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Figure 2. A: Histogram
<br />Volume
<br />(km')
<br />showing number (No.)
<br />N 21
<br />00,L
<br />of tephra ages, prepared
<br />m a
<br />from Tables DR2 and DR3
<br />0o
<br />(see footnote 1). B: Histo-
<br />0.3
<br />gram showing number of
<br />Explosive
<br />different dated lava flows
<br />11
<br />per century in each erup-
<br />260
<br />tive period. Numbers indf-
<br />o
<br />tate how many different
<br />Z
<br />flows were dated per pe-
<br />5.5
<br />riod. From 1800 CE, only
<br />Effusive
<br />flows outside the caldera
<br />are counted, because in-
<br />ZLC
<br />tracaldera flows are not
<br />E
<br />recognizable for earlier
<br />=i
<br />periods. Analytical data
<br />j
<br />and original figure from
<br />which histogram was pre-
<br />pared are in Table DRi -sco 0 500 '000 1500 2000
<br />and Figure DR11; entire Calendar year (BCE negative)
<br />calendar ranges were
<br />used except where constrained by stratigraphy. C: Histogram of esti-
<br />mated volume (not corrected for pore space) erupted subaerially dur-
<br />ing each eruptive period. Gray is dominantly effusive period. Black is
<br />dominantly explosive period. UA—Uwekahuna Ash; KT—Keanakako'i
<br />Tephra Member.
<br />tivity. Instead, almost all summit eruptions were explosive: sporadic, vio-
<br />lent, and brief. In contrast. most eruptions during the intervening periods
<br />were frequent, effusive, and sometimes lasted for decades. This suggests
<br />a cyclicity in summit activity. shifting from mostly effusive to mostly ex-
<br />plosive and back again.
<br />DISTRIBUTION OF LAVA FLOW AGES
<br />Did the rest of the volcano, beyond the extent of marker ash beds for
<br />the two dominantly explosive periods, behave similarly? To address this
<br />question, we compiled and calendar -calibrated all 93 known r'C ages of
<br />10auca lava flows younger tJhan ca. 500 BCE (only a few ages are older
<br />than that -.Table DR 1.1. Figure 2B and Figure DR1 show the distribution of
<br />the calendar -calibrated flow ages with time for the past 2500 yr, with the
<br />two periods of major tephra production indicated. Lava flow ages cluster
<br />in periods between explosive eruptions. This is best shown for the past
<br />1000 yr. Numerous ages plot in the 100G-1500 CE time interval, when
<br />the present summit shield was under construction (Holcomb, 1987; Neal
<br />and Lockwood. 2003); few are within die 1500-1800 CE period, when
<br />the Kea nakako'i tephra was deposited, and numerous lava flows have
<br />been observed to form since 1800 (1823 CE is the actual age of the oldest
<br />known post-Kcanakiko'i flow). The number of flow ages is small before
<br />1000 CE, but there appears to be an increase in the period 200-500 BCE
<br />(Fig. 2A), from 0.8 flows/100 yr to 3.3 flows/100 yr, bracketing the 200
<br />BCE -1000 CE age of the Uwekahuna Ash.
<br />Three factors could weaken this pattern. The spatial distribution of
<br />lava flow ages is uneven (Fig. 1). The lower east rift zone is underrepre-
<br />sented. For example, flows from Heiheiahulu (Holcomb, 1987) and the
<br />so-called 1790 flow (Moore and Trusdell, 1991), both ascribed to the 18i1
<br />century, are not dated.
<br />Second, young flows cover old ones, so die temporal record becomes
<br />obscured with age. Few ages, however, plot in the 1200 -yr -long period
<br />of tephra production, in contrast to die many flow ages in the following,
<br />much briefer, 500 -yr -long period.
<br />Third, flows along die Puna Ridge, the 75 -km -long submarine ex-
<br />tension of the east rift zone, etre not dated well enough for our analysis
<br />(Smith ct al., 2002). Palagonite rind thicknesses suggest ages for dredged
<br />samples of 700-24.000 yr, mostly 2000-7000 yr (Clague et al., 1995).
<br />This suggests relatively little eruptive activity during the past 2500 yr. The
<br />large flows at the base of the ridge are not young, based on sediment cover
<br />(Clague et al., 1995). We discuss only the subaerial edifice in this paper,
<br />but suspect that our conclusions apply to the Puna Ridge.
