most likely thin and that the caldera's collapse was primarily related to eruptions out along the fissure swarms.