Plate tectonics 101—What happens when plates slide past each other?
Oceanic crust is created at divergent plate boundaries and destroyed at convergent boundaries, but transform boundaries cause neither; rather, they transform motion between other types of plate boundaries, enabling plates to move about efficiently on our spherical planet. In this post, we will explore various examples. The majority of transform boundaries are found in the ocean, where they separate segments...
Plate tectonics 101—what happens when plates move toward each other?
When plates move away from each other (divergent boundaries; see last post on December 31), new oceanic crust is created. Since Earth is not expanding, ocean crust must be destroyed elsewhere. This happens at convergent boundaries, where plates move toward each other. Here, oceanic crust is destroyed, or rather, it gets recycled back into Earth's depths. The places where oceanic...
Plate tectonics 101—what happens when plates move away from each other?
It's winter in the midst of a pandemic, and travel is restricted. So it seems a good time to go back to some geologic basics, the knowledge of which are sometimes assumed in these blog posts. The next four posts will explore the essentials of plate tectonics—the underpinning of our modern geologic understanding. Plate tectonics refers to the processes that...
The geology of Point Reyes National Seashore
Although not as widely known as Yellowstone and Yosemite, Point Reyes National Seashore is particularly loved by residents of the San Francisco Bay Area. Just an hour's drive from downtown San Francisco, Point Reyes feels a world away and provides a welcome refuge from urban life. Most also know that the San Andreas fault (SAF) extends through the park and...
The geology of Yosemite National Park
With stunning vistas of shear rock walls, cascading waterfalls, rounded granite domes and jagged spires, it's no surprise that Yosemite is one of the most popular national parks in the U.S. While most visitors are drawn to Yosemite Valley, the park has numerous other wonders beyond the valley and even the park boundaries in the Sierra Nevada (=snowy mountain range...
The geology of Yellowstone National Park, and its precursors in Oregon and Idaho
Yellowstone has the distinction of being the first national park in the U.S., and probably in the world. President Ulysses S. Grant signed the bill to protect more than one million acres—mostly in Wyoming—in 1872. Yellowstone was set aside because of its hydrothermal displays—the park contains more than 10,000 features, including the world’s greatest concentration of geysers, hot springs, mud...
The Geology of Crater Lake National Park
Like most of our national parks, the spectacular beauty of Oregon's only national park is a result of the geologic processes that created it. But whereas most of our parks were created by gradual processes over long periods of time—for example, erosion by the Colorado River to create the Grand Canyon—Crater Lake was created in a few days during a...
South Sister—the third highest volcano in Oregon and one of its youngest
The Three Sisters volcanoes, located in the Three Sisters Wilderness west of Bend, Oregon, are part of the Cascade Range of volcanoes formed above the Cascadia subduction zone. "For magnificence of glacial scenery, for wealth of recent lavas, and for graphic examples of dissected volcanoes, no part of this range surpasses the area embracing the Sisters and McKenzie Pass", said...
Lassen Volcanic National Park—the southernmost Cascade Range volcano
The Cascadia Subduction zone extends along the entire coast of Washington and Oregon, but also along the northern coast of California. South of Mendocino, California, the plates change from converging with each other (the ocean plate loses and slides beneath the continental plate in the subduction zone) to sliding past each other along the San Andreas fault (a transform-type plate...
How does the Rogue Valley fit into the larger tectonic picture?
In the last three posts, we explored the geologic makeup of the Rogue Valley region. Most people know that there is a subduction zone off the coast of the Pacific NW and you might wonder: how does the Rogue Valley region fit into this larger picture? We will explore the tectonic setting in this post. This diagram shows the present...
Sedimentary rocks tell stories about the Rogue Valley region’s geologic history
In the previous two posts, we explored the distribution of rock units in the Rogue Valley region, and how the different resistances of these rock types to weathering and erosion has created the topographic variations we see in the landscape. The different rock types also produce a large variety of soil types that are important for gardeners and vintners in...
Why is the Rogue Valley a valley?
In the April 19 post, we examined the geologic units that are exposed in the Rogue Valley region, and saw that the units are all tilted toward the NE, with the oldest units located to the SW (between the valley and the coast), and the younger units located toward the NE (toward the active volcanoes of the High Cascades like...
About the Blogger
Karen (here with Mt. Shasta in background) is a geology professor emerita who aims to provide a "pocket geologist" for world travelers. Follow the blog to explore the landscapes of our planet and figure out what causes them to look the way they do.