Egmont, New Zealand, North Island

Eruptive style and history

Egmont is both explosive and variable. Both lava flows and pumiceous tephra can be found. Numerous eruptions have occured in the past two thousand years with possible eruptions in the 17th and 18th century. Egmont is the newest member of a chain of four volcanoes forming the Taranaki group. The setting of the Taranaki group is unusual. They occur in a deep (6-7 km?) sedimentary basin which has been well studied by seismic reflection and borehole data. This eliminates uncertainties associated with volcanoes intruded into previously magmatized regions.

Chemistry and proposed magma system

There is significant variety in the eruptive products of Egmont as evidenced by the different eruptive styles. The pumiceous tephra is typically dacite in composition while the lava flows are more basaltic-andesite. Magmas contain xenocrysts from a melt zone near the base of the crust.

The most primitive basalts are high-Mg. These pond at the base of the crust and fractionate ol-clpx-etc. to become high-Al basalts (calc-alkaline?) Much of this magma completely solidifies at this level, crystallizing amphibole and underplating the crust with mafic material. Rising magmas entrain wall rock xenoliths. Some of the entrained material is amphibole. Much of this amphibole decompressively melts leaving only (resorbed?) remnants.

High-level magma chambers are responsible for changing the composition of melt from basaltic andesite to dacite. At this level, hydrous melt intersects the amphibole stability curve and fractionates amphibole. The Fantham's peak lavas are basalts while the summit vents erupt basaltic-andesites to dacites. The suggestion is that denser less evolved magmas do not reach the summit peak.

Local Seismicity

Cavill et al. 1996 discuss 15 months of data from a 5 station array installed in 1993 to establish baseline seismicity. They recorded 133 events of mag. 1.5-3.4. Five events exceed mag. 3. No seismic activity could be linked to volcanic processes. It is most likely due to a nearby fault.
Overall seismicity is quite low for a passive study.

Postscript version of plot.

Notes and Questions

• Gravity survey claims mountain is very accessible. Cavill et al. say vehicular travel is very limited.
• Gravity survey 1997 gravity survey included 450+ stations. Bouguer anomalies exceed 350 g.u. Presumably this is due to the contrast between the sedimentary basin and the injected material.
• Does the vent height/composition correlation imply a connection between vent height and the presence of a magma chamber?
• "Simple" stratigraphy make this a good location for small scale active studies

Selected references

Thrasher and Cahill.1990. Subsurface maps of the Taranaki basin region, NZ. NV Geol. Surv. Rep. G142

Cavill et al. 1996. Results of New Seismic Monitoring Network at Egmont Volcano, NZ. Tectonic and hazard implications. NZJ Geol. Geophys.

Locke and Cassidy. 1996. Egmont Volcano, New Zealand: 3D structure and its Implications for evolution. JVGR 76 (1997) 149-161.

Stewart et al., Evolution of High-K arc magma, Egmont Volcano, Taranaki, NZ: Evidence from Mineral Chemistry. JVGR 74 (1996) 275-295.