Laboratory Manual for Introductory Geology 4e

minerals melt, and (3) how plate-tectonic settings produce specific rock types.
The first two questions are addressed in this section and question three is dealt
within section 5.5.
5.4.1 Where and Why Do Rocks and Minerals Melt?
Some science-fiction movies and novels suggest that the rigid outer shell of the
Earth floats on a sea of magma. In fact, most of the crust and mantle is solid rock.
Melting occurs only in certain places, generally by one of the following three
processes:
1. Decompression melting: Stretching of the lithosphere at a divergent boundary
(mid-ocean ridge) or continental rift lowers the confining pressure on the asthenosphere
below. The amount of heat in the asthenosphere cannot by itself
overcome the chemical bonds in the rock and the confining pressure, but
once the pressure decreases, the heat already present in the rock is sufficient
to overcome the bonds and cause melting. This process, called decompression
melting, requires no additional input of heat.
2. Flux melting: Some minerals in subducted oceanic crust contain water in
their structures. The water is incorporated in two processes: reactions of
oceanic rock with seawater shortly after its erupting from an ocean ridge,
or from the island are itself, and weathering of rocks erupted from islandarc
volcanoes. When the subducted crust reaches a critical depth (about
150 km), water and other volatiles are released from these minerals and rise
into the asthenosphere, where they cause melting. In this process, called
flux melting, no additional heat is needed because the added water lowers
the melting point of the rock.
3. Heat-transfer melting: Iron in a blast furnace melts when enough heat is added
to break the bonds between atoms, and some magmas form the same way
in continental crust. Some very hot mafic magmas that rise into continental
crust from the mantle bring along enough heat to start melting the crust,
much as hot fudge melts the ice cream it is poured on. This process, called
heat-transfer melting, occurs mostly in four settings: (1) oceanic or continental
hot spots, where mantle plumes bring heat to shallower levels; (2) continental
volcanic arcs, where mafic magma transfers heat from the mantle to
the continental crust of the arc; (3) during late stages of continental collision,
when the base of the lithosphere peels off and the asthenosphere rises to fill
that space, melts, and sends large amounts of basaltic magma upward; and
(4) in continental rifts, where mafic magma from the asthenosphere rises to
the base of thinned continental crust.
5.4.2 How Do Rocks and Minerals Melt?
We can’t directly observe melting in the mantle or in a subduction zone, but we can
study it in the laboratory. In the 1920s, N. L. Bowen and other pioneering geologists
melted minerals and rocks and chilled them at different stages of melting and
crystallization to learn how magma forms. They learned that magma melting is very
different from the way ice melts into water, and their results paved the way for our
understanding of the origins of the four compositional groups.
■ Most rocks melt over a range of temperatures. This is because some of their
minerals have lower melting points than others. The minerals with low melting
124 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY