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Key Terms:
Converge = Two tectonic plates pushing towards each other (--><--). The pressure pushing form each side causes the plate with the higher density to subduct.
Subduct = The tectonic plate with the higher density gets pushed under the less dense plate. Results in volcanic arcs and ocean trenches
Seaway = shallow layer of sea water connected to the ocean that covers a portion of the continental plate for a time.
Flux = Melting process under the earths surface that is caused during subduction. Is characterized by the lowering of a material's melting point by adding water to it's composition. Creates magma and results in volcanic activity.
Magma = Hot, molten rock beneather the Earth's surface. Either cools to create solid rocks, or erupts as lava in a volcano.
The volcanoes in Lassen are a result of the subduction of the Juan de Fuca oceanic plate beneath the North American continental plate.
About 700 million years ago, the Juan de Fuca oceanic plate converged in an eastern direction towards the North American continental plate. Due to the pressure, the Juan de Fuca plate subducted beneath the continental plate, creating mountain and volcanic ranges.
Image of the modern day Sierra Nevadas
During this time, the Sierra Nevada and Klamath mountains were created, and a depression was formed between them. This depression became a seaway, filling with ocean water. This seaway is known as the Lassen Strait and connected the California ocean to Oregon.
As the earth continued to shift, the ocean eventually receded, leaving the seaway dried. In it's wake, the ocean water left layers of sand, mud, and limestone that had built up over the years.
As the tectonic plates converged and the oceanic plate subducted. Lava began to rise towards the surface, creating volcanoes. The heat and melting that caused these volcanoes was due to flux melting.
Flux melting occurs when an oceanic plate subducts under another tectonic plate. The water from the ocean is added to the rock, reducing the melting point and causing melting to occur closer to the surface. As the new material melts, the heat and pressure increase and the magma pushes towards the surface, thus resulting in a volcano.
Simple cross sectional diagram of a oceanic plate subduction beneath a continental plate, creating volcanoes.
These volcanoes were developed throughout the Ice Age. There was an incredible amount of volcanic activity along the Cascades and Sierra Nevada up until about 11 to 12 million years ago. The volcanic rock was deposited upon itself and pilled up to about 10,000 feet, forming the Western Cascades. These cascades were formed along San Andrea's fault.
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