Laboratory Manual for Introductory Geology 4e

GEOTOURS EXERCISE 5
Exploring the Nature of Batholiths
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Exploring Geology Using Google Earth
1. Visit digital.wwnorton.com/geolabmanual4
2. Go to the Geotours tile to download Google Earth Pro and the accompanying
Geotours exercises file.
Expand the Geotour05 folder in Google Earth
by clicking the triangle to the left of the folder
icon. Check and double-click the Half Dome
placemark to fly to this impressive landmark
on the flanks of Yosemite Valley in Yosemite
National Park. Here, one gets a sense of the
expansive and spectacular glacially-shaped
exposures of the Sierra Nevada batholith that
forms much of the Sierra Nevada Mountains
in California. The batholith is composed of
many separate plutons, which intruded into
the area as part of a Mesozoic-age subduction
system.
(a) Given that these rocks crystallized
at depths of ~20-25 km beneath a
continental volcanic arc, what most likely
is their texture: coarse-grained, finegrained,
porphyritic, glassy, porous, or
fragmental?
texture
(b) Check and double-click the Rock Type placemark to take a closer look at these rocks. Here, pink orthoclase-rich
dikes intrude the mostly light-colored rock that forms Half Dome. To which of the four major compositional groups
(felsic, intermediate, mafic, or ultramafic) do these rocks likely belong? Hint: Use your igneous classification chart to
see how light-colored rocks with orthoclase are classified.
compositional group
(c) Check and double-click the Joints placemark. Visitors commonly note the rounded appearance of the back side of
Half Dome. It was not polished by glaciers, but rather fractured off in sheets much like how one might peel layers
from an onion. This process occurs because these rocks formed at ∼20-25 km depth, but experienced a removal
of confining pressure when they were exposed at the surface (think of how a sponge expands once you stop squeezing
it with your hands). What is this process called? Hint: Look up jointing processes associated with plutons in your
textbook.
process
140 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY