{"id":195,"date":"2021-09-16T19:28:48","date_gmt":"2021-09-16T19:28:48","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accphysicalgeography\/chapter\/4-6-volcanoes-in-british-columbia-physical-geology-2nd-edition\/"},"modified":"2026-05-26T13:43:04","modified_gmt":"2026-05-26T13:43:04","slug":"4-6-volcanoes-in-british-columbia-physical-geology-2nd-edition","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/chapter\/4-6-volcanoes-in-british-columbia-physical-geology-2nd-edition\/","title":{"raw":"4.6 Volcanoes in Colorado \u2014 Physical Geology \u2013 2nd Edition","rendered":"4.6 Volcanoes in Colorado \u2014 Physical Geology \u2013 2nd Edition"},"content":{"raw":"
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4.6 Volcanoes in Colorado<\/h1>\r\nIf you were to drive around Colorado in search of volcanoes, you would find several spots of extrusive igneous rocks, but you would be very hard-pressed to find physical volcanoes! Furthermore, if you review section 4.1, recall that we are nowhere near a plate boundary! We are also not over a hot spot, like Yellowstone National Park is. It turns out that Colorado has been host to a variety of volcanic activity in the \"recent\" geological past; a testament to the changing dynamics of plate tectonics.\r\n\r\nThe majority of Colorado's recent volcanic past is associated with a very unusual episode of subduction, off the modern-day California coast. Starting in the Mesozoic Era, a now non-existent tectonic plate, called the Farallon Plate, was subducting underneath California (where the San Andreas Fault is now). What made this subduction zone unique is that it occurred at a much shallower angle than normal, resulting in flat-slab subduction<\/strong>. The consensus on why this happened largely revolves around the westward-moving North American plate started to move somewhat faster, which flattens the angle of the subducting Farallon Plate. Also, it is widely thought that the Farallon Plate had thick, low-density oceanic plateaus that resisted sinking which also contributed to slab flattening. The shallow subduction caused intense compression between the two plates, leading to a thicker continental crust than a usual subduction zone.\r\n\r\nA good model to visualize shallow (or flat) slab subduction is to put your foot slightly underneath an edge of an area rug. The rug represents the overriding North American plate, and your foot is the subducting Farallon Plate. If you take your flat foot and dragged it forward deeper under the rug, you'll notice the rug will probably develop folds or wrinkles. Before doing this though, guess to yourself \"will the folds appear at the edge of the carpet or more towards the center?\" If you hypothesized \"near the center\", you are correct! The edge of the carpet is the California west coast. The center of the carpet, where the folds would appear are the Rocky Mountains of Colorado. Of course, the tectonic history and the formation of the Rockies is way more complex than what is presented here, but this model will suffice to explain the state's unusual volcanism.\r\n\r\nThis flat slab subduction not only thickened the North American lithosphere, but also eliminated the mantle wedge between the two plates present during a normal subduction scenario. Recall the asthenosphere behaves like a hot plastic that flows very slowly. The flat slab subduction would force the asthenosphere underneath the base of the sinking Farallon Plate, where it would pond, accumulating an anomalous amount of heat that remains trapped there. Around 50 Ma ago, the flat-slab subduction stopped, and the front of the Farallon Plate snapped off in a process called delamination<\/strong>. We'll discuss this process more in Chapter 10.\r\n\r\nOnce the space that the Farallon Plate occupied became open, the ponded asthenosphere material rushed in and started to make its way to the surface. However, the flat slab subduction beforehand made the continental crust thicker than average, making the ascending magma travel slower than anormal subduction zone. This slower speed allowed the magma to incorporate more silica-rich continental crust material. When the felsic magma finally erupted, it did so with power comparable or even exceeding Yellowstone's most recent eruption 640,000 years ago. One grand example found in Colorado is the Fish Canyon Tuff located in the Wheeler Geologic Area, near the town of Creede (Figure 4.6.1.). The eruption that produced this formation occurred roughly 28 Ma ago erupted over 1,000 times the material that the 1980 Mt. St. Helens eruption made!\r\n\r\n \"File:My\r\n\r\nFigure 4.6.1a (left) The Fish Canyon tuff has weathered out to produce this canyon in the Wheeler Geologic Area, near Creede, CO. Figure 4.6.1b (right) A close-up of a portion of the Fish Canyon Tuff in the Wheeler Geologic Area. Here, the specific rock exposed is rhyolite; a hard rock that is popular amongst recreational climbers.\r\n\r\n \r\n\r\nNot only were these eruptions mega-powerful but occurred in swarms all throughout the western U.S. in a geologic event known at the Mid-Tertiary Ignimbrite Flare-Up <\/strong>(The term ignimbrite<\/em> is Latin for rain of fire<\/em>). This event occurred from 40-18 Ma ago and is thought that over 230 eruptive centers have been identified in this part of the country. The collective calculated volume of material erupted from the flare-up episode is thought to be enough to cover the lower 48 states over 55 meters!\r\n\r\nAnother noteworthy eruption was centered around present day Mt. Harvard, near Buena Vista. The eruption that took place here around 36.1 Ma ago also produced copious quantities of rhyolite and welded tuff that produced the Thirtynine Mile Volcanic Field. The most significant deposits are observed in the town of Florissant, specifically with Florissant Fossil Beds National Monument (Figure 4.6.2.). The pyroclastic flow traveled all the way to present-day Castle Rock where rhyolite from the eruption can be observed at Castlewood Canyon State Park.\r\n\r\nThe two highlighted areas above are only two of many such areas of significant pyroclastic deposits produced by the Mid-Tertiary Ignimbrite Flare-up.\r\n\r\n\"File:Lahar\r\n\r\n<\/div>\r\n<\/div>\r\nFigure 4.6.2 A preserved lahar deposit from the Thirtynine Mile Volcanic Field observed in an outcrop along Highway 24 in the town of Florissant, CO. Note the vast array of clast sizes and the ashy fine, muddy matrix.\r\n\r\n \r\n\r\nThe most recent volcanic activity in Colorado occurred roughly 4,000 years ago near the town of Dotsero. Here, basaltic lava flows cut across Interstate 70. If you get off the only exit to Dotsero and look north, you'll notice a rusty-red high mount (Figure 4.6.3). This is the cinder cone that the 4,000-year-old eruption had produced.\r\n\r\n\"File:Dotsero\r\n\r\nFigure 4.6.3. The Dotsero cinder cone as seen from the top of one of its flanks. Note the rusty red color of the rock. This is composed of the igneous rock scoria<\/strong>.\r\n\r\n \r\n\r\nIf we revisit our above discussion of the Farallon Plate, recall that the shallow subduction resulted in increased compression between that and the North American Plate. This anomalous compression is like winding up a coiled spring with loads of stored up energy. When the subduction stopped, the spring recoils and stretches back out! That is what is currently happening to a section of the western North American crust; it is undergoing crustal extension even though we are not near an actual divergent boundary! This said stretching section of the crust is called the Rio Grande Rift <\/strong>(Figure 4.6.5), and it started around 37-35 Ma ago. If you observe this map, the Dotsero cinder cone is at the very northern tip of the rift, where basaltic lava has squeezed out towards the surface as a result of localized crustal thinning.\r\n\r\n<\/div>\r\n\r\n\r\nFigure 4.6.4. A map showing the extent of the Rio Grande Rift. The rifting is continuing today.\r\n
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Media Attributions<\/h3>\r\n
    \r\n \t
  • Figure 4.6.1a,b, 4.6.2, 4.6.3, 4.6.4: Wikimedia Commons<\/li>\r\n<\/ul>\r\n<\/div>\r\n","rendered":"
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    \n
    \n

