And back to another classic Washington locale- the North Cascades as viewed from the top of Maple Pass!
Geologically, the North Cascades are divided up into three provinces, or domains (depending on which term you’d rather use). The Western Domain (west of Marblemount) is a mix of accreted rocks, primarily sedimentary and metamorphosed sedimentary and volcanic rocks that were originally part of or on top of the ocean floor during the Mesozoic, about 250- 80 million years ago before being added to the North American continent.
The Central Domain, or Crystalline Core, is a beautiful set of igneous and metamorphic rocks that are a representation of the deep and sometimes tumultuous core of a vast mountain range. A mountain range similar to the southern Cascades today, but that formed around 200- 100 million years ago and a couple thousand kilometers to the south! (Likely, this is the Baja-BC Hypothesis and the “tropical” origin of some of the rocks, Mount Stuart is a great example that we’ve talked about before!)
East of the Crystalline Core is the Methow Domain, mainly Cretaceous (~140- 80 million year old) rocks of an ocean basin and the sediments deposited in that basin. Though, unlike the Western Domain and Crystalline Core, much of the Methow Domain is surprisingly not metamorphosed… but quite deformed nonetheless.
Separating these three domains are large crustal-scale faults, some with a very significant component of vertical offset between rocks on either side. And it’s this vertical offset that allows us to peer into the deep core of a mountain range!
Volcanic mountain ranges like the Cascades form when a piece of the oceanic crust is colliding with a piece of continental crust. Since the oceanic crust is slightly denser, it is shoved underneath the continent in a process of subduction. This process also generates magma, which works its way up through the continent and forms the volcanoes that we are so familiar with in the Pacific Northwest.
But what’s going on really deep within the continent? Well, a few things! Magma that doesn’t work its way to the surface can cool and crystallize, forming large bodies of granitic-adjacent* composition rocks known as batholiths (*I’ll have to do a post later on all the varieties of “granite”). Rocks that were already present in the continent are squeezed by the collision and forced downward due to the thickening of the crust. These rocks and parts of the batholiths are subject to pretty intense pressures and temperatures in the lower parts of the crust, and will experience varying degrees of metamorphism- depending on how deep the rocks got! This process typically forms different types of gneiss, a high-grade metamorphic rock readily identified by parallel or wavy banding of light- and dark-colored minerals.
And later on, geologically speaking, if these rocks are uplifted along faults and eroded by glaciers (hey, like in the North Cascades!) then we get to observe this history of melt, crystallization, and intense metamorphism that was taking place over 20 kilometers below the surface!
Looking southwest from Maple Pass, one is staring into the Crystalline Core of the North Cascades. Many of the jagged, glaciated peaks in the background are varieties of gneiss and other rocks that formed deep within the crust. And for good measure, there’s also Glacier Peak- one of the modern Cascades volcanoes! Fun to think that the processes that initially formed the Crystalline Core of the North Cascades millions of years ago are still going on today! Bet if you dug 20 or so kilometers down you’d find a pretty geologically violent environment, with enough pressure and heat to partially melt and deform rocks…
And it’s this “violent” environment that’s likely the next topic, along with one of my personal favorite rocks that forms there- migmatite!