Lava - water interactions

The Honister slate mines in the Lake district are rightly famous as for centuries, the finely laminated Ordovician slates have provide a time honoured roofing material, and in more recent years, they have provided trinkets for the tourists at the visitor centre.  The exceptional beauty of the slate has also seen it used as a municipal decorative stone - visit St Nics shopping arcade in nearby Lancaster - you'll see some there.  The slates are the remains of ancient ash fall deposits, squashed, metamorphosed and then exhumed by the weather and the years.

The original fall deposition features can often be seen on this commemorative plaque a few hundred yards from the visitor centre.  There is clear evidence from different grain sizes and textures and things like graded bedding that there is more than one eruptive episode represented - the course to fine  upward grading indicates a degree of gravitational settling, possibly in still or only weakly flowing water. The evidence for water is also there further up the path were it is lined by large jagged boulder size chunks of a rock called Peperite.

Peperite at Honister slate Mine

Large jagged chucks of often vesicular Andesite are nested in the blue grey remains of what was once wet layered ash/mud at the bottom of a lake.  The Andesite intruded into the wet mud with the inevitable reaction. In the open air, with the lack of confining pressure, the lava acts in a phreatomagmatic manner - explosively, producing a fine ash.  Confined by water and heavy wet mud, the lava fractured on a much larger scale.

Cartoon of Peperite formation

Beyond the old miners cottages, across a bubbling mountain stream, the hills to the south offer evidence of the explosive nature of the andesite eruptions with fall and gravity flow deposits.  Bomb sags are a frequent occurrence, as are quasi-sedimentary structures like cross-bedding.  The different sizes of clast form distinct gravity driven layers which have now been heavily faulted. 

Bomb sag in a fall deposit

Airborne fall deposits and later faulting.

Miners Cottage - Honister Pass