Chicxulub crater

The Chicxulub crater is more than 110 miles wide and was made by a large asteroid that struck Mexico's Yucatán Peninsula about 66 million years ago. The asteroid impact is believed to have been a primary cause of a mass extinction that brought the age of dinosaurs to an end. About half of the crater is located on land while the other half lies beneath the Gulf of Mexico. Named for the Mexican town of Chicxulub, a term that means "tail of the devil" in the Mayan language, the crater was discovered in 1978 during a search for oil. Years later, the Chicxulub crater was linked to the cosmic impact scientists believe resulted in the death of about three-quarters of the species on Earth.

Background

Earth was born around 4.5 billion years ago when a giant, rotating cloud of gas and dust clumped together to form the sun and planets. The first life-forms are believed to have developed on Earth sometime around 3.8 billion years ago. During the next few billion years, life evolved and developed into more complex forms. The first creatures with backbones appeared around 530 million years ago; the first fish and land-dwelling amphibian-like creatures came onto the scene around 500 million years ago; and plants began sprouting up on land around 465 million years ago. Scientists believe the first dinosaurs evolved around 240 million years ago.

At five periods during its history, Earth has been subject to mass extinctions that decimated the majority of species on the planet. The first occurred around 443 million years ago at the cusp of the Ordovician and Silurian periods. About 85 percent of all sea life on Earth died off in an event most likely caused by a global ice age. The Late Devonian extinction occurred around 359 million years ago and wiped out 75 percent of the planet's species. This event may have been a series of smaller extinctions, although its cause is unknown. The largest mass extinction in history occurred around 248 million years ago at the end of the Permian period. Known as the Great Dying, about 96 percent of life on Earth went extinct. The exact cause is unknown. About 75 percent of the species on Earth died out around 200 million years ago at the end of the Triassic period.

Overview

The last major extinction killed about 76 percent of all life on earth around 66 million years ago at the juncture of the Cretaceous and Tertiary periods. Scientists believe a large asteroid impact caused this extinction and brought about the end of the 175-million-year reign of the dinosaurs. The Chicxulub crater is the remnant of that impact.

The asteroid that slammed into the Yucatán Peninsula is estimated to have been about six miles wide and moving at about 40,000 miles an hour. Upon impact, it released the energy equivalent of 100 trillion tons of TNT and punched a hole in Earth several miles deep, instantly vaporizing thousands of miles of rock. The asteroid triggered a fireball that incinerated anything within hundreds of miles and released a powerful tsunami that may have reached a height of more than 1,000 feet. The impact gouged out so much of the planet's surface that it forced large amounts of dust, steam, and ash into the atmosphere. The skies were darkened for months, and after the global wildfires subsided, temperatures across Earth plummeted. So much debris was thrown into the atmosphere that much of the planet's plant life would have died from lack of sunlight. Larger creatures such as the dinosaurs would have been hard-pressed to survive. Some scientists believe the impact was directly responsible for ending the age of the dinosaurs, while others see it as the crushing blow among many factors that led to their extinction.

Millions of years of erosion and geological activity covered the Chicxulub crater under a layer of younger limestone rock. It was further hidden by the fact that much of the impact site was below the waters of the Gulf of Mexico. In 1978, two geologists working for the Mexican oil company Pemex noticed an underground symmetrical pattern on magnetic survey maps of the Yucatán Peninsula. Comparing their discovery with earlier findings, the geologists determined that an impact crater spanning more than 110 miles was located in the region with its center near the town of Chicxulub.

In the early 1980s, father and son geologists Luis and Walter Alvarez discovered a strange layer of the element iridium spread out worldwide in the planet's crust. Iridium is extremely rare on Earth but is more common in space objects. The duo noticed the layer corresponded to the approximate geological period of the Cretaceous-Tertiary extinction and theorized that a large object such as an asteroid or comet may have been the cause. At first, their idea was met with skepticism since other factors had long been assumed to have caused the extinction and there was no telltale sign of a large crater on Earth's surface. In 1991, scientists studying the Chicxulub crater determined it was formed about 66 million years ago, a date that would place its impact near the time of the dinosaurs' extinction.

Further research on the site has solidified the theory that the Chicxulub crater is in fact the site of the dinosaur-killing impact. Scientists have discovered the presence of surface minerals produced at pressures too high for normal geological activity on Earth's surface. They have also found small traces of melted rock and layers of carbon soot left behind by the fires that spread across the globe after impact. In 2016, scientists began drilling into the peak ring of the Chicxulub crater to obtain core rock samples. Chicxulub is the only crater on Earth with an intact peak ring, a geological formation caused by the uplift of melted rock at the time of impact. The scientists' findings suggest the impact churned up granite from deep within the planet and brought it to the surface.

The crater is surrounded by an outer layer of around 100 sinkholes called cenotes. These were believed to be caused by groundwater weakening the surrounding limestone, resulting in a collapse. The cenote ring around the Chicxulub crater was named a World Heritage site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 2012.

Bibliography

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