BATHOLITH – EVAN SHERIFF
INTRODUCTION
Batholiths are large intrusive igneous structures which have a significant area exposed at
the surface as well as extending deep into . Batholiths are a single large structure comprised of
multiple masses of intrusive igneous rock, called plutons which arise from intruding magma
cooling in Earth’s crust. Batholith formation is associated with subduction zones which have the
conditions necessary to form magma. Batholiths are commonly found across the Western US and
Canada as well as throughout the world near subduction zones.
ORIGIN AND DISCUSSION
Batholiths are formed of multiple intrusions of igneous rock masses, plutons, which
combine to form an extensive structure and are commonly extant in the center of many mountain
ranges including the granitic mountains of the western US (Johnson et al., 2017). A batholith is
defined as plutonic terrane exposed at the surface for an area comprising at least 100 km2, its
discrete units comprising the individual plutons. A batholith is generally felsic, principally
formed of medium to coarse grained granitic rock (Bateman, 1988). A batholith is alternatively
defined as discordant plutonic mass with large surface exposure, extending deep into the Earth
with no known floor, largely composed of granodiorite (Hall, 2007).
Batholith formation is associated with subduction zones where active melting of parent
rock occurs to form underground reservoirs of molten rock. From these magma chambers, the
magma rises along a path called a diapir, the exact process of diapir intrusions are an area of
active geologic inquest and debate. It is not clear what becomes of the native rock during diapir
intrusion. It is posited that extant rock may be forcible displacement by the increased magma
volume, or it may be melted and consumed into the magma known as ballooning, or the rock
may be broken and settle into the magma which is known as stoping (Johnson et al., 2017). Other
geologists have demonstrated that diapirs would take 100,000 to 1 million years to reach the
surface while others believe that the diapir would solidify before reaching the upper crust. Given
these discrepancies diapir, many geologists believe plutons form as blobs in the crust being fed
by dikes over several thousand years (Hall, 2007).
EXAMPLES
The Sierra Nevada Batholith is ~650 km long, ~100-130 km wide, forming the core of
the Sierra Nevada Mountain range in California and lying within the batholith is the majestic
Yosemite National Park. The plutonic rocks of the batholith formed during the Mesozoic, along
the western edge of the Paleozoic North America Craton with the Pacific Plate subducting
beneath the North American Plate. Magmatism and thus intrusions began ~210 Ma, coincident
with the breakup of Pangea, and continued until ~85 Ma. It has been suggested that the diapir
emplacement occurred by a combination of forcible displacement, ballooning, and stoping.
Beginning in the early Cenozoic, tilting and tectonic uplift began forming the Sierra Nevada
range and uplifted the batholith to the surface (Bateman, 1988).
