Page 483 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch18_455-476.indd Page 460 9/3/10 6:21 PM user-f465
tiL12214_ch18_455-476.indd Page 460 9/3/10 6:21 PM user-f465 /Users/user-f465/Desktop

Oceanic crust Continental crust

Crust
Uppermost mantle Lithosphere
Mantle Asthenosphere

(part of mantle)

Mantle continues downward

6,370 km
Inner
core

Outer core 2,900 km

Mantle

Crust

FIGURE 18.7 Earth’s interior, showing the weak, plastic layer called the asthenosphere. The rigid, solid layer above the asthenosphere
is called the lithosphere. The lithosphere is broken into plates that move on the upper mantle like giant ice sheets floating on water. This
arrangement is the foundation for plate tectonics, which explains many changes that occur on Earth’s surface such as earthquakes, volcanoes,
and mountain building.

Evidence from the nature of meteorites indicates that evolved over time. There is also another, more detailed structure
Earth’s core is mostly iron. Earth has a strong magnetic field that can be described. This structure is far more important in
that has its sources in the turbulent flow of the liquid part of understanding the history and present appearance of Earth’s sur-
Earth’s core. To produce such a field, the material of the core face, including the phenomena of earthquakes and volcanoes.
would have to be an electrical conductor, that is, a metal such as The important part of this different structural description
iron. There are two general kinds of meteorites that fall to Earth: of Earth’s interior was first identified from seismic data. There is
(1) stony meteorites that are made of silicate minerals and (2) iron a thin zone in the mantle where seismic waves undergo a sharp
meteorites that are made of iron or of a nickel-iron alloy. Since decrease in velocity. This low-velocity zone is evidently a hot,
Earth has a silicate-rich crust and mantle, by analogy Earth’s elastic semiliquid layer that extends around the entire Earth.
core must consist of iron or a nickel-iron alloy. It is called the asthenosphere after the Greek for “weak shell”
(Figure 18.7). The asthenosphere is weak because it is plastic
and mobile and yields to stresses. In some regions, the astheno-
A MORE DETAILED STRUCTURE sphere is completely liquid, containing pockets of magma.
There is strong evidence that Earth has a layered structure with The rocks above and below the asthenosphere are rigid,
a core, mantle, and crust. This description of the structure is lacking a partial melt. The solid layer above the asthenosphere
important for historical reasons and for understanding how Earth is called the lithosphere after the Greek for “stone shell.” The

460 CHAPTER 18 Plate Tectonics 18-6