My completely uninformed opiniun is that impacts are far more common and less deadly then is currently thought. Locally (couple thousand miles) very bad but not the cause of great extinctions. Plese do not ask my to back that up. I can't.
Asteroids and TsunamisTsunami can travel at around 400 mph in deep water. When they reach shallow water they slow down, and that's when the real danger begins. The front of the wave slows first and the effect is like a pile-up on a freeway, with the rear of the wave catching up to the front. The wave increases in height from this bunching effect. The final height of the wave depends on several factors, but the shape of the sea floor has the greatest impact.
by Michael Paine
5 November 1999Out ThereIn the spring of 1986, I published my explanation of the black spots in a scientific journal: The Earth's atmosphere was being bombarded by house-sized, water-bearing objects traveling at 25,000 mph, one every three seconds or so. That's 20 a minute, 1,200 an hour, 28,800 a day, 864,000 a month and more than 10 million a year. These objects, which I call "small comets," disintegrate high above the Earth and deposit huge clouds of water vapor into the upper atmosphere. Over the history of this planet, the small comets may have dumped enough water to fill the oceans and may have even provided the organic ingredients necessary for life on Earth.
by Louis A. Frank
and Patrick Huyghe
Scientists reacted to my announcement as if I had plowed through the sacred field of established science with a bulldozer. I had. If the small comets were real, one scientist commented, textbooks in a dozen sciences would have to be rewritten... I spent more than a year answering the objections of critics. But I didn't convince them. It was 10,000 to 1 -- actually 2, myself and John Sigwarth, whose task as my graduate student assistant had been to help me resolve this black-spot mystery. "We have taken a representative poll of current opinion in this field," an editor at Nature wrote in rejecting a small-comet paper we submitted to them in 1988, "and the verdict goes against you." It was my first encounter with taking polls as a way of doing science.
Ah, but a mile thick ice sheet doesn't continue to pour into the crater until all the heat is absorbed converting it to steam.
A wall of ice surrounding a red hot crater is much more benign than a wall of ocean encroaching on a 60 mile wide geyser of live steam.
With liquid water, the heat is fairly quickly transported to the atmosphere as steam, water vapor, salts, and sea bottom ooze swept into the mix.
Ice OTOH doesn't transport the energy planet wide, the crater ends up radiating most of the heat out to space, limiting the damage to the local vicinity.