Expert: C. Robert Reszka, Jr. - 1/3/2005

Question
where does rock come from?
What is the difference between magma and lava?

Answer
Hi Donna,
This is a very large question.  Let's see if we can simplify it some.  First let's say what a rock is:  Generally speaking a rock is made of minerals or other small pieces of rock that have been welded together.  How does that welding happen?  Just add a lot of temperature and pressure.
Rocks are divided into three types; IGNEOUS, SEDIMENTARY, and METAMORPHIC.  There are many types of rocks within each of those classifications.  But let's tackle each one, briefly.

IGNEOUS – this is a rock that's made up of melted rock material.  That could be other rocks that already existed or a group of minerals in a magma (we'll get to what that is shortly).  So you melt rocks, or anything that makes rocks, and when it cools enough to solidify you get an Igneous rock.

METAMORPHIC – Heat an existing rock really hot, but not hot enough to melt, and the minerals in it (and the rock itself) will change.   When that happens you get a Metamorphic rock.  This usually happens when the rock gets deeply buried (thousands of meters).

SEDIMENTARY – You get a Sedimentary rock when you break up ANY kind of other rock and you force all the pieces back together.   You usually do that by burying them and increasing the temperature and pressure.  However, you don't heat things up so much that you change anything in the little pieces (unlike the metamorphic rock).

So….melt rocks completely and you get Igneous, heat them up a lot but don't melt them and you get Metamorphic, break them into little pieces and put them back together and you get Sedimentary.

Check out this site.  It has some good info on the “Rock Cycle” which talks about the different types of rock (nice pictures too).

http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html


As to the difference between lava and magma:  They are really the same thing, just melted rock.  But if it's underground it's called Magma, if it's flowing on the surface it's called Lava.  That's all there is to it.  It's easier to say “magma” than, “underground lava”.  Or “lava” instead of, “that magma that flows out of volcanoes”.
Hope this helped.
Bob