Re: Can Sedimentary Rock melt and become igneous rock?

Julia,

Sedimentary rocks definitely do melt into igneous rocks, however, as they heat up, but before they get hot enough to melt, they will recrystallize and react to form new minerals. These reactions change the texture and minerals of the rock and so technically the sedimentary rocks become metamorphic rocks first and then they melt to become igneous rocks.

I can think of two exceptions, fulgurites and pseudotachylites, both of which involve extremely rapid heating. Metamorphic reactions take time, lots of time, so with rapid heating, there is not time for metamorphism to occur. Fulgurites are formed when lightning strikes the earths surface and melts the sand or soil. For an example, see:
http://geology.about.com/library/bl/images/blfulgurite.htm
The melt cools very quickly and forms glass (like obsidian).

Pseudotachylites are thin seams of glassy material formed when friction during earthquake motion melts rocks in a fault zone. They can also be formed due to friction or shock during large impacts. For example, see:
http://www.lpl.arizona.edu/~rlorenz/pseud.html

When metasedimentary (metamorphosed sedimentary) rocks melt in large amounts, they usually produce a distinctive sort of grantite known as an S- type granite. The other main type is the I-type, produced from the remelting of older igneous rocks. For more on the differences, see:
http://www.geologyone.com/granitetypes.htm

The rock cyle is one of the most fundamental concepts in geology. Unfortunately, the classic diagram leaves out the most important process in geology, namely the role of plate tectonics and the fact that the rock cycle is not purely cyclical, but tends to evolve over time. A very nice web site has been put together by geologist Lynn S. Fichter at James Madison University. Please share this site with your teacher:
http://csmres.jmu.edu/geollab/Fichter/Wilson/PTRC.html

You should really explore this site in some depth. There's a lot of good information and it makes a nice comparison to the traditional picture.

Enjoy,

Dave Smith