#accretionary lapilli

[drrockclub]

Lapilli, meaning ‘little stones’ in Latin, can have diverse origins, but the example shown on a photograph is more precisely called accretionary lapilli.

Accretionary lapilli form during a volcanic eruption in the ash cloud. When big volcanic eruption occurs, a volcano will blow a huge cloud into the atmosphere. Such cloud will contain massive boulders, smaller rocks, volcanic ash and gases etc. Rocks are heavy, so they fall back down fast, however, ash and gases do not. The most abundant and important in this process gas is a water vapour.

Mix fine ash and water vapour/steam together and what you get is an airborne mud. Water vapour sticks ash particles together, so as the huge turbulent ash cloud grows and mixes, more and more ash particles come together to form little balls (lapilli) which then rain down like hail once they get too heavy.

This layer formed after such rock rain in Hawai’i Island (Big Island).

[earthstory]

Accretionary Lapilli

This photo is from an outcrop of the Erill Castell formation in the Spanish Pyrenees showing examples of accretionary lapilli. The rocks date back to 272 - 303ma to the Carboniferous - Permian boundary and are associated with the formation of the supercontinent Pangea.

The force of the colliding continents led to thickening of the continental crust and the growth of mountain ranges. This also caused the crust to thicken downwards by penetrating deep into the mantle, where increased temperatures caused the rocks to melt. This process produced large volumes of magma which began to rise towards the surface using faults and fractures as conduits. The magma pooled within chambers in the upper crust, later erupting to form volcanoes. Accretionary lapilli form during phreatomagmatic eruptions –where the magma comes into contact with either meteoric (water produced by precipitation) or seawater leading to a very explosive reaction! (the magma boils the water causing it to expand and turn into steam). This can occur due to the presence of crater lakes or from a volcano that has become shallowly submerged by seawater, and if you’re very unlucky it can be a combination of the two. This was the case in Santorini when Thera erupted, leading to the extinction of the Minoan civilisation.

The lapilli form either from the magma itself or as fragments of the original crater broken off during the eruption. As they pass through the eruption cloud the fragments become rounded by abrasion with other volcanic debris. Due to the moisture in the air the grains become encased in ash and dust to form darker outer rims as can be seen below.

I came across this outcrop on my Independent Mapping Project and thought it was pretty cool, hopefully you do too!

Watson

Photo: Watson Further Reading: The formation of super continents - http://science.nasa.gov/science-news/science-at-nasa/2000/ast06oct_1/ http://www.sciencedirect.com/science/article/pii/S0040195113001479 Accretionary Lapilli: http://www.glgarcs.net/topics/aclapilli.html

References: The Geology of Spain by The Geological Society. Accessed online:http://bit.ly/1D0pBPp Geological Magazine article: http://bit.ly/1zke616