Question 1
Oceanic lithosphere subducts (sinks into the asthenosphere) because ________.
◦ the upwelling magma at divergent boundaries pushes the lithospheric plate into the asthenosphere
◦ old oceanic lithosphere is more dense that the asthenosphere
◦ the pressure from two plates colliding forces one plate into the asthenosphere
◦ the convection of the mantle pulls the lithosphere into the asthenosphere
Question 2
Mount St. Helens and the other Cascade volcanoes are ________.
◦ a row of young, active, shield volcanoes built as western North America moved over a hot spot deep in the mantle
◦ young, active stratovolcanoes built on a continental margin above a sinking slab of oceanic lithosphere
◦ old, deeply eroded stratovolcanoes built before the Pacific Ocean existed
◦ old, deeply eroded, basaltic shield volcanoes built when western North America was over the present-day site of the Hawaiian hot spot
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