Breaking Breccia: Investigating Hydrothermal Eruptions at Rotokawa

By: Dr. Geoff Lerner

Shane Cronin and Geoff Lerner have been working on learning more about hydrothermal eruptions in the Taupo Volcanic Zone. Over numerous trips to the Rotokawa geothermal field and other nearby geothermal areas, they have observed and sampled the sequence of hydrothermal breccias, looking to recreate the history of steam-driven eruptions in the area. By looking at characteristics of the breccias like grain size, sorting, and composition, they can correlate different breccia outcrops in order to learn about the vent locations of these eruptions and determine the size of these eruptions.

In June and July, Geoff and Shane traveled to Germany to work with colleagues at Ludwig Maximilians University–Munich (including former Auckland VGP postdoc Cristian Montanaro) running experiments on breccia material to determine the conditions under which hydrothermal eruptions take place. In the experiments, sample material from the Rotokawa field is placed in a special chamber (“autoclave”) which can be set to a variety of temperatures and pressures to simulate pre-eruptive conditions. When the chamber is depressurized, a lab-created eruption takes place. This eruption is measured for a number of characteristics and filmed with high-speed cameras so that visual analysis of the eruption in slow motion can be conducted.

View across the steamfield to Mt Tauhara

 

A small eruption crater in the Rotokawa steamfield

 

The “Twin Towers”, a bubbling geothermal feature in the steamfield

 

A section of hydrothermal breccias deposited by eruptions at Rotokawa

 

Tephra deposits sometimes serve as important marker beds between breccias

 

Site A-Hole, a hydrothermal feature near the steamfield

 

Cores from Rotokawa breccia material prepared for experiments in Munich

 

Preparing the Fragmentation Lab for an experiment

 

Sample material in the chamber prior to an experiment with the high speed camera looking on

 

Video of a laboratory eruption of breccia material

 

The aftermath of a lab eruption

 

 

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