MORO ROCK
Moro Rock is a large exfoliation dome in the Sierra
Nevada Mountains of eastern California, USA. It is accessible by road and
hiking trail in the western part of Sequoia National Park, a little east of
Generals Highway (Rt. 198).
Moro Rock (looking ~NE), as seen from the valley of the Middle Fork of the
Kaweah River.
Moro Rock exfoliation dome (looking ~S) - exfoliation domes are common in the
Sierra Nevada Mountains. They form by large-scale spheroidal weathering
of granites and granitoids of the Sierra Nevada Batholith. The
batholith represents a Late Jurassic to Late Cretaceous-aged mass of cooled
magma chambers originally beneath a chain of subduction zone stratovolcanoes.
Pressure release from erosional unroofing of the
batholith resulted in the rocks having curved sheeting joints. Spheroidal
weathering followed, akin to peeling the layers from an onion (“exfoliation”).
The end result is a rounded mountain top - an exfoliation dome.
Moro Rock exfoliation dome - view of the summit (looking ~S) and the valley of
the Middle Fork of the Kaweah River beyond.
Moro Rock exfoliation dome - view of the summit (looking ~S) and the valley of
the Middle Fork of the Kaweah River beyond.
Giant Forest Granodiorite (Giant Forest Pluton) - Moro Rock is the type
locality of the Cretaceous-aged Giant Forest Granodiorite, one of numerous
specific igneous intrusions in the Sierra Nevada Batholith. Granodiorite
is dominated by quartz and sodic plagioclase feldspar and some potassium
feldspar. The Giant Forest Pluton is also rich in hornblende amphibole (=
black crystals) and relatively rich in small, ~honey-colored crystals of
titanite (a.k.a. sphene - CaTiSiO5).
Geologic unit & age: Giant Forest Granodiorite, Sequoia Intrusive Suite,
97-102 million years (late Albian Stage to early Cenomanian Stage,
mid-Cretaceous).
Xenolith in Giant Forest Granodiorite - xenoliths are pieces of “foreign-rock” that have
fallen from the walls of a cooling magma chamber. This particular
xenolith is rich in mafic minerals, which is why it is dark-colored. Mafic
xenoliths, or “inclusions”, are fairly common in the Giant Forest Granodiorite
at Moro Rock.
Great Western Divide - the high ridge of mountains seen to the east from
the summit of Moro Rock is the Great Western Divide, a major drainage divide in
the Sierra Nevada Mountains. The highest mountains in the ridge peak out
at 12,000 to 12,600’ elevations.
Middle Fork of the Kaweah River as seen from the summit of the Moro Rock exfoliation
dome (looking ~SW).