<br />Acknowledging these caveats, we think that the clear pattern for the
<br />summit area holds for the entire subaerial edifice. We interpret th-_ sub-
<br />aerial volcano to have undergone alternating periods of mostly explosive
<br />and mostly effusive eruptions for the past 2500 yr. Successive periods con-
<br />stitute explosive -effusive cycles of varying duration.
<br />MAGMA SUPPLY DROPS DURING PERIODS OF MAINLY
<br />EXPLOSIVE ACTIVITY
<br />How do eruptive volumes and rates of magma supply compare be-
<br />tween the explosive and effusive periods? The volume of magma erupted
<br />during periods of dominantly explosive activity is far less than that dur-
<br />ing effusive periods (Fig. 2C), and the calculated magma supply rate is
<br />correspondingly lower, only 1%-2% of the effusive rate (Table 1). Our
<br />estimates of flow volumes (Table 1; Fig. 2C) are compromised by variable
<br />flow thickness and coverage by later flows. A simple comparison of tephra
<br />and flow thickness at the summit area, however, illustrates the disparity
<br />between effusive and explosive volumes.
<br />TABLE 1. ESTIMATED VOLUME OF LAVA ERUPTED ON SUBAERIAL KTLAUEA
<br />DURING DOMINANTLY EFFUSIVE AND DOMINANTLY EXPLOSIVE PERIODS
<br />Calendar age
<br />range
<br />Volume
<br />(km')
<br />Magma supply rate
<br />(km°/yr•)
<br />Dominant style
<br />500r200 BCE
<br />>0.6
<br />not calculated
<br />Effusive
<br />200 BCE -1000 CE
<br />0.3
<br />2.5 x 10-`
<br />Explosive
<br />1000-1500 CE
<br />11
<br />2.2 x 10-2
<br />Effusive
<br />1500-1800 CE
<br />0.15
<br />5 x 10-'
<br />Explosive
<br />1600 -present
<br />5.5
<br />2.6 x 10-1
<br />Effusive
<br />Note: Effusive volumes estimated using mapped areas of flows and areas
<br />projected beneath younger flows, as shown on geologic maps (Wolfe and Morns,
<br />1996; Neal and Lockwood, 2003), assuming an age consistent with 14C data
<br />and unit label; thickness was estimated from field observations and topographic
<br />gradient. Explosive volumes were estimated from area and average thickness of
<br />juvenile tephra. Volumes were not adjusted for vesicularity (lava flows) or pore
<br />space (tephra).
<br />'Average supply rate to ground surface for entire period. Not calculated for earli-
<br />est period, which began before 500 BCE and so is incomplete.
<br />The 140 -m -high wall of Kilauea Caldera is made almost entirely of
<br />flows erupted between )000 and 1500 CE (Neal and Lockwood. 2003),
<br />when the Observatory shield was built (Holcomb, 1987); its total thick-
<br />ness is more, because the base of the shield is covered by caldera fill. In
<br />contrast, the maximum exposed thickness of the Keanakako'i tephra is
<br />only -11 m (McPhie et al., 1990; Swanson et al., 2012x). The flow thick-
<br />ness is several meters thick 5 km southwest (downwind) of the summit,
<br />and the tephra is only several centimeters.
<br />A similar comparison can be made for the Uwekahuna Ash on
<br />Kilauea's south flank. It is at most a few tens of centimeters thick, thin-
<br />ning to only a few centimeters at the coastline (Fiske et al.. 2009), but the
<br />overlying and underlying flows are each at least several meters thick.
<br />The striking difference in erupted volume between periods domi-
<br />nated by effusive and explosive activity must reflect a major disruption
<br />to the supply system that lasts for centuries. The disruption could take
<br />place anywhere between the point of melt accumulation in the mantle and
<br />the shallow storage system beneath Krlauea's summit. Perhaps increased
<br />magma supply to Mauna Loa volcano causes a drop in supply to Kilauea
<br />(Gonneimann et al., 2012). Once magma enters the Kilauea plume, it
<br />could be diverted away from the shallow reservoir, perhaps as intrusions
<br />into the crust or lower shield (Lin et al., 2014). A subhorizontal mantle
<br />pathway of magma transport at -30 km depth, interpreted by Wright and
<br />Klein (2006; see also Wolfe et al., 2003), might be a zone within which
<br />magma could stall or. -be divertedil Magma probably did not bypass the
<br />summit reservoir system and immediately erupt on the Puna Ridge, be -
<br />632 www.gsapubs.org I July 2014 1 GEOLOGY
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