November 2009
Billie Silvey
Someone may say of a deal or an organization, “It’s as solid as
the earth beneath your feet.â€�  That may be reassuring to some
people, but I live on the Pacific Rim where the earth isn’t always
such a solid thing.

Here in Los Angeles, we’re threatened by earthquakes along the
San Andreas Fault, the line where the Pacific Plate meets the North
American Plate.  There’s a good bit of friction there.  The land
west of the fault lies on the Pacific Plate, which is moving slowly to the
northwest.  The land east of the fault is moving southwest, with a rate of
slippage that averages approximately one and a third inches a year.

The westward motion of the North American Plate creates
compressional forces which create the Coast Ranges.  The northwest
motion of the Pacific Plate creates significant compressional forces
where the North American Plate stands in its way, creating the
Transverse Ranges in Southern California.

How do I know?  Geologists, who study of the history of our planet--
the materials it’s made from, the structure of those materials, and
the processes acting on them—told me so.

Geologists study processes such as landslides, earthquakes, floods and
volcanic eruptions so people can avoid risk.  They map areas which
have been affected by natural disasters to help prepare for new ones in
the future. They study the materials of the earth, including oil, metals
and water.  They locate rocks that contain important metals and plan
mines to produce them and methods to extract them.

They also study earth’s history.  For instance, by understanding
how climate has changed in the past, they can better understand how
our climate is changing today and what the results might be.  And by
studying earthquakes in the past, they can better predict what might
happen the next time the pressure builds and things start moving on the

The earth is less stable than we might think, because it’s in three
parts.  The land we stand on is part of the crust, the outermost solid
silicate shell which is chemically distinct from the underlying mantle.  
The mantle is about 1,800 miles thick and constitutes about 84 percent
of Earth's volume.  The mantle is a slow-moving, highly viscous layer
that separates the crust and the solid core, which occupies about 15
percent of Earth's volume.

So, next time someone tries to sell you on a deal that's as solid as "the
earth beneath your feet," take warning.  Sometimes, at least around
here, the earth moves.
Rock of Ages
Rocks & Minerals