Although taking its name from the Cascade Range, this term is a geologic grouping rather than a geographic one, and the Cascade Volcanoes extend north into the Coast Mountains, past the Fraser River which is the northward limit of the Cascade Range proper. The arc has formed due to subduction along the Cascadia subduction zone.
The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 miles (1,100 km). There are some sedimentary rocks in the North Cascades as well the Methow Valley contains a thick layer of Cretaceous sedimentary rocks such as shale, sandstone, and conglomerate.Oregon, Washington, California, British ColumbiaĤ6★1′1.9″N 121☄5′35.6″W / 46.850528°N 121.759889°W / 46.850528 -121.759889
Baker and Glacier Peak, are made of basalt and andesite as well as volcanic breccia, a pyroclastic rock composed of volcanic fragments from an explosive eruption. The volcanoes of the North Cascades, such as Mt. Numerous magmatic intrusions left behind intrusive rocks such as granite, diorite, and gabbro, as well as causing contact metamorphism. The collision that accreted these terranes created intense pressure, leading to metamorphic conditions that formed rocks such as slate, phyllite, schist, gneiss, and marble. The North Cascade Range, although it has a few prominent volcanoes, is predominantly an uplifted block of accreted terranes. The old volcanic remnants of the Western Cascades provide us with a glimpse of what the current High Cascades will look like in about 18 million years. The oldest outcrops, 18–40 million years old, contain many pyroclastic rocks. The Western Cascades are made up of old, extinct, and highly eroded volcanoes. Overall, the flows from Belknap Crater-and its sister craters, Little Belknap and Yapoah-cover nearly 200 square kilometers (77 square miles) of the surrounding area.
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Today, the caldera forms a lake that contains a few smaller volcanic cones as well as the Big Obsidian Flow, a 1300-year-old patch of volcanic glass that covers about 10 square kilometers (4 square miles). The volcano’s caldera is thought to have first formed 500,000 years ago, when the original cone collapsed following a particularly large eruption that emptied the underlying magma chamber. This broad shield volcano emitted a wide range of flows, ranging from basalt to rhyolite. Newberry Volcano in Oregon is the widest volcano of the Cascades, with a diameter of 32 kilometers (20 miles). A shield volcano has a low profile and gradual slope because it is made of lava flows that were more fluid than the “sticky” silica-rich lavas that build up stratovolcanoes. The Cascades also contain a few ancient shield volcanoes. Another common rock is dacite, a gray volcanic rock that lies between andesite and rhyolite in terms of its silica content. The most common rock produced by these volcanoes is andesite, a fine-grained rock of intermediate silica content. The original volcanoes are long since worn away, and their sediments can now be found in the Central Valley of California.
As a result of this shallow subduction angle, volcanism slowed and the North American plate was heavily uplifted. During the Laramide Orogeny in the late Cretaceous (70–80 million years ago), the angle of subduction became shallower, possibly due to an increased rate of plate convergence. While these volcanoes ruptured the surface, massive bodies of magma slowly cooled deep below, creating a huge granite batholith. Oceanic lithosphere at the edge of the North American continent melted while subducting at a steep angle, and the magma rose back through the continental crust to form a volcanic arc. In contrast, the plutonic core of the Sierra Nevada began to form over 200 million years ago during the Nevadan Orogeny. The Cascades are younger mountains, having first appeared 36 million years ago, and are still forming today. The rocks of the Cascades and the Sierra Nevada are primarily igneous, since they resulted from melting above a subduction zone.
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