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FAQ about Highlands Geology

Q: When I look around in the Highlands, sometimes I think I’m seeing ancient volcanoes. Is this true?

A: In reality, there are no volcanoes around there. That is the first point to get across…

A more subtle point to get across is how, in that area, there are rocks that are related to volcanoes. This where it gets interesting, though a bit complicated. The rocks there relate to volcanoes in a couple of different ways.

Down near Tonasket and in a few places along the sides of the Okanogan River valley, including Coleman Butte and Shell Rock by the towns of Omak and Okanogan, are some outcrops of Eocene volcanic rock. Those volcanic rocks are the remains of lava flows and leftovers from eruptions of ash and pumice. These volcanic rocks are incomplete parts of volcanic centers that erupted 45 million (+/- 5 million) years ago and have since largely eroded away. That is the first type of rock in that area that relates to volcanoes. Those are volcanic rocks, but the volcanoes themselves have otherwise eroded away.

Higher up in the Highlands east of the Okanogan Valley, the high points that can be seen from the Havillah area are not made of volcanic rocks. The hills and mountaintops around Havillah are made of granite. (Or, as a side note, I should say that they are mostly made of rocks that are close relatives of granite, having a generally similar appearance and originating the same way as granite. Though some of these rocks have different amounts of key minerals and different mixtures of major elements than true granite, the distinctions may be too technical for those who are not rock aficionados or geology students, so we will just call them all granite.)

A couple of the local bodies of granite are known as the Mt. Bonaparte Pluton and the pluton of Mt. Hull. These plutons originated as intrusions of molten magma miles deep inside the crust of the Earth. The magma (molten rock) crystallized (turned solid) down there to become granite. Far above, on the Earth’s surface, there may have been volcanoes related to these deep intrusions of molten rock that were becoming granite down below. The granite of the Mt. Bonaparte Pluton has an Eocene age (in the range 45-55 million years old) overlapping the age of some of the volcanic rocks in the area, though chemical analyses have not yet established any direct connection.

There is also a lot of gneiss in that area. Gneiss is a metamorphic rock, pronounced “nice.” Most of the gneiss in that area started out as granite but went through another stage of heating and squeezing deep in the crust, perhaps as a result of plates continuing to push on the crust in the general area and terranes being shoved onto the edge of the continent up above. The gneiss looks kind of like granite because it typically has same minerals as granite. But the gneiss is layered and folded, like a good metamorphic rock that got squeezed and pushed around like taffy, whereas granite is a massive rock without layers.

Pretty complicated. Like I said, there are volcanic rocks, of Eocene age, west of there, but not in those high hills or mountains that Roger refers to.

There are igneous rock holding up some of those high points that can be seen in the Havillah area, but the igneous rock is granite, an intrusive igneous rock, not a volcanic rock.

In general, the high parts of the Okanogan Highlands in that area tend to be made of granite, or gneiss that used to be granite, because those are hard, massive rocks, made of mostly hard minerals all intricately crystallized together in an interlocking way. Granite and gneiss like that have fewer weak layers or cracks compared to most rocks. So, after millions of years of erosion, the higher hills and mountains tend to be on the parts of the crust made of that kind of rock.

Thank you for relaying community members’ interest in possible volcanoes in his area. It leads to some in-depth geology!

– Dr. Dawes

education, FAQ, geology, volcanoes

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