The extinction of the dinosaurs occurred during the end of the Cretaceous Period, about 65 million years ago, and caused the loss of up to 70 percent of all life on the planet. Although this event was not the only mass extinction in Earth’s history or the most severe, the extinction of the dinosaurs has spawned a variety of theories, from the credible to the less credible.

ice Age

If an Ice Age were to occur during the Cretaceous Period, a considerable amount of the planet’s water and large areas of land would have been trapped in the ice. Since dinosaurs were thought to be cold-blooded, they likely would not have been able to cope with such extreme conditions and their numbers would have eventually dwindled.

disease

Disease is another suggestion for the extinction of the dinosaurs. However, it is hard to see how a disease could be so prevalent as to wipe out large swaths of the animal population, travel great distances, and kill both terrestrial and marine species as well as plant life.

Climate change

If there were a gradual change in the earth’s climate with a cooler, drier environment and adapting plant life, the dinosaurs might not have been able to adapt fast enough to survive. Fossil records of dinosaur skulls show that they could only have had small brains. This indicates that dinosaurs were associated with automatic bodily responses rather than the cognitive decision-making responses that warm-blooded animals are known to give. Therefore, a changing environment might have been difficult for the dinosaurs to adapt to, and they may not have been able to compete effectively with warm-blooded animals.

supernova

A supernova is a star that explodes with enough energy to keep our sun burning for six billion years. If the Earth were hit by radiation resulting from a nearby supernova, it would have destroyed all dinosaur life. There is no evidence that a supernova caused the extinction of the dinosaurs, and it is hard to see how anything could have survived such an event.

The volcano theory

Volcanic activity is known to have been widespread during the Cretaceous era, and is a credible reason why the dinosaurs became extinct. The Deccan traps in western India and areas in the Pacific Rim are likely sites of increased volcanic activity at this time.

The consequences of large-scale volcanic activity would have led to a nuclear-type winter with ash and gases added to the atmosphere, depleting sunlight and creating harsh conditions for dinosaur species. Hydrochloric acid in particular would have broken down the ozone layer allowing harmful ultraviolet rays to penetrate the surface.

The high level of volcanic activity would also have emitted large volumes of greenhouse gases such as C02 and methane, increasing the Earth’s temperature and drastically changing the planet’s climate. The effects on the dinosaurs would have been catastrophic as they struggled to acclimatize to changing conditions, and large-scale extinction of dinosaur species would have been inevitable.

Cretaceous sediments support the theory of volcanic activity as they contain higher than normal concentrations of minerals and sodium, both associated with volcanoes. Cretaceous sediments may also contain volcanic rock.

The asteroid theory

In the late 1970s, Luis and Walter Alvarez and scientists from the University of California were studying rocks from the KT boundary (or Cretaceous-Tertiary boundary) in Gubbio, Italy. Within the clay layer of the KT boundary they discovered iridium concentrations thirty times higher than normal. The two natural sources of this rare element are asteroids and lava from the Earth’s core, which led the team of Luis and Walter Alvarez to first suggest that an asteroid collided with Earth causing the extinction of the dinosaurs. Alvarez’s team estimated that the asteroid would have to be ten kilometers across to contain enough iridium to cause the iridium concentrations in the clay layer.

More evidence has emerged supporting an asteroid impact. The KT boundary has two layers. The top layer is three millimeters thick and includes soot believed to have come from global fires caused by the impact.

The lower level is two millimeters thick and contains ejecta from the asteroid impact site. This includes quartz crystals known as shock crystals that have been physically altered by high temperatures and intense pressure. The layer also contains tektites and microtektites, which are often made of molten glass. Impacted crystals and tektites are closely associated with asteroid impact sites.

In 1990, scientist Alan Hildebrand, after studying data collected by geophysicists searching for oil, noticed a ring structure called Chicxulub off Mexico’s Yucatan Peninsula. Chicxulub is a crater 180 kilometers in diameter and dates back to about 65 million years ago, the same time as the extinction of the dinosaurs. A 10-kilometre-diameter object impacting the site was estimated to have caused the crater, which fits Alvarez’s team’s calculations of the size of the asteroid to create the KT boundary findings.

The Effects of the Asteroid Impact on the Dinosaurs

It is estimated that the speed of the asteroid would have been about 100,000 kilometers per hour and the initial impact would have destroyed everything within a radius of 500 kilometers. The intense shock wave that followed would have caused large-scale fires, while trillions of tons of debris, gas and water vapor would have been thrown into the atmosphere.

Earthquakes, tidal waves, volcanic eruptions and strong winds would have followed. Soot and debris in the atmosphere would have blocked out the sun for months, causing fluctuating temperatures and severely disrupting photosynthesis. The global fires would have emitted large volumes of CO2, increasing global temperatures and causing a chain reaction that would create sulfur dioxide falling as acid rain.

The fires would have destroyed up to twenty-five percent of all vegetation, and with photosynthesis unable to function effectively, the plant-eating dinosaurs would have starved to death. This, in turn, would have led to large-scale die-offs of carnivores, driving the dinosaurs to extinction. Smaller animals such as scavengers and birds, which might have traveled longer distances in search of food, would have been more likely to have survived.

Effects on marine life

Oxygen levels in the sea would have decreased as deep sea water was drawn to the surface by huge undercurrents. The plankton would have died as a consequence, leading to a collapse of the food chain and widespread die-off. Acid rain may also have increased the acidity of the sea, killing vulnerable species.

Problems with the asteroid theory

While an asteroid impact has gained ground over most other theories, problems with the theory remain. Paleontologists have yet to find dinosaur fossils dating to the time of the impact, and some evidence suggests that dinosaurs may have