A group of us from this Society visited this unique area on the 24th June 1984. It covers an area of 215 square kilometers created to preserve this fragile landscape. Lava extruded from fissure vents in this Rift Zone which started erupting around 15,000 years ago and last erupted about 2,000 years ago, producing the landscape we see today.

The flows of basaltic lava are classified according to their outward appearance. The most common type of flow here is pahoehoe (ropey) lava. When it was erupted it had a very low viscosity, hence all the gases escaped in a fire fountain type of eruption and the lava poured out over the landscape in flat rivers. As the surface crust cooled and hardened, it formed its characteristic textured surface of crumpled ropes, caused by the still flowing lava beneath it.

At our first stop we walked around a trail through a landscape of pahoehoe. Near the trail were some cinder crags, remainents of part of a cone wall which had broken off and been floated there on a lava flow.

The pahoehoe flowed in channels under its own hardened crust, and so when the flow ceased, the lava often drained from these channels and left empty lava tunnels behind. We found tunnels varying in size, from tiny hand sized ones to the Indian Tunnel Cave which we went in, which was over 10 metres in diameter. Roof collapse let in enough light to walk through a short section of it. Some of the lava tunnels contained stalactites, formed by the dripping of molten lava before it cooled. The floors still carried the marks of flowing lava.

Further from the vent, when the pahoehoe lava started to cool, it thickened and produced large clinker type lumps, producing what are known as aa flows, with a jagged, rubbly, irregular surface. These types of flow are also produced when the lava from the vent is more viscous and contained less gas. We saw patches of aa lava standing out in sharp contrast to the smoother surface of the pahoehoe lava.

The small cinder cones were produced when gases trapped in the lava caused the eruption of porous, vesicular lumps of lava which built up around the vents into low hills. We walked around the North Crater Trail which took us over some cinder cones. The lava was initially blackish in colour when first erupted, but the iron content of the cinders has oxidized, turning them a reddish purple colour. Some of the cinders too, had developed a thin glassy film which gave them an iridescent blue colour.

At the end of the trail we came to the Spatter Cones. With less gas than in the cinders, a pasty lava is thrown out in lumps that partially welded together and built up the spatter cones.

These spatter cones were created by eruption along the line of the fissure, marking its length into the distance, and building up the steep sided cones. The hard paths have been built around them so that visitors can walk up them to look down into the vents, without causing serious erosion. On looking down into one of the vents, we were surprised to see snow still lingering, while the sun heated the surrounding rocks to about 20 C. The insulating properties of the rock ensured no heat ever reached the depths of the vent.

At our last stop in the Park, a short walk took us to see the tree moulds.

Trees overcome by the lava flows had left casts of themselves. The flowing lava had solidified around burning trunks and later the charred remains rotted away leaving only their casts. Some of the casts were in their upright position, others had been pushed flat before leaving their mark in the cooled lava. The radio-carbon dating of charcoal remnants found in some casts has allowed some dates to be estimated for the eruptions. The age of trees which grew on top of the lava flows have established dates by which eruption had ceased.

Low annual rainfall has caused much slower weathering over this area than is the case with many volcanic sites, hence the remarkable preservation of many of the flows. Gradually though, plant life is taking over the flows, often starting growth in cracks and crevices where dust has been blown in by the wind and trapped to start the first soil off, before the breakdown of the lava itself begins. In many places flowers and shrubs were already growing. If no further volcanic activity occurs, the gradual recolonisation will continue until the area is again hidden beneath vegetation.

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