Pumice – Floating rocks

The mystery of Pumice – floating rocks!

Most of us know rocks as being hard heavy objects that easily sink in water. The closest thing we see to rocks on top of water might be when we skip stones on a pond.  But one rock, pumice, is so full of gas bubbles that it easily floats.  And lately its been something I have been seeing amongst the seaweed and driftwood on my local beach (or at least my holiday beach in NSW, Australia).


White and light – pumice!

Pumice is a type of rock formed from the rapid cooling of volcanic lava that contains huge amounts of gas and steam.   The gas and steam get ‘frozen’ into the cooled lava forming a lava froth.  Under a microscope, pumice can be seen to be a frothy volcanic glass.

The pumice I am finding on the beach here came from a huge underwater eruption of the Havre Seamount that took place in June 2012.  This volcano lies north-west of New Zealand, between New Zealand and Tonga.    The eruption produce enough pumice to form a floating raft that was estimated to be 20,000 km2 (that’s HUGE – and bigger than some countries).  The raft was dispersed by the tides, currents and winds and now the rock has been washed up on beaches along the South Pacific and Tasman Sea shores.


White pumice amongst the pebbles on a beach.


Pumice washing in the waves

While it’s a novelty for traveling rockhounds like me, ecologists have been concerned how these events may introduce invasive species of marine life as they hitch a ride on the floating rocks.


It floats! And it travels!

Pumice has many uses.   It can be used a an abrasive – people buy pieces of it an beauty stores as a stone to rub off rough parts of their feet.  It can be added to concrete mixtures as a light weight aggregate and the ancient Romans used pumice in their building aggregates.  It’s even used to wash jeans – those stone-washed keepsakes!  (See here for more information on the use of Pumice)

Pumice soils and farming
In some places on the planet people are farming on soil that is developed on pumice layers.  In the central North Island of New Zealand for example, the pumice rich soils are used for growing commercial pines and dairy farms.   The soils seem to hold water for plants, but can be nutrient poor.

Pumice rich soils in the Cascades in the USA Northwest are also used for commercial forests.  See here for a article on the properties and use of those soils.

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Rocks, soil and geology of your farm




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About Gaz

I'm an Australian transplanted to rural Maine where I live on a small property with my wife and two youngest children. Life about family, work and trying the make the planet a better place for everyone.
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