    4.6 Volcanoes in Colorado<\/h1>\n

    If you were to drive around Colorado in search of volcanoes, you would find several spots of extrusive igneous rocks, but you would be very hard-pressed to find physical volcanoes! Furthermore, if you review section 4.1, recall that we are nowhere near a plate boundary! We are also not over a hot spot, like Yellowstone National Park is. It turns out that Colorado has been host to a variety of volcanic activity in the “recent” geological past; a testament to the changing dynamics of plate tectonics.<\/p>\n

    The majority of Colorado’s recent volcanic past is associated with a very unusual episode of subduction, off the modern-day California coast. Starting in the Mesozoic Era, a now non-existent tectonic plate, called the Farallon Plate, was subducting underneath California (where the San Andreas Fault is now). What made this subduction zone unique is that it occurred at a much shallower angle than normal, resulting in flat-slab subduction<\/strong>. The consensus on why this happened largely revolves around the westward-moving North American plate started to move somewhat faster, which flattens the angle of the subducting Farallon Plate. Also, it is widely thought that the Farallon Plate had thick, low-density oceanic plateaus that resisted sinking which also contributed to slab flattening. The shallow subduction caused intense compression between the two plates, leading to a thicker continental crust than a usual subduction zone.<\/p>\n

    A good model to visualize shallow (or flat) slab subduction is to put your foot slightly underneath an edge of an area rug. The rug represents the overriding North American plate, and your foot is the subducting Farallon Plate. If you take your flat foot and dragged it forward deeper under the rug, you’ll notice the rug will probably develop folds or wrinkles. Before doing this though, guess to yourself “will the folds appear at the edge of the carpet or more towards the center?” If you hypothesized “near the center”, you are correct! The edge of the carpet is the California west coast. The center of the carpet, where the folds would appear are the Rocky Mountains of Colorado. Of course, the tectonic history and the formation of the Rockies is way more complex than what is presented here, but this model will suffice to explain the state’s unusual volcanism.<\/p>\n

