#spherule

Impact

This is a complicated figure from a recently published paper featuring a really neat geologic site. This is an outcrop of the debris from the asteroid impact at the end of South America after the impact.

Start with the map in the upper right. The map shows the modern day locations of the Chicxulub impact crater on the Yucatan Peninsula and the tiny island of Gorgonilla off the coast of Colombia, where this outcrop was found.

The vertical column at the far left shows the sedimentary sequence. These rocks are off the coast of South America, near the Andes Mountains today. 65 million years ago, there were volcanoes nearby just like we find along this coastline today. This sequence of rocks therefore includes a basaltic lava flow and tuffs that have mixed with sediments. Tuffs are ash that exploded out of volcanoes, so volcanoes were feeding this site and the type of sediment deposited was alternating between sandy and muddy.

Then, right in the middle of this sequence, a whole bunch of spheres of glass appear. These are debris from the impact itself and they’re shown at the top in the figure labeled A. When the asteroid hit the Yucatan Peninsula, it vaporized and melted some of the target rocks. That molten rock splashed out of the crater like water bursting from a balloon and it rained down around the entire surrounding area. It cooled off rapidly as it flew through the air, creating spheres of hot glass that pelted the landscape. That spherule layer is the direct debris of the impact.

The final 2 frames show what has happened to the other sedimentary layers. The spherule layer is colored green in the center image – note that it’s a little bit bent. The zoomed in image below it shows that the once simple sedimentary beds have been bent and disturbed. When the impact took place, it caused geologic disruption throughout the entire region. It is thought that earthquakes continued shaking the land for tens of minutes after the impact, long enough for blast debris to rain down and still be disrupted. These bent sedimentary layers were shaken up by the energy released after the initial blast.

The scientists at this site characterized pollen samples in the sediments before and after the impact to analyze the types of plants living in these areas. They found that the plant populations shifted dramatically after the impact and one group of ferns that has been thought to occupy areas that are hit with disasters shows up just after the impact layer.

-JBB

Spherule Layers

This layer is found in a sequence of sedimentary rocks in the Barberton Greenstone Belt of South Africa – some of the oldest rocks exposed on continents in the world. This layer is estimated to be 3.24 billion years old. It is a geologic record of what I would call…a very bad day.

These are impact-generated spherules. This layer formed when an asteroid, at least several kilometers in diameter, slammed into the Earth. The energy of the impact superheated the target rocks and the impactor, vaporizing part of it and also splashing much of the flash-melted rock outward as small droplets. These droplets would have then splashed down on the earth and cooled as they traveled through the atmosphere, creating layers rich in spherules. Many of these layers contain elements that are abundant in asteroids but are rare at Earth’s surface such as iridium; a chemical fingerprint of these as impact spherules.

Large impacts capable of producing these spherules were much more common early in the Earth’s history when there were more asteroids around to hit planets and when gravitational stirring in the giant planets may have thrown asteroids into the inner part of the solar system.

Many of the exact processes that led to the formation of the terrestrial planets are still not understood, including the frequency of impacts in the early solar system. We have some evidence for a pulse of asteroid impacts about 4 billion years ago from the ages of rocks picked up on the moon, but this proposed “Late Heavy Bombardment” concept remains controversial as we don’t have exact ages for craters, just ages when some rocks were reheated.

By counting the number of these layers found in early Archean sequences on Earth and measuring their thicknesses, scientists are able to estimate the number and size of impacts in the early solar system. A recent effort to do just that led to an estimate of cratering frequency similar to that observed on the moon. The distribution of impact sizes also suggests that the objects hitting the Earth (and the Moon) early in the planet’s history represented a population of objects similar to those observed in the asteroid belt today. In other words, it looks like the objects being thrown at the Earth came from a population similar to those found in the solar system today.

-JBB

Image credit: http://bit.ly/1QyjczX