Supplemental Material; Oceanic Islands

Electronic Supplementary Material

Calculation of volumes and proportions.

Mount Murphy

Justification of conical shape. Mt. Murphy is deeply dissected by glacial erosion (Fig. 2b), which somewhat obscures its’ original constructional morphology. An original conical shape is most easily visualized by observing the five interfluvial ridges, identified by Sechrist Peak, Hawkins

Peak, Boyd Head, Eisberg Head, and Kay Peak (Fig. 2c). Each rises in elevation to the summit of

Mt. Murphy. The points where each ridge meets ice level defines a somewhat eccentric circular plan view of the originally conical volcano. To compensate for the eccentricity, two radii, measured along A-A’ and B-B’ (Fig. 4) were used in volume calculations.

Constraints and assumptions regarding structure. Pre-Cenozoic basement is exposed in the northwest segment of Mt. Murphy, from ice level to ~2000m, where it appears to form a horst block that was overridden by a mantle of volcanic rock that has subsequently been largely removed by erosion (Smellie 2001). Proceeding counterclockwise around the west flank of the mountain (Fig. 2c), the basement/volcanic contact is exposed at ~500 m elevation, and then lies below ice level at the base of the southwest (Sechrist Peak) ridge, where the volcanic section is most completely exposed and best studied. Thicknesses measured and lithologies analyzed at this locality are the data used for volume calculations (Table 2). These include a 1400m section of mildly alkaline (hy-ol normative) basalt, overlain by an 800m summit section of trachyte and benmoreite flows (LeMasurier et al. 1990a, 1994). The lowermost 546 m of the summit section has been sampled (Table 3; Fig. 4), but the remainder is snow-covered except for inaccessible exposures shown at the 2600m level on the Mt. Murphy (1977) topographic map. The unvisited

1 upper 257m is assumed to be felsic, consistent with the structure of the other MBL volcanoes

(LeMasurier and Thomson 1990).

Volcanic rocks shown on the Eisberg Head and Boyd Head ridges have been visited and mapped by Smellie (2001), and these results are incorporated in Fig. 2c. Hawkins Peak is completely snow-covered and has not been visited, but the morphology of this ridge is similar to that of Boyd Head. For purposes of volume calculation it is assumed to be volcanic. It is also assumed to be basaltic, because all of the exposed volcanic rock between 400m and 1900m that has been visited is basalt (LeMasurier et al. 1994; Smellie 2001).

Constraints, assumptions, and corrections involved in volume calculations. Volumes (Table 2) were calculated from the formula for a cone (V=1/3πr2h) with surface slopes of 6.5° - 8.0°. Ice level at the base of Mt. Murphy is 800m on the up-glacier (south) side, 200m on the down- glacier side, and the lowermost basalt/basement contact is at ~500m. Total volcano volume was calculated using 500m as the base elevation, and height h =2.203km (2703 m summit minus 500 m base elevation). Uncertainty in estimating “r” is a major source of error, therefore two cases were calculated, using measured radii of 16km (Fig. 4, A-A') and 20km (Fig. 4, B-B').

To correct for the volume of the basement horst, a 108° segment of the cone (see Fig.

2c), equivalent to 30% of its total volume, was subtracted from the raw total volume figure. The result (Table 2) is probably a conservative figure for the total volume of volcanic rock, because the base of the southwest ridge volcanic section lies below ice level (i.e. below 400m) and cannot be included, nor can the mantle of basalt that originally rested on the horst block.

The volume of felsic rock (Vss) was calculated as a summit cone with h = 0.803km (2703

– 1900m), and radii of 5.8 km and 7.3 km. The basalt/felsic contact is exposed at the 1900m

2 level (Fig. 2c, sample locality 35), where it dips 10° SW, concordant with the constructional slope. This suggests that the contact dips quaquaversally around the summit. However, other exposures of the contact are in a cliff face at the headwall of a cirque, where they could only be observed in a helicopter fly-by. From this perspective the contact appears to be horizontal. The actual contact may be “tent-shaped,” but in the absence of constraints other than those above, the wavy line in Fig. 2a seems reasonable as a gross approximation, and it facilitates volume calculations. Again, in the absence of other constraints, the base radii of the summit section

were calculated trigonometrically, as in Fig. 2a, to be consistent with base radii (r 1) of 16 km

and 20 km, a height (h1) = 2.203, and a basalt/felsic contact at 1900m. The resulting figures were then checked against the map view (Fig. 2c) for consistency. Final results are shown in

Table 2.

