With very (very!) broad definitions, rocks come in 2 main types: Crystalline (composed of interlocking crystals of one or more minerals); and clastic (composed of pieces of other rocks that have been cemented together). And different types of rocks can broadly be explained and classified by the Rock Cycle.

Liquid rock (magma in Earth’s interior, lava on the surface) cools and crystallizes to form igneous rocks; rocks at the surface break down into smaller rocks and can become cemented together to form sedimentary rocks; and rocks in Earth’s crust can be subject to intense pressures and temperatures which produces a change in mineral composition and forms metamorphic rocks!

However- it’s very easy to interpret the rock cycle that we tend to learn as 3 distinct types of rock, and not focus on the in-betweens. In reality, the rock cycle is more of a continuum. For example, when rocks melt to form magma it is a long process with many different stages. Each different mineral has a different melting temperature, so as a body of rock heats up different minerals will melt at different times producing a mush of liquid and solid rock- a process called “partial melting.” And the same process works in reverse! As magma cools different minerals will crystallize from the melt at different times, again producing a mush that is part igneous rock and part magma.

It’s actually some of these in-between rocks that can tell us a huge amount about a region’s geologic past!

Take for instance, the Thunder Knob trail just off of Highway 20 in the North Cascades. We’re still in the Crystalline Core of the mountain range- igneous and metamorphic rocks that formed in the Cretaceous period deep below an ancient mountain range similar to the modern Cascades. Definitely look down at your feet for parts of this hike!

These rocks are banded gneiss, a high-grade (very thoroughly altered) metamorphic rock characterized by wavy bands of light- and dark- colored minerals. To get an idea of the intense environment where these rocks formed, consider that the original location of these rocks were over 20 kilometers deep within the crust! So intense, that the temperatures there were high enough to start melting some of the minerals in the rock.

But not enough to melt the entire rock!

This particular type of partially-melted metamorphic rock is named a migmatite. Many of the lighter-colored mineral bands in the rock melted during its formation, then cooled in place. With some bigger exposures of this rock (the one across the highway from the Diablo Lake overlook is one of my favorites!), one can notice these swirly bands of light colored minerals, almost as if the rock had once been in a giant, heated blender!

Also something specific to the gneiss and migmatite around Diablo and Gorge Lakes, many portions of it are garnet-bearing….. so if you’re a fan of small, shiny, dark red garnets this is a great location to look for them! Not gemstone quality, but pretty nonetheless 🙂