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Some Background Details

In 1908, the Siberian region of Tunguska made headline news when an explosion knocked down 80 million trees in an area of only 770 square miles. The mystery was that there was no crater, which led to the assumption that an "air burst" was responsible. The explosion (named the Tunguska Event) was theorized to have happened four to six miles above the actual lands of Tunguska.

The Question

Is the idea of a large scale Tunguska air burst realistic? How powerful and how fast would it need to be to cause a mass extinction?

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The Wiki article about it says that it's thought to have been a meteor that burst in mid-air. This one was between 60 to 190 meters, I'm assuming in diameter. If it was larger and burst with a proportionate amount of energy, the range of the blast would also increase. I'm not sure how fast the blast would occur.

They estimate that the Tunguska event released at most the equivalent of a 30 megaton TNT blast, an answer I found on Yahoo answers (not the most reliable) said about 45,000 megatons would blow up the Earth entirely. It would have to be a pretty large asteroid/comet.

Maybe you could consider an air burst shower event?

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If you have an impactor a lot bigger or a lot faster than a Tunguska-size one, it will reach the ground, rather than exploding in mid-air. The Chelyabinsk meteor was somewhat smaller than the Tunguska event, but probably similar apart from that.

The kind of mass extinction that is due to an impact event, such as the Cretaceous-Paleogene extinction event happens because of "impact winter", huge amounts of fine dust, ash or other debris being thrown into the sky, and taking many months to settle out (like ash from Icelandic volcanoes, if you remember that). This blocks significant amounts of sunlight and causes cold weather, reducing photosynthesis and thus availability of food for animals. If this continues for a year or two, you get mass die-offs.

The nearest thing we have to a modern example is the "year without a summer" which happened in 1816, apparently due to a large volcanic eruption rather than an impact, and caused widespread famine in the Northern Hemisphere.

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Not by itself

But perhaps a mass extinction could result when the Tunguska-like event triggers a large, more catastrophic event or series of events.

As awe inspiring as Tunguska was, the damage caused compared to the size of earth is very small. Earth has 196 million square miles of surface area. 770 miles of destruction is absolutely nothing at all.

Possible Precipitates

Tunguska-like events only affect the atmosphere and a small portion of the surface.

Volcano

Perhaps if Earth was very unlucky, the air burst happened in just the right place that the overpressure cracked the rock above a large scale magma chamber. With the weakened cap, the magma chamber erupts and causes a massive extinction that way.

Atmospheric Composition Change

If the air burst released the right kind of pollutants into the right place in the atmosphere, it might change enough atmospheric chemistry that life will struggle to keep up. (I know this is really vague but I'm not a chemist nor a meteorologist.)

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I was going to say no, but just possibly ...

Incoming is a snowball or comet made of lots of bits of meteor-sized rock held together by ice. Passing the sun the ice melts and evaporates leaving a cloud of debris on slightly divergent trajectories. The cloud has expanded to a radius of about 7000km when it encounters planet Earth.

No single piece is large enough to make landfall but an entire hemisphere is exposed to the equivalent of multi megatonne nuclear air bursts.

Mass extinction? Seems certain for terrestrial species that have restricted geographic ranges on just that hemisphere. Effect on the rest of the planet? Not sure. That's a lot of dust and NOx suddenly present. Something like a nuclear winter may follow.

An unlikely scenario but I can't say impossible.

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