Mt. Murphy is much more deeply dissected than other MBL volcanoes, and a summit caldera, obvious in the others, cannot be recognized. This implies that the estimates of felsic

volume (Vss) and percent felsics may be biased by summit degradation. By assuming that the original form of Mt. Murphy was similar to that of Mt. Takahe, which is undissected (Fig. 7),

Andrews and LeMasurier (1973) estimated the height and average radius of the Mt. Murphy

“fresh cone” at 2.57 km and 17.5 km, respectively. The total volume thus calculated is 824km3.

The volume of the summit section was calculated assuming the basalt/trachyte contact at

1900m. The raw total volume was then reduced by 30%, as described above, yielding 577 km 3 total volume and 13.4% felsics (Table 2).

3 Toney Mountain

Toney Mountain is a linear volcanic massif that consists of a plateau of hawaiite flows (Fig. 3a, foreground), adjacent to and presumably surmounted by a felsic cone that rises from ice level (1200m) to the 3595 summit (Fig. 3a, background). The upper 200m of the hawaiite section is well exposed up to the 1800m summit of the plateau in the foreground of Fig. 3a (LeMasurier et al. 1990b). The hawaiite/comendite contact is not exposed, and may be downfaulted beneath the felsic cone. The contact of the hawaiite section with crystalline basement, at ~3000 below sea level, is “unambiguous”

(Bentley and Clough 1972), and suggests that the hawaiite section is ~5 km thick. It is assumed that a section this thick continues beneath the felsic cone, and a suggested structure is shown in Fig. 3c.

The total volume (Vt) shown in Table 2 was calculated as a rectangular block of hawaiite underlying just the exposed portion of the massif, and extending down to -3 km, plus the volume of the felsic cone. The hawaiite volume was calculated as two blocks, (1) a 50km x 17km x 4.2km block (see Fig.

3b) that assumes hawaiite lies just beneath the felsic rock at ice level (1200m), therefore 3km below sea level, plus 1.2km above equals 4.2km, for a volume of 3570km 3. (2) The Cox Bluff plateau, 12km x 6km x

0.6km (1800-1200m) yields a 43.2km3 volume, which, added to the sub-ice level block, yields a total hawaiite volume of 3613km3. This approach avoids attempting to estimate the lateral extent of the lower slopes of the probable coalescing hawaiite shield volcanoes that underlie Toney Mountain proper.

Felsic rock volume was also computed as two blocks. (1) A more or less rectangular block between 1200m and 2200m calculated as 38km x 13km x 1km = 494km 3, plus (2) a summit cone with h =

1.4km (3600 – 2200m) x r = 5km yields 36.65km3, for a total felsic volume of 530.7km3. This yields 14.7% felsics (530.7/3613) - obviously a very gross approximation.

4 Crary Mountains

The Crary Mountains include the conspicuous Mt. Frakes and Mt. Steere phonolitic cones, and the underlying, predominantly basaltic, sequence that extends from Mt. Rees southeastward beneath

Mounts Steere and Frakes, and then discontinuously to Boyd Ridge (Fig. 4) (LeMasurier et al. 1990c;

Panter et al. 2000). The base of the volcanic section lies below ice level. The accessible east-facing cliff exposures beneath Mt. Steere, between ice level (1600 m) and ~2400 m, consist of alternating basanite and trachyte lavas and hydrovolcanic deposits, with more basanite toward the base, and more trachyte toward the top of this interval (Wilch and McIntosh 2002). Isolated exposures of trachyte on the west flank of Mt. Steere, and of phonolite on the south flank of Mt. Frakes, extend down to the 2400m level

(Fig. 4b) (LeMasurier et al. 1990c). It seems reasonably clear that the base of this volcanic complex is basanite, and the summit sections of Mt. Frakes and Mt. Steere are felsic, but the mafic – felsic alternations beneath the summit cones, and the unknown volume below ice level, allow only a gross approximation of what is most likely a maximum value for felsic proportions, assuming a sub-ice shield volcano profile.

