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The potassium-argon (K–Ar) dating method is often used to date volcanic rocks (and by extension, nearby fossils). The darker recent lavas were clearly visible and each one easily identified (with the aid of maps) on the northwestern slopes against the lighter-coloured older portions of the cone (Figures 4 and 7).

In using this method, it is assumed that there was no daughter radiogenic argon ( For volcanic rocks which cool from molten lavas, this would seem to be a reasonable assumption. Inset: Andesite of the June 30, 1954 flow, Mt Ngauruhoe, seen at 60 times magnification under a geological microscope. All flows were typically made up of jumbled blocks of congealed lava, resulting in rough, jagged, clinkery surfaces (Figure 8).

Blocks weighing up to 1,000 tonnes were hurled 100 m (330 feet).

However, the most violent explosions occurred on 19 February 1975, accompanied by what eye-witnesses described as atmospheric shock waves.

These flows are still distinguishable today on the northwestern and western slopes of Ngauruhoe (Figure 4).

However, Mt Ngauruhoe is an imposing, almost perfect cone that rises more than 1,000 metres (3,300 feet) above the surrounding landscape to an elevation of 2,291 m (7,500 feet) above sea level (Figure 3).

Eruptions from a central 400 m (1,300 foot) wide crater have constructed the cone’s steep (33°) outer slopes.

Because argon is a gas, it should escape to the atmosphere due to the intense heat of the lavas. The samples were sent progressively in batches to Geochron Laboratories in Cambridge, Boston (USA), for whole-rock potassium-argon (K–Ar) dating—first a piece of one sample from each flow, then a piece of the second sample from each flow after the first set of results was received, and finally, a piece of the third sample from the 30 June 1954 flow.

The 18 August flow was more than 18 m (55 feet) thick and still warm almost a year after congealing.

Explosions of ash completed this long eruptive period. Cannon-like, highly explosive eruptions in January and March 1974 threw out large quantities of ash as a column into the atmosphere, and as avalanches flowing down the cone’s sides.

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