MadSci Network: Earth Sciences

Re: Why is the surface of the earth floating on lava

Date: Sun Feb 18 17:05:57 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 981224131.Es

Dear Nitin,

In order to answer your question we need to establish a few terms and 

(1)Lava is molten rock that erupts at the Earth’s surface at volcanoes.  
Molten rock that has not erupted at the surface is called magma.
(2)A cross-section through the Earth would show that it is divided into a 
thin outer lithosphere, a thick layer called the mantle, and the core.
(3)The lithosphere actually consists of the crust and the top part of the 
mantle, which are both composed of solid, rigid rock and are fused 
together as a rigid unit.  The thickness of the lithosphere varies, but 
averages about 50 km.
(4)Below the lithosphere, between about 50 km and 250 km depth, is a layer 
of the mantle called the asthenosphere.  The asthenosphere is not rigid 
but can flow, rather like toothpaste being squeezed from a tube.
(5)Below the asthenosphere is the rest of the mantle, which extends down 
to about 2900 km to the boundary with the outer core.
(6)The mantle is composed of a rock called peridotite, which is rich in 
iron and magnesium.
(7)Heat is continually being produced within the Earth’s mantle and 
lithosphere.  This heat causes the mantle to churn, rather like water 
boiling in a pan only, of course, much more slowly.  The main point is 
that the rocks of the mantle actually flow.
(8)Mid-ocean ridges are regions where new ocean crust is created by 
upwelling of magma from the mantle.  Subduction zones are regions where 
old ocean crust dives back down into the mantle.

Here is a website with a diagram of the lithosphere and the upper part of 
the mantle, including the asthenosphere, that you may find helpful:    

Perhaps the asthenosphere is the zone you are thinking about when you 
describe the lithosphere as floating on lava.  Although the lithosphere 
does not float on an ocean of liquid magma, it can be said to float on the 
asthenosphere.  The asthenosphere is a weak layer of the mantle which is 
thought to consist of about 95% solid rock (peridotite) and 5% magma.  
That may not sound like a lot of melt, but it is enough to make the 
asthenosphere mechanically weak, so that it can flow like a fluid.  Think 
of the asthenosphere as a sort of slush of mostly solid rock mixed with a 
small amount of liquid magma.  The asthenosphere exists because at the 
depth where it occurs, roughly between 50 and 250 km, the conditions of 
temperature and pressure are just right to cause the rock to partially 
melt.  Below the asthenosphere the mantle is hotter but also under greater 
pressure, and melting does not occur.  Above the asthenosphere the mantle 
is under lower pressure but the temperature is also lower, and again 
melting does not occur.

You are correct that hot, liquid magma is less dense than the rock that 
surrounds it, and that being less dense it will tend to rise to the 
surface.  But because the asthenosphere lies beneath the lithosphere there 
are only certain regions where magma from the asthenosphere is able to 
find its way to the surface.  Also, other parts of the mantle in addition 
to the asthenosphere can partially melt under certain conditions and this 
magma can reach the surface.  Magma from the asthenosphere reaches the 
surface along mid-ocean ridges, where heat flow is very high and where the 
depth from the surface to the asthenosphere is only about 50-60 km.  Magma 
from very deep in the mantle can reach the surface at hot spots, such as 
Hawaii and Réunion Island, where plumes of very hot material from the 
boundary between the mantle and the core reach the surface.  Hot spot 
plumes will punch through the entire mantle and the crust on their way to 
the surface.

At mid-ocean ridges and hot spots the lava erupted at the surface is 
mostly a type of lava called basalt.  Basalt is formed by the partial 
melting of the peridotite of the mantle, and its composition – rich in 
iron and magnesium – reflects its origin in the mantle.  Away from mid-
ocean ridges and hot spots, magma from the asthenosphere is unable to 
reach the surface because the asthenosphere lies deeper, heat flow is 
less, and the magma is confined under higher pressure by a greater 
thickness of overlying lithosphere.

Volcanoes associated with subduction zones, such as the volcanoes of 
Indonesia, produce lavas from melting of rocks of the lithosphere, 
including rocks of  the crust or of lithospheric mantle lying above the 
asthenosphere.  But whatever the type of volcano, the lava erupted comes 
from pockets of magma which form as a result of local melting of the rocks 
of the mantle or lithosphere.  It does not come from anything resembling 
an ocean of magma beneath the surface.

A final point is that because the mantle flows, any magma which is lost to 
the surface will be replaced by fresh material rising from deeper in the 
mantle.   The whole process is a cycle, in which melting of fresh magma 
takes place continually at mid-ocean ridges and hot spots, rises to the 
surface to form new crust, and is replaced by old crust being carried back 
down into the mantle at subduction zones.


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