TimoHi Timo,
There was an eruption (apparently) on the Gorda Ridge (off the Oregon/Washington coasts, on February 28 of this year, but it would not have affected the tides. The tides are caused by the gravitational action of the Moon and Sun on the oceans.
If you'd like to learn more about this eruption, check:
http://www.pmel.noaa.gov/vents/eruption.html
Sincerely,
Scott Rowland
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