Geology
Mount Fee is one of the southernmost volcanoes in the Mount Cayley volcanic field. This volcanic zone forms the central portion of the larger Garibaldi Volcanic Belt, which extends from the Silverthrone Caldera in the north to the Watts Point volcano in the south. The volcanic belt has formed as a result of ongoing subduction of the Juan de Fuca Plate under the North American Plate at the Cascadia subduction zone along the British Columbia Coast. This is a north-south trending fault zone about 1,000 km (620 mi) long, extending 80 km (50 mi) off the Pacific Northwest from Northern California to southwestern British Columbia. The plates move at a relative rate of over 10 mm (0.39 in) per year at an oblique angle to the subduction zone.
The edifice of Mount Fee is the remains of a volcanic feature that has been significantly eroded by glacial ice. It likely represents a dissected stratovolcano (also known as a composite volcano) that was larger in area and higher in elevation than its current form. Stratovolcanoes can reach heights of 2,500 m (8,000 ft) and consist of alternating layers of lava flows, volcanic ash, cinders, blocks and bombs. During the glacial periods, much of the volcano's original outer cone of pyroclastic material was eroded away by moving layers of ice and rock. The removal of the ejected volcanic material has exposed the dacite lava that forms the narrow north-south trending ridge of Mount Fee. Black Tusk, a pinnacle of dark volcanic rock to the southeast, is also interpreted to be the remains of a deeply eroded volcano that was once covered with pyroclastic material. The present day edifice of Mount Fee contains several lava spines that reach heights of 100 m (330 ft) to 150 m (490 ft) above the main ridge.
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