Mountain Terrain, Weather, and Hazards

Plutonic (Intrusive) Rocks
1-15. Slow crystallization from deeply buried magmas generally means good climbing, since the minerals
formed are relatively large and interwoven into a solid matrix. Weathering develops protrusions of resistant
minerals, which makes for either a rough-surfaced rock with excellent friction, or, if the resistant crystals
are much larger than the surrounding matrix, a surface with numerous knobby holds. Pieces of foreign rock
included in the plutonic body while it was rising and crystallizing, or clusters of segregated minerals, may
weather differently than the main rock mass and form chicken heads.
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Intrusions are named according to location and size. Large (100 square kilometers or larger)
masses of plutonic rock are called batholiths and small ones stocks. Most plutonic rock is in the
granite family, differing only in the amounts of constituent minerals contained. A core of such
batholiths is in every major mountain system in the world. In the Alps, Sierras, North Cascades,
Rockies, Adirondacks, and most other ranges this core is at least partly exposed.
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Small plutonic intrusions are stocks, forced between sedimentary strata, and dikes, which cut
across the strata. Many of these small intrusive bodies are quickly cooled and thus may look like
extrusive rock.

Volcanic (Extrusive) Rocks
1-16. Explosive eruptions eject molten rock so quickly into the air that it hardens into loose aerated masses
of fine crystals and uncrystallized glass (obsidian). When this ash consolidates while molten or after
cooling, it is called tuff, a weak rock that breaks down quickly and erodes easily. Quieter eruptions, where
widespread lava flows from large fissures, produce basalt. Basaltic rocks are fine-grained and often
sharp-edged.

Jointing Rocks
1-17. In plutonic rocks, joints or cracks are caused by internal stresses such as contraction during cooling
or expansion when overlying rock erodes or exfoliates. Some joints tend to follow a consistent pattern
throughout an entire mountain and their existence can often be predicted. Therefore, when a ledge suddenly
ends, the joint―and thus the ledge―may begin again around the corner. When molten rock extrudes onto
the surface as a lava flow or intrudes into a cold surrounding mass as a dike or sill, the contraction from
rapid cooling usually causes so much jointing that climbing can be extremely hazardous. Occasionally, this
jointing is regular enough to create massed pillars with usable vertical cracks such as Devil’s Tower in
Wyoming.

Sedimentary Rocks
1-18. Sedimentary rocks are born high in the mountains, where erosion grinds down debris and moves it
down to rivers for transportation to its final deposition in valleys, lakes, or oceans. As sediments
accumulate, the bottom layers are solidified by pressure and by mineral cements precipitated from
percolating groundwater. Gravel and boulders are transformed into conglomerates; sandy beaches into
sandstone; beds of mud into mudstone or shale; and shell beds and coral reefs into limestone or dolomite.
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Though in general sedimentary rocks are much more friable than those cooled from molten
magmas, pressure and cementing often produce solid rocks. In fact, by sealing up internal cracks,
cementing can result in flawless surfaces, especially in limestone.
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Most high mountain ranges have some sedimentary peaks. Ancient seafloor limestone can be
found on the summits of the Himalayas and the Alps. The Canadian Rockies are almost
exclusively limestone. With the exception of the Dolomites, in general sedimentary rocks do not
offer high-angle climbing comparable to that of granite.

METAMORPHIC ROCKS
1-19. These are igneous or sedimentary rocks that have been altered physically and or chemically by the
tremendous heat and pressures within the earth. After sediments are solidified, high heat and pressure can
cause their minerals to recrystallize. The bedding planes (strata) may also be distorted by folding and
squeezing. Shale changes to slate or schist, sandstone and conglomerate into quartzite, and limestone to

26 July 2012

TC 3-97.61

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