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Crater Hogback
More recent flows and deposits Laccolith

Older ash deposits

Older lava flows Dike
Sill

Batholith

Stock Dike

Sedimentary rock

FIGURE 19.24 Here are the basic intrusive igneous bodies
that form from volcanic activity.
A

intrusive rocks, igneous rocks that were described in chapter 17.
A large amount of magma that has crystallized below the surface
is known as a batholith. A small protrusion from a batholith is
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called a stock. By definition, a stock has less than 100 km (40 mi )
of exposed surface area, and a batholith is larger. Both batholiths
and stocks become exposed at the surface through erosion of the
overlying rocks and rock materials, but not much is known about
their shape below. The sides seem to angle away with depth, sug-
gesting that they become larger with depth. The intrusion of a
batholith sometimes tilts rock layers upward, forming a hogback,
a ridge with equal slopes on its sides (Figure 19.24). Other forms
of intruded rock were formed as moving magma took the paths
of least resistance, flowing into joints, faults, and planes between
sedimentary bodies of rock. An intrusion that has flowed into a
joint or fault that cuts across rock bodies is called a dike. A dike
is usually tabular in shape, sometimes appearing as a great wall
when exposed at the surface. One dike can occur by itself, but
frequently dikes occur in great numbers, sometimes radiating
out from a batholith as do spokes around a wheel. If the intru-
sion flowed into the plane of contact between sedimentary rock
layers, it is called a sill. A laccolith is similar to a sill but has an
arched top where the intrusion has raised the overlying rock into
a blisterlike uplift (see Figure 19.24).
Where does the magma that forms volcanoes and other
volcanic features come from? It is produced within the outer
100 km (about 60 mi) or so of Earth’s surface, presumably from
a partial melting of the rocks within the crust or the uppermost
part of the mantle. Basalt, the same rock type that makes up
the oceanic crust, is the most abundant extrusive rock, both on
B
the continents and along the mid-oceanic ridges. Volcanoes that
FIGURE 19.23 (A) A schematic cross section of an idealized rim the Pacific Ocean and Mediterranean extrude lava with a
composite volcano, which is built up of alternating layers of slightly different chemistry. This may be from a partial melting
cinders, ash, and lava flows. (B) A photo of Mount Shasta, a of the oceanic crust as it is subducted beneath the continental
composite volcano in California. You can still see the shapes of
crust. The Cascade volcanoes are typical of those that rim the
former lava flows from Mount Shasta.
Pacific Ocean. The source of magma for Mount St. Helens and
the other Cascade volcanoes is the Juan de Fuca Plate, a small
plate whose spreading center is in the Pacific Ocean a little west
of the Washington and Oregon coastline (Figure 19.25). The

492 CHAPTER 19 Building Earth’s Surface 19-16