Geology Reference
In-Depth Information
the cycle. In this province, the very fluid basaltic lavas
predominate, but rhyolitic extrusions and domes are
situated along major fracture zones. The association of
basalt and rhyolite is is rare and usually occurs where
the earth's crust thins because it is undergoing tension
and being stretched.
Comparatively young eruptions and intrusions
here between the upper Miocene and Recent occur in
a broad northern belt of approximately 100 centers
trending northwest across the Lava Plains and Owyhee
Uplands. One of the most striking aspects of the High
Lava Plains is the uniform decrease in the age of these
volcanic eruptive centers geographically from east to
west. Within the Harney Basin in the east, rhyolitic
eruptions date back to 10 million years ago in the late
Miocene, while near Newberry Crater in the west many
lavas were extruded less than 1 million years ago. The
eruptive zone moved steadily from the southeast
toward the northwest at slightly more than one mile
per 100,000 years. Such a progression of eruptions
might be seen as an earth crustal plate moving over a
hot spot, but two important aspects of local geology
seem to preclude this notion.
First, it is well established that the North
American plate, upon which this province rests, has
moved progressively westward since well before Mio-
cene time. The age progression should then be reversed
with younger volcanics appearing in the east instead of
in the west, as is the case with the Yellowstone hot
spot. Additionally, another broad belt of rhyolitic and
silicic domes to the south in the Basin and Range
province between Beatys Butte in the southeast and
Age progression of silicic volcanic centers in the
High Lava Plains and Basin and Range from the
oldest (upper Miocene) in the southeast to the youn-
gest (Pleistocene and Holocene) in the northwest.
Contoured lines of simultaneous volcanic activity are
in millions of years. (modified after Walker and Nolf,
1981).
