Page 48 - visitorGuide

■ Views of Mount St. Helens
These three-dimensional views were made from a digital elevation model
produced at the U.S. Geological Survey Western Mapping Center. Before (left)
and after (right) the eruption of 18 May 1980.

Gift from U.S. Geological Survey Western Mapping Center, through Nicolas R. Serbu.

On the wall opposite the memorial are three specimens. From the top:

■ Acasta Gneiss
The Acasta Gneiss, from Northwest Territories, Canada, is an assemblage of
4-billion-year-old metamorphic rocks. Individual mineral grains more than 4
billion years old have been found in younger rocks in western Australia, but
the Acasta Gneiss is currently the oldest known whole rock in the world.This
specimen was collected from an island in an unnamed lake drained by the
Acasta River.The Acasta Gneiss was dated by uranium-lead radiometric anal-
ysis using naturally radioactive zircon grains in the rocks.This dating method
indicates when the rocks were formed, so the rocks from which they were
formed are older than the date indicated by the ion microprobe analyses.

Collected and donated by Donald G. Hadley.

■ Fossilized Whale Vertebra

The group of huge marine animals we call whales may be up to 75 million

years old, but this fossilized vertebra is probably just a few million years old.

The size of this specimen illustrates just how large whales are and that they
are supported by the buoyancy of the sea.

Gift from Richard P. Snyder, U.S. Geological Survey, Denver, Colorado.

Volcanic Bomb

■ Volcanic Bomb

A twist of molten lava was thrown

into the air from the throat of a vol-
cano.The lava cooled and solidified in

flight before dropping back down to the

ground as this volcanic bomb. It was found
on the eastern border of the Lunar Crater

Quadrangle, central Nevada (38°22′N, 116°W).
Gift from Richard P. Snyder, U.S. Geological Survey, Denver, Colorado.

Proceed forward and turn left, returning to the spot where you began this floor’s tour. To
begin touring the third floor, proceed up the stairs and turn to p. 43.

Once you have completed the third floor tour, proceed down the stairs to the second
floor–east wing where you will see the Orbicular Gabbro specimen on your left. A floor
plan is provided on p. 36.

Second Floor Exhibits–East Wing

■ Orbicular Gabbro
This specimen of orbicular gabbro was found near El Cajon, in southern Cali-
fornia. Gabbro, a term describing any dark, coarse-grained igneous rock, forms
deep in Earth’s crust from the slow cooling and crystallization of iron- and
manganese-rich fluid magmas. The minerals in the rings you see formed in

38 VISITOR GUIDE