How Old is an Old Rock?
Mountaineer/Progress 22 July 2004
by Horst Meister, correspondent
1.7 billion years isn't old if you're a rock, but it is almost
half as old as the earth. Geologists Chris Bucklin and
Bruce Garbaccio said that the oldest rocks in the San
Gabriel mountains were about that old.
Garbaccio said that scientists date rocks by measuring the
decay of radioactive elements contained within the rock
and by noting their positions relative to each other. If a
layer of sandstone is on top of a layer of granite, it doesn't
take much to deduce that the granite is probably older than
the sandstone.
Determining the absolute age of a rock takes a little more
work. Uranium's rate of atomic decay is known, and
uranium turns to lead as it decays. By comparing the ratio
of lead to uranium in a rock sample, scientists can come up
with a pretty good idea of how old a rock is. This technique
is called radiometric dating.
The old stuff is called Mendenhall Gneiss.
Next oldest on the age list at 1.2 billion years are the
granitic anorthosite and gabbro that forms the bulk of the
San Gabriel mountains.
Geologists refer to granitic rock as intrusive rocks.
The earth largely consists of igneous rocks; rocks that
when deep inside the earth are molten into a plastic state.
The interior of the earth is hot. Temperatures range from
1,400 to 2,300 degrees. At those temperatures, rocks turn
into a thick liquid. This plasticized rock is under great
pressure and it sometimes rises to the surface through
cracks in the earth's crust (volcanoes), or the intense heat
and pressure of the magma weakens the overlying crustal
rock until they give way. Igneous rocks form 'when the
magma cools down and becomes solid.
There aren't any volcanoes in the San Gabriels, but there
are a few ancient volcanic domes in the desert north of the
mountains.
Scientists divide igneous rocks into two groups: extrusive
rocks and intrusive rocks. Lava is an extrusive rock; it
reaches the surface through cracks in the crust.
Intrusive rocks form from magma that fails to break the
surface of the Earth. It may push up the surface into a huge
blister, like Cima Dome in the Eastern Mojave Desert.
Sometimes the hot rocks spread out in vast sheets between
layers of older rocks.
Beneath the surface, the molten rock cools and hardens,
slowly. Rocks formed in this way have coarse crystaline
grains that are clearly visible to the unaided eye. These
crystalline rocks include the granites, syenites, and
gabbros. The granites that form much of the Sierra Nevada
and the San Gabriel mountains are intrusive rocks that
were once magma.
Bucklin said that one of the best places to see intrusive
rocks in the San Gabriels was the Williams Rocks, which
are about a mile west of the tunnels on the Angeles Crest
Highway.
Garbaccio said that these rocks formed as underground
magma domes. After they hardened, the overlying softer
rock was eroded away, leaving the intrusive rock domes
standing, presumably for the convenience of the rock
climbers who flock to the Williams Rocks every weekend.
Locally, Swarthout Valley is formed of gneiss and Pelona
Schist.
Table Mountain on the north side of Swarthout Valley is
mostly gneiss. Gneiss is a banded, coarse-grained
metamorphic rock. Gneiss is formed deep underground,
when heat and pressure change the original rock deep
within the earth's crust.
Gneiss is a hard rock that, unlike many, layered rocks like
schist, does not readily break along the bands of layers.
Gneiss is the reason why the drainages incised into the
flanks of Table Mountain are so small; gneiss resists
erosion far better than the Pelona Schist that forms the bulk
of Blue Ridge.
Schist is a medium to coarse-grained rock that splits easily
into parallel layers. Like gneiss, schist is a metamorphic
rock that forms when heat and pressure alter the shape and
mineral content of solid rock that is below the surface of
the earth.
Schist is easily eroded because it has flaky or elongated
minerals that are lined up in parallel layers, and the rock
readily breaks along those layers. Schist often contains a
large amount of mica. Pieces of cleaved schist have shiny
surfaces because the mica flakes reflect light.
The easily eroded Pelona Schist that forms the bulk of Blue
Ridge is responsible for the many, drainages that penetrate
deep into Blue Ridge, and it's those drainages that are the
source of Swarthout Valley's water. Pelona Schist is about
60 million years old.
Swarthout Valley is bisected by the San Andreas fault.
Table Mountain is on the North American plate, while
Blue Ridge is on the Pacific Plate. The two halves of
Swarthout Valley are moving in opposite directions at a
blistering rate of a quarter inch a year. The gneiss that
forms Table Mountain and the Pelona Schist of Blue Ridge
formed in separate locations and were moved to their
present location by fault action.
Bucklin’s and Garbaccio’s talk was sponsored by the
Volunteers of the Angeles National Forest as part of the
Volunteers' annual summer programs held at Grassy
Hollow.
Questions for “How Old is an Old Rock?”
1.
How old are the oldest rocks in the San Gabriel Mountains?
2.
Which layer of rocks is most likely older, the top or the bottom layer?
3.
Uranium slowly turns to _______________ at a known rate. Determining the age of a rock by comparing the ratio of the
end-product element to the starting element is called ____________________ dating.
4.
What are the oldest rocks in the San Gabriels called?
5.
What two types of rock are next in age?
6.
How old are they?
7.
Where are ancient volcanic domes found near the San Gabriels?
8.
What kind of intrusive igneous rocks form much of the San Gabriels (and the Sierras as well)?
9.
Where are the intrusive rock domes found in the San Gabriels that attract weekend climbers?
10.
What two kinds of rock mainly form the Swarthout Valley (where Wrightwood is)?
11.
Table Mountain (on the northwest end of Swarthout Valley is mostly what kind of rock?
12.
Blue Ridge (runs all along the south side of Swarthout Valley) is mostly what kind of rock?
13.
How does schist form?
14.
What are some of its characteristics?
15.
What is responsible for the many drainages (“canyons”) that are the source of Swarthout Valley’s water?
16.
Swarthout Valley is bisected by the ____________________ fault. Table Mountain is on the ____________________
plate; Blue Ridge is on the ____________________ plate.
17.
How fast are the two plates moving relative to one another?
18.
The gneiss and pelona schist formed in separate locations. How did they get together in the Swarthout Valley today?