Plate tectonics are the phenomenon of the plates of the earth’s lithosphere moving around and colliding, causing tectonics (upheavals) that form the texture of the earth that we see today, evident in canyons, trenches, and volcanoes. Unable to collide together anymore`, subduction zones are formed when one plate slides beneath the other. Movements between the two plate boundaries are designated based on the relative movement to each other.

Convergent boundaries move toward each other and eventually start to move upward together. Divergent boundaries are when parts of the lithosphere pull apart, exposing the asthenosphere and allowing magma to force its way up through the weakened areas. Transverse plate boundaries move in parallel to each other, either in the same or opposite directions, as well as travelling toward each other. They are the most destructive type of boundary, causing erosion wherever the two plates meet.

The Cascade Ridge was formed by a transverse plate boundary, with overlapping sheets and lots of offshore sediment as the plates slide back and forth and subduct. The coast range was formed from friction caused by the oceanic plate descending under the continental plate, and the minerals floating up to the top suspended in liquid. The High Plateau or Columbia Plateau is a wide swath of basalt between the Rocky Mountains and Cascade Range, schismed by the Columbia River. It was formed over millions of years as lava flowed out of the earth, slowly building and flattening.

There are three basic types of rock.


Igneous rock is directly formed by cooled magma, forming crystals. It is an exothermic reaction (expelling heat) and changes to a solid state from a liquid one.


Sedimentary rock is often referred to as secondary, due to its composition by layers of geologic debris. Many different forces and different components make up sedimentary rock.


Metamorphic rock could include just about any rock that has undergone a physical and chemical transformation. Metamorphic rock has a loose definition in that rock needs to move to a new environment in order to be considered metamorphic rock.


In addition to different types of rocks, they can be further categorized into where in the layers of the earth they were formed. Mostly igneous rocks, intrusive rocks are formed by magma crystallization deep in the earth. It’s slower to cool than at the surface, allowing crystals to grow. Extrusive rocks are formed at the surface of the rocks, and form grains rather than crystals, due to the increased speed of cooling.


Types of igneous rock

Name % Silica Intrusive/extrusive
Granite >63% Intrusive
Rhyolite >63% Extrusive
Gabbro 45-52% Intrusive
Basalt 45-52% Extrusive
Diorite 52-63% Intrusive
Andesite 52-63% Extrusive

Intrusive rock is fine

Extrusive is coarse

Viscosity is a liquid’s ability or inability to move under stress; it also could be thought of as malleability. Water has a low viscosity and high malleability, while something like molasses has a high viscosity and low malleability. A higher amount of silica translates into a higher viscosity of magma, because of the incredibly strong bonds between silica and oxygen. Viscosity affects the look of volcanoes because of the speed of lava flow. As seen below, slower flowing lava creates gentler slopes over time, whereas fast lava flows produce a steep slope over time.


Types of volcanoes:

Type How it’s formed Examples in the PNW
Shield Formed from basaltic flow. Slow lava flow over time. Broad and gently sloping. Belknap Shield Volcano, Newberry Caldera, Mt. Washington & 3-fingered Jack
Cinder cones Built of fragmented material, contain different sized rocks, and fine volcanic ash. Three Sisters
Stratovolcanoes Layers of tephra and other material. High and steep. Mts. Rainier, Shasta, Hood, St. Helens
Pyroclastic “Fire broken” in Greek. Lava erupts in fragments.
Lava domes Single mass of lava from periods of activity. Pilies up over vent. Crater Lake,



A field guide

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