Basic Geologic Principles Relevant to the PNW
Layers of the Earth and Convection
One of the core (pun intended) tenets of Geology is the theory of Plate Tectonics. Plate Tectonics seeks to understand how major landforms are created on Earth, and it turns out that major landforms are created by subterranean movement of large slabs of earth- or tectonic plates. Before wrapping your head around what a tectonic plate is, it’s important to understand that the Earth is composed of 3 main layers- the solid core, the molten mantle, and the solid crust. Tectonic plates compose the third, outermost layer of the earth- the crust. Rather than the crust being one continuous solid entity floating atop the mantle, the crust is “cut up” and composed of several slabs of solid rock that continuously move around, interacting with one another. These slabs are propelled by convection in the molten part of the earth (mantle). Convection in Geology is the phenomenon of heat transfer through the mantle from the core to the crust (naturally, heat rises- the core is VERY hot, the crust, not so much). This heat transfer creates currents in the mantle like currents in the ocean- consider these slabs of solid rock that move atop the mantle akin to boats on the ocean. Now, imagine if these “boats” were as large as continents, while constantly moving due to subjugation of currents in this proverbial ocean. Now imagine if these “boats” were made of solid rock and were floating on an ocean of molten rock. That’s plate tectonics. The aforementioned slabs of solid rock (“boats”) that float atop the mantle (“proverbial ocean”) are tectonic plates.
Chemistry
There are two main types of tectonic plates, both types distinct due to a litany of physical, chemical, and structural characteristics; continental and oceanic plates (also referred to as continental and oceanic crust). Generally speaking, continental plates (or crust) are chiefly composed of silicon-rich rock known as granite and rhyolite, while oceanic plates (or crust) are chiefly composed of iron-rich rock known as gabbro and basalt. At a fundamental level of chemistry with regard to the Periodic Table, Silicon is element 14 on the periodic table, whilst Iron is element 26. Silicon’s atomic mass is 35.45 amu, while Iron’s is 55.85 amu. Iron is denser and heavier than Silicon at an atomic scale, and this is reflected on a planetary scale in regards to plate tectonics. Part of the reason I love science so much is how phenomena at minute scales are reflected on colossal scales, just like this. Continental plates (silicon-rich) are much lighter and more buoyant than the heavier, denser oceanic plates (iron-rich). As such, when continental and oceanic plates collide, oceanic plates are subducted- key word- underneath continental plates because they’re so much heavier, denser, and colder.
Physics & Types of Plate Boundaries
All of the action with regards to Plate Tectonics happens in areas where tectonic plates collide with, slide past, or spread apart from another. In the middle of a large tectonic plate, things are actually quite boring, landscape-wise (think of the Great Plains, smack dab in the middle of the North American Plate), but at plate boundaries, interesting things occur, geologically speaking. As aforementioned, tectonic plates can collide with each other, slide past one another, or spread apart from each other. Each type of plate boundary is unique and has defining characteristics. Areas where tectonic plates collide with one another are called Convergent Plate Boundaries. Plate Boundaries where the involved tectonic plates slide past one another are called Transform Plate Boundaries, and areas where plates are spreading apart from each other are called Divergent Plate Boundaries. Each type of Plate Boundary of course has more complicated components to it, but these are the basic types of tectonic plate boundary. Furthermore, each type of boundary has defining geologic structures and characteristics.
​
The type of plate boundary we are interested in for all intents and purposes of discussing the Pacific Northwest’s fundamental geology is the Convergent Plate Boundary- where two tectonic plates collide. Even more specifically, we are interested in an Oceanic-Continental Convergent Plate Boundary. This type of plate boundary forms a Subduction Zone, or a collision between two tectonic plates where one plate is forced underneath the other. At subduction zones, the colder, denser, oceanic plate is subducted underneath the warmer, lighter, continental plate. During this process, several geologic structures are formed and a litany of geologic phenomena ensues. Defining geologic structures of this type of plate boundary include Volcanic Arcs, Accretionary Wedges, Fore-Arc Basins, and Offshore Trenches, while processes such as Volcanic Eruptions, Earthquakes, and Slow Slip [also known as Episodic Tremor and Slip (ETS)] are present throughout the region. In Washington and the Pacific Northwest in general, the oceanic Juan De Fuca Plate and the continental North American Plate are the involved parties. The subduction zone here is referred to as the Cascadia Subduction Zone. Three foreboding words if you’re a geologically-conscious resident of the Pacific Northwest, like myself.
Diagram of the features of a Subduction Zone, taken from the National Park Service (NPS).