When I think of natural catastrophes on a massive scale (continent- or world-wide impact) that can kill millions of people, I usually think of:

1. Meteorite impacts — Asteroids of greater than one kilometer diameter that either kick up sun-obscuring dust in the atmosphere, or, if they hit the oceans, cause massive tsunamis 35 meters (about 100 feet) high.
2. Supervolcanoes — Like the eruptions of the Yellowstone Caldera that occur every 600,000 years or so. The largest eruptions of the Yellowstone Caldera spewed out over 240 cubic miles (over 1000 km³) of volcanic ash, which spread out over much of the North American Continent.

Here’s another megacatastrophe for you to ponder: giant landslides on the slopes of the Hawaiian Islands, or other volcanic islands. The underwater portions of these volcanic edifices have slopes that are in many cases steeper than the rocks can support. The largest underwater landslides, which occur on average every 350,000 years, have rock volumes close to 1000 km³, and likely produce tsunamis up to 100 m high, which would hit the coastlines all around the Pacific. The Indian Ocean tsunami of December 26, 2004, by contrast, produced maximum tsunamis with a height of about 30 m (around 100 feet).

Here is a map showing the extent of two of the larger slides; one of which originated on the northeast slope of Oahu, and the other on the north slope of Molokai. The debris from these slides extends about 200 km (125 mi) away from the islands.

Here is a color version of the same area:

The grayscale map is from a technical article in the Journal of Volcanology and Geothermal Research. The color map is from the ESRI Map Book Volume 20.

Just another reminder of how fragile our lives are.

Grace and Peace