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Intrusive Rock Types

 
  
 
(a) Batholiths. The largest and deepest intrusive bodies are batholiths, which are the solidified
remnants of magma chambers. Batholiths may be as much as 2,000 kilometers long and 200 kilometers wide, and
they broaden downward to unknown depths. They often serve as the cores of folded mountain ranges.
(b) Dikes. These are tabular bodies of igneous rock formed from the solidification of magma as it
travels upward through fractures in the overlying country rock. Due to the mode of formation, dikes cut across
the structure of adjacent rocks; however, they terminate before reaching the earth's surface. Depending on the
size of the original fracture, the thickness of a dike can range from a few centimeters to over a kilometer, and it
may extend laterally for several kilometers. Dikes are commonly found radiating from a volcanic center, similar
to the way the spokes of a wheel radiate from the hub.
(c) Volcanic Necks or Volcanic Pipes. If the upward movement of magma along a fracture is not
halted at some depth below the surface but is, instead, allowed to continue until lava pours out onto the surface to
form a volcano (see Lesson 1.B.1.b.(1)(a), page 1-12), the tabular intrusive igneous rock body formed is called a
volcanic neck or volcanic pipe. These cylindrical masses may be as much as several thousand meters in diameter.
(d) Sills. A sill is a tabular mass of rock oriented parallel to the bedding planes of the enclosing
country rock. Most sills are connected to a dike system and are formed when the magma, rising along a fracture,
encounters a weak sedimentary layer and subsequently invades that layer. These intrusive bodies rage from a few
centimeters to 1,000 meters in thickness; most are approximately 30 meters thick.
(e) Laccoliths. A laccolith is similar in origin to a sill. It is a large mass of igneous rock formed
along a bedding plane. However, in this case, the magma pushes upward on the overlying rock formations,
creating a domelike structure. The laccolith itself then takes on a lenticular shape.
(2) Intrusive Rock Types. For military purposes, intrusive igneous rocks are grouped into the following
broad categories (see figure 1-2, page 1-5):
(a) Granite. This is the most predominant of the two intrusive igneous rock types and is composed
of essentially light-colored minerals, such as orthoclase and quartz. In fact, it is common practice to apply the
term "granite" to any light-colored, coarse-grained, intrusive igneous rock. Because of its mineral content,
regions composed of granite exhibit a fairly uniform, light-colored tone on aerial photographs, except in areas of
extensive surface roughness, where darker photo tones may be present. In addition, numerous joints, or fractures,
which are represented by dark lines, may produce a "scrabbled" or scratchy photo tone in granitic regions.
(b) Gabbro-Diorite. Gabbro and diorite are actually two different intrusive igneous rock types that
are so similar in appearance that they are often grouped into one category called "gabbro-diorite." Gabbro-diorite
is composed of predominantly dark-colored minerals, such as plagioclase, pyroxene, and hornblende. In the field,
any dark-colored, intrusive igneous rock type is generally referred to as gabbro-diorite. These rock types are
represented by dark photo tones on aerial photographs.
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