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Rock Box

These volcanic dacite boulders originated from
Lassen Peak and were transported here during the
May 1915 eruptions. Four types of dacite boulders
are described below.

Gray Dacite - unaltered
pieces of Lassen Peak dug
out of the mountain's
interior by glacial plucking.

Red Dacite - oxidized
pieces of lava from Lassen
Peak's exterior. Iron
minerals throughout the hot
lava were oxidized upon
contact with air.

Black Dacite - contains a
high amount of volcanic
glass. These rocks came
from a small dacite dome
that was emplaced from
May 14 to 19, 1915 in the
crater at the summit of
Lassen Peak.

Altered White to Tan
Dacite - oxidation by steam
and extremely hot water
created a much lighter
colored rock. Alteration
occurred when venting
steam came in contact with
Lassen Peak's older lava.

Flow-Banded Pumice -
dark and light banding of
these rocks formed when
two chemically different
types of magma were
mixed by not blended.
During the May 22, 1915
eruption, bombs of this
pumice were showered into
the air and fell back onto
the steep slopes of Lassen
Peak generating destructive
flows of pumice and hot
gases that swept through
the Devastated Area.

Don't miss the rest of our virtual tour of Lassen Volcanic Park in 1699 images.