    This flat slab subduction not only thickened the North American lithosphere, but also eliminated the mantle wedge between the two plates present during a normal subduction scenario. Recall the asthenosphere behaves like a hot plastic that flows very slowly. The flat slab subduction would force the asthenosphere underneath the base of the sinking Farallon Plate, where it would pond, accumulating an anomalous amount of heat that remains trapped there. Around 50 Ma ago, the flat-slab subduction stopped, and the front of the Farallon Plate snapped off in a process called delamination<\/strong>. We’ll discuss this process more in Chapter 10.<\/p>\n

    Once the space that the Farallon Plate occupied became open, the ponded asthenosphere material rushed in and started to make its way to the surface. However, the flat slab subduction beforehand made the continental crust thicker than average, making the ascending magma travel slower than anormal subduction zone. This slower speed allowed the magma to incorporate more silica-rich continental crust material. When the felsic magma finally erupted, it did so with power comparable or even exceeding Yellowstone’s most recent eruption 640,000 years ago. One grand example found in Colorado is the Fish Canyon Tuff located in the Wheeler Geologic Area, near the town of Creede (Figure 4.6.1.). The eruption that produced this formation occurred roughly 28 Ma ago erupted over 1,000 times the material that the 1980 Mt. St. Helens eruption made!<\/p>\n

    \"image\" \"File:My<\/p>\n

    Figure 4.6.1a (left) The Fish Canyon tuff has weathered out to produce this canyon in the Wheeler Geologic Area, near Creede, CO. Figure 4.6.1b (right) A close-up of a portion of the Fish Canyon Tuff in the Wheeler Geologic Area. Here, the specific rock exposed is rhyolite; a hard rock that is popular amongst recreational climbers.<\/p>\n

     <\/p>\n

    Not only were these eruptions mega-powerful but occurred in swarms all throughout the western U.S. in a geologic event known at the Mid-Tertiary Ignimbrite Flare-Up <\/strong>(The term ignimbrite<\/em> is Latin for rain of fire<\/em>). This event occurred from 40-18 Ma ago and is thought that over 230 eruptive centers have been identified in this part of the country. The collective calculated volume of material erupted from the flare-up episode is thought to be enough to cover the lower 48 states over 55 meters!<\/p>\n

    Another noteworthy eruption was centered around present day Mt. Harvard, near Buena Vista. The eruption that took place here around 36.1 Ma ago also produced copious quantities of rhyolite and welded tuff that produced the Thirtynine Mile Volcanic Field. The most significant deposits are observed in the town of Florissant, specifically with Florissant Fossil Beds National Monument (Figure 4.6.2.). The pyroclastic flow traveled all the way to present-day Castle Rock where rhyolite from the eruption can be observed at Castlewood Canyon State Park.<\/p>\n

    The two highlighted areas above are only two of many such areas of significant pyroclastic deposits produced by the Mid-Tertiary Ignimbrite Flare-up.<\/p>\n

    \"File:Lahar<\/p>\n<\/div>\n<\/div>\n

    Figure 4.6.2 A preserved lahar deposit from the Thirtynine Mile Volcanic Field observed in an outcrop along Highway 24 in the town of Florissant, CO. Note the vast array of clast sizes and the ashy fine, muddy matrix.<\/p>\n

     <\/p>\n

    The most recent volcanic activity in Colorado occurred roughly 4,000 years ago near the town of Dotsero. Here, basaltic lava flows cut across Interstate 70. If you get off the only exit to Dotsero and look north, you’ll notice a rusty-red high mount (Figure 4.6.3). This is the cinder cone that the 4,000-year-old eruption had produced.<\/p>\n

    \"File:Dotsero<\/p>\n

    Figure 4.6.3. The Dotsero cinder cone as seen from the top of one of its flanks. Note the rusty red color of the rock. This is composed of the igneous rock scoria<\/strong>.<\/p>\n

     <\/p>\n

    If we revisit our above discussion of the Farallon Plate, recall that the shallow subduction resulted in increased compression between that and the North American Plate. This anomalous compression is like winding up a coiled spring with loads of stored up energy. When the subduction stopped, the spring recoils and stretches back out! That is what is currently happening to a section of the western North American crust; it is undergoing crustal extension even though we are not near an actual divergent boundary! This said stretching section of the crust is called the Rio Grande Rift <\/strong>(Figure 4.6.5), and it started around 37-35 Ma ago. If you observe this map, the Dotsero cinder cone is at the very northern tip of the rift, where basaltic lava has squeezed out towards the surface as a result of localized crustal thinning.<\/p>\n<\/div>\n

    \"image\"<\/p>\n

    Figure 4.6.4. A map showing the extent of the Rio Grande Rift. The rifting is continuing today.<\/p>\n

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    Media Attributions<\/h3>\n
      \n
    • Figure 4.6.1a,b, 4.6.2, 4.6.3, 4.6.4: Wikimedia Commons<\/li>\n<\/ul>\n<\/div>\n

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