Re: How long does lava take to cool ?

Dear Lizelle,

Your question has no simple answer.  There are different kinds of lava, 
each with different properties depending on its chemical composition.  The 
chemical  composition of a lava determines what minerals will crystallize 
from it as it cools, and different minerals crystallize at different 
temperatures.  For example, lavas erupted at mid-ocean ridges in places 
like Iceland, or at hot spots in places like Reunion Island in the Indian 
Ocean, are basaltic in composition; they contain minerals like olivine and 
pyroxene which have high crystallization temperatures.  Basaltic lavas are 
typically erupted at temperatures of around 1100ºC.  Other types of lava 
erupted at destructive plate boundaries, such as at volcanoes in 
Indonesia, have a chemical composition richer in silica; silica-rich 
minerals such as quartz and certain feldspars tend to have lower 
crystallization temperatures than olivine and pyroxene, and these lavas 
tend to erupt at lower temperatures than basaltic lavas.

So, the chemical composition of a lava influences its temperature as it 
erupts and also the temperature at which it starts to solidify as it 
cools.  Other factors that influence how long it takes for a lava to cool 
include the rate at which it erupts, the volume of lava erupted, and 
whether it is erupted into the air or under water.  The rate of eruption 
is simply the volume of lava erupted per second.  The eruption rate 
influences the rate of cooling because a high eruption rate will 
continually add a large volume of fresh, hot lava to a lava flow, and this 
will tend to encourage it to stay liquid for longer and to flow farther.  
A good example is the continual eruption that has been taking place on 
Hawaii for some years – you may have seen pictures of lava flowing from 
the crater for miles until it reaches the sea, or flowing like an 
underground river in lava tubes where the surface of the lava flow cools 
to form a crust that keeps the heat in for the lava flowing beneath.

The volume of lava affects the rate of cooling according to a basic 
principle of physics which says that the ratio of surface area to volume 
of an object determines the rate at which heat is lost.  A large object 
has a low surface to volume ratio, which means that it has a relatively 
small surface area in relation to its size from which it can lose heat.  
So a large object tends to cool more slowly than a small one.  Think of a 
cup of hot water:  if you leave the cup standing the water will cool 
eventually to room temperature, but if you tip the cup over so the water 
flows out onto a table top, the spilled water will cool much more quickly 
because you have greatly increased its surface area in relation to its 
volume.  With lavas, this means that a large volume of lava will cool more 
slowly than a small one.  A lava lake, where lava erupted in a crater 
remains inside the crater, would cool more slowly than a lava flow.

If a lava erupts into water it will cool much more quickly than if it 
erupts on land into the air.  The reason is that water is denser than air, 
and transports heat away from a lava much more efficiently than air.  
Lavas erupted under water frequently form what are called “pillows”, which 
are rounded blobs in which the surface of the lava in direct contact with 
the water is quenched within seconds to form a solid crust while inside 
the lava will remain liquid for longer as it cools more slowly.  Lavas 
erupted under water tend not to flow very far before they solidify.  On 
land, however, lavas can flow for miles before they solidify and stop.  
With eruptions of spectacular size called continental flood basalts, lava 
flows can reach thicknesses of tens of meters and flow for hundreds of 
miles before they cool.  Thankfully no such eruption has occurred during 
human history, but many examples from the distant geological past are 
known to geologists.

I have not exactly answered your question, have I?  I’ve told you that 
lavas can take different amounts of time to cool depending on a number of 
factors.  Lava flows like those being erupted on Mount Etna as I write are 
of a size and composition where they will flow some miles down the 
mountain before they solidify.  In the case of Etna today the solidifying 
of such flows might take several weeks, and afterwards they would remain 
warm or hot to the touch for much longer, perhaps several months.

I hope this answers your question.

Best wishes,

David Scarboro