Hi guys,
This has some really cool implications for the potential for eruption in the PNW. The angle of the subducting slab has a profound effect on volcanism. Goes something like this:
Oceanic plates can still retain some heat, even after traveling long distances. These younger slabs are typically less dense, and have a shallower angle of subduction. As you can imagine, a shallower subduction angle requires a much longer horizontal distance of travel to reach the same distance of a steeper subducting slab. This means it's not going to experience as much partial melting, and is therefore not going to induce much volcanism. The subducting slab under the Andes is a good example. On the other hand, a steeply subducting slab will experience a more severe pressure-temperature gradient, and therefore will experience more partial melting. Although it's theorized that mostly what occurs is not partial melting of the slab itself, but the mantle wedge! Water is squeezed out of the rock in metamorphic mineral reactions, and travels up through the mantle wedge, lowering the melting temperature. This has been confirmed from isotopic signatures that are indicative of mantle sources.
So back to Oregon and Washington. Just to the west of the "high" or "modern" cascades, are the "western" or "old" cascades. These are the remnants of large stratovolcanoes similar to those you see in today's cascade range; however, they have mostly been worn away. It turns out that the reason volcanism has shifted eastwards in the pacific northwest is because the angle has decreased. If you think about it, what is being subducted is much younger, and closer to the spreading source; therefore, the angle should decrease.
cool stuff!
-Jesse Walters
This has some really cool implications for the potential for eruption in the PNW. The angle of the subducting slab has a profound effect on volcanism. Goes something like this:
Oceanic plates can still retain some heat, even after traveling long distances. These younger slabs are typically less dense, and have a shallower angle of subduction. As you can imagine, a shallower subduction angle requires a much longer horizontal distance of travel to reach the same distance of a steeper subducting slab. This means it's not going to experience as much partial melting, and is therefore not going to induce much volcanism. The subducting slab under the Andes is a good example. On the other hand, a steeply subducting slab will experience a more severe pressure-temperature gradient, and therefore will experience more partial melting. Although it's theorized that mostly what occurs is not partial melting of the slab itself, but the mantle wedge! Water is squeezed out of the rock in metamorphic mineral reactions, and travels up through the mantle wedge, lowering the melting temperature. This has been confirmed from isotopic signatures that are indicative of mantle sources.
So back to Oregon and Washington. Just to the west of the "high" or "modern" cascades, are the "western" or "old" cascades. These are the remnants of large stratovolcanoes similar to those you see in today's cascade range; however, they have mostly been worn away. It turns out that the reason volcanism has shifted eastwards in the pacific northwest is because the angle has decreased. If you think about it, what is being subducted is much younger, and closer to the spreading source; therefore, the angle should decrease.
cool stuff!
-Jesse Walters
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