Thanks for the question! In most situations,
sedimentary rocks would metamorphose as they are
heated and the metamorphic rock would begin to
melt when the temperatures are high enough. There
are only three ways that I know of in which a
sedimentary rock could be heated up fast enough to
melt without ever becoming a metamorphic rock...
and all three of them are amazing:
-Meteorites hit the Earth so fast that they can
instantaneously melt rocks that they collide with,
the small pieces of igneous rock produced this way
are called tectites. Of course, the larger
the impact, the higher the energy and the more
magma can be made. Perhaps the best example of
this happening at a large scale is in the Sudbury
basin in Canada.
-Earthquakes can actually cause rocks to melt.
Earthquakes happen with the force of the moving
teconic plates becomes greater than the strength
of the rocks, causing the rocks to fracture and
slide past each other for some short distance
(10's of feet for the absolute largest
earthquakes, but usually much less). As the rocks
are sliding past each other along the fault, the
friction is so strong that it can actually melt
the rock within a few centimeters of the fault,
this melted zone (technically an igneous rock) is
called a pseudotachylite.
-Lightning strikes actually melt the ground
they strike, by flash heating it to over 3000 F.
The type of rock formed this way (never more than
a few inches large) is called a fulgurite
(these can be spectacularly shaped, I recommend
doing an image search for some).
Tectites, pseudotachylites, and fulgurites
are all technically igneous rocks, however
they are definitely odd-ball rock types. Usually
sediments have to metamorphose before they can
Hope this was interesting!