(3) Relationship of Topography to Intrusive Igneous Rocks.
(a) Landforms Developed on Intrusive Igneous Rocks. Extensive formations of intrusive igneous
rock may be found throughout the cores of many eroded mountain ranges and in some areas of folded rock. In
general, these massive batholithic intrusions are topographically expressed as domelike hills. In humid or
temperate regions, the tops of the hills are gently rounded, and the side slopes are relatively steep, producing a
"knobby" topography. In arid or semiarid regions, the hills are more angular. Figure 1-4 illustrates the
topography of a batholith in the Rocky Bar, Idaho, area.
Intrusive igneous rocks, such as granite and gabbro-diorite, are often more resistant than the surrounding rock
material. Therefore, in areas that have undergone extreme weathering and erosion, intrusive bodies may stand out
in great relief. For example, volcanic necks, which originally crystallized below the earth's surface, may form a
tower rising 1,000 meters above the surrounding country in highly eroded regions. Likewise, when exposed by
the erosion of surrounding material, dikes take on the appearance of huge walls. Figure 1-5, page 1-10, is a
topographic map of one such exposed volcanic neck and its associated radiating dikes.
(b) Drainage Developed on Intrusive Igneous Rocks. Because of the uniform nature of intrusive
igneous rocks, dendritic (treelike) drainage patterns tend to develop on massive formations such as batholiths (see
figure 1-4). However, a ringlike, or annular, drainage pattern may develop where streams flow away from the
peak of a dome in a series of concentric fractures. In areas where extensive jointing has occurred, rectangular
drainage patterns may develop (see figure 1-6, page 1-11).
In arid regions where soils are shallow, gullies are seldom present. However, in humid regions, intrusive igneous
rocks weather to form soil deposits consisting primarily of fine sands and fine, inorganic silts and clays. Gentle,
U-shaped gullies are commonly formed on these types of soils.
(c) Vegetation in Areas of Intrusive Igneous Rocks. Vegetation on these rock formations is
usually heavy in humid and tropical regions, while in arid regions, vegetation is sparse on the dome tops, with
scattered vegetation aligned with the joints. Large areas of bare rock are also common in arid and semiarid
(4) Engineering Properties of Intrusive Igneous Rocks. Both granite and gabbro-diorite are relatively
tough, hard, and durable rock types. Furthermore, they are chemically stable, have good surface character, and
produce a good crushed shape. As a result, these intrusive igneous rock types provide strong foundations and
serve as excellent sources of construction material for both building stone and aggregate. Overall, intrusive
igneous rocks display exceptional engineering properties.
b. Extrusive Igneous Rocks.
(1) Characteristic Extrusive Rock Bodies. Several different types of extrusive igneous rock bodies can
be distinguished based on form (see figure 1-3, page 1-6). The most important of these are volcanoes and lava