So, there’s the ring of fire around the Pacific Ocean.
Why no ring of fire around the Atlantic? What’s the difference ?
The Pacific Plate is a large tectonic plate whose edges are the Ring of Fire - that’s where the magma comes up, more or less.
The Atlantic is instead characterized by a large Mid-Atlantic Ridge in the middle between the North American and Eurasian Plates, so the magma comes up in the middle instead of around the edges.
Just to elaborate - the Pacific is shrinking as the plates around it come together - the Ring of Fire is where the collision is happening. The Atlantic is growing - and the mid-Atlantic ridge is where the expansion is happening.
Note that the east coast of North America has a large continental shelf, while the west coast doesn’t.
Of course, just as I was going to post the first response, SDMB goes down. :rolleyes: 
In addition to what’s already been posted, note that there is volcanic activity; for example, in Iceland and Italy.
And Iceland is on the Mid-Atlantic Ridge (Italy is in the area affected by the collision of the European and African plates)
There are volcanoes in the Caribbean, but that is down to the interaction of the Caribbean plate and the South American plate.
Iceland is a hotspot, similar to the Hawaiian islands. Hotspots are something different than plate tectonics as far as where the volcanism comes from.
Nope, sorry. It’s smack dab on the Mid-Atlantic Ridge. Go look at an undersea map of the Atlantic.
The Ridge, by the way, is not fixed over a single area on the Earth as hotspots are. There was one (perhaps more) hot spot that started on North America and now is on the other side of the Ridge. This is because the Ridge drifts somewhat to the west. I forget the name of it off-hand, but it came up in a thread not too long ago.
This site has a map of the Ridge Mid-Atlantic Ridge - Wikipedia - it goes through the middle of Iceland.
The Great Meteor/New England hotspot track.
No reason it can’t be both… but Iceland is definitely over a hot spot- what we see there isn’t normal mid-oceanic ridge behavior.
http://geologymatters.org.uk/2011/08/18/the-iceland-hot-spot/
and the most interesting (“Our model suggests that the initiation of the Iceland hotspot predates the opening of the North Atlantic by at least 70 m.y.”)
https://www.researchgate.net/publication/235982076_Iceland_hotspot_track
So was the Pacific once much bigger?
Iceland is essentially in the (long, drawn-out) process of being created by volcanic activity. There’s a very nice graphic in (IIRC) the Volcano Museum in Reykjavik of the two sides of the island being ripped apart form each other and filled in up the middle by successive eruptions.
If it keeps going at the current rate, it’ll end up as wide as Australia some day. In … oooh … just about a hundred million years or so
In some ways of looking at it, you could say that the Pacific was nearly the whole deal.
Thanks. Learn something new everyday.
Oh, yes.
From the first cite:
Well depending on how you define “The Pacific” it was once all the ocean that was.
I remember a photo from last year where a Doper was diving off the coast of Iceland and had his hands touching the two different plates, which I thought was incredibly cool. Certainly put my touching the Vishnu schist when I took a mule to the bottom of Grand Canyon into the shade.