For this exercise the basalt/felsic contact was chosen at the 2400m level, the horizon of the lowest good outcrops of felsic rock. Total exposed volume was calculated as two rectilinear segments, plus two cones, plus two back-to-back wedges for Boyd Ridge, as shown in Fig. 4b. The Mt. Rees block was calculated as: 10km x 10km x 0.9km high = 90km3. The Steere-Frakes block as: 22km x 16km x 0.8km high (2400m-1600m) = 281.6km3. The Steere and Frakes volcanoes were each calculated as cones with r

= 6km and h = 1.2km (3600m-2400m) yielding V=45.2km3 for each. Boyd Ridge was calculated as back- to-back wedges, each 10km long, along the ridge top, 0.8km high, and each with a 3.5km base width north and south of the ridge line. Together the two wedges equal a rectilinear block 10 x 3.5 x 0.8 =

28km3. The sum of the above yields a total volume of 490km 3. Total felsic volume equals the sum of the

Frakes and Steere cones, which is 90.4km3. This divided by 490km3 yields 18.4% felsics.

Computed Base Elevations

Mt. Bursey is a late Miocene trachyte/phonolite/pantellerite shield volcano. The exposed volume of Mt.

3 Bursey (shown as the volume of the summit section, Vss, in Table 2) has been estimated at 290 km

(LeMasurier 1990b) In Table 2, the 10% case yields a base elevation of -1644 m, and total volume of

2900 km3. The 15% felsics case yields a base elevation of -1083 m and total volume of 1933 km3.

Mt. Takahe is a perfectly conical, undissected late Quaternary trachyte/pantellerite shield

3 volcano (Fig. 7). Its’ exposed volume (Vss) has been estimated at 780 km (LeMasurier and Rex 1990b) and 897 km3 (Andrews and LeMasurier 1973). For Table 2 this volume has been recalculated to 552 km 3 using the most recent maps to re-estimate height and radius. Following the example in Table 1, if felsics are 10% of total volume the base elevation is -2030 m, and total volume would be 5520 km 3. For the

15% case, base elevation would be -1340 m and total volume 3680 km3.

Mt. Sidley is an early Pliocene (4-5 Ma) phonolitic shield volcano, the highest volcano in

Antarctica. Its breached caldera wall (Fig. 8) exposes a 1200 m section that provides the best evidence in

MBL that summit sections are composed entirely of felsic and intermediate rocks (LeMasurier 1990c;

Panter et al., 1994). It lies ~250 km from the coast where ice level is 2200 m asl, and no basalt is exposed around its base. The total volume estimate of 250 km3 (LeMasurier 1990c), recalculated to 185 km3 in

Table 2, is therefore a minimum estimate for the volume of the summit section. If a basalt section underlies Mt. Sidley near ice level, the 10% case yields +381m for a base elevation and total volume of

1850 km3; the 15% case yields +851 m base elevation and total volume of 1233 km3.

Supplemental References

LeMasurier WE (1990b) Mt. Bursey. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU. Antarct. Res Ser 48: 221-224.

6 LeMasurier WE (1990c) Mt. Sidley. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic plate and Southern Oceans. AGU. Antarct. Res. Ser 48: 203-207.

LeMasurier WE, Rex DC (1990a) Mount Siple. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU. Antarct Res Ser 48: 185-188.

LeMasurier WE, Rex DC (1990b) Mt. Takahe. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU. Antarct. Res. Ser 48: 169-174.

LeMasurier WE, Kawachi Y, Rex DC (1990a) Mount Murphy. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU. Antarct Res Ser 48: 164-168.

LeMasurier WE, Kawachi Y, Rex DC (1990b) Toney Mountain. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU, Antarct Res Ser 48: 175-179.

LeMasurier WE, Kawachi Y, Rex DC (1990c) Crary Mountains. In: LeMasurier WE, Thomson JW (eds) Volcanoes of the Antarctic Plate and Southern Oceans. AGU Antarct Res Ser 48: 180-184.

Mount Murphy Antarctica (1977) 1:250,000 Reconnaissance Series, Department of the Interior, Geological Survey, Reston, Virginia 22092

Panter KS, McIntosh WC, Smellie JL (1994) Volcanic history of Mount Sidley, a major alkaline volcano in Marie Byrd Land, Antarctica. Bull Volcanol 56: 361-376.

Panter KS, Hart SR, Kyle P, Blusztanjn J, Wilch T (2000) Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica. Chem Geol 165: 215-241.