Answer to Question 1
Answer: Volcanic ash, sometimes deposited thousands of miles away from the erupting volcano, is frequently preserved in the rock record. These ash deposits accurately reflect the chemistry of the source volcano, which must be fairly explosive to produce the eruption column in the first place. Composite volcanoes are most likely to be preserved in terms of their ash deposits.
Answer to Question 2
Answer: Volcanoes can be triggered by the subduction that occurs at convergent boundaries, either (1) at an oceanic lithosphereoceanic lithosphere subduction zone, or (2) a subduction zone where oceanic lithosphere is subducting under continental lithosphere. Magma is produced when subducted slabs give off water, which lowers the melting temperature of the neighboring/overlying mantle. Volcanoes may also occur at divergent boundaries, either at (3) continental rifts, or (4) oceanic ridges. Decompression melting of the underlying mantle is typically the cause here. Lastly, rising mantle plumes and hot spots can cause volcanism, far from any plate boundary. This intraplate volcanism may occur either beneath (5) oceanic lithosphere, e.g. Hawaii, or (6) continental lithosphere, e.g. Yellowstone. Addition of heat from Earth's deep interior and the decompression of rising warm rock (due to convection) both play a role here.
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Earth’s Oldest Preserved Rocks – 3.8 Billion Years Old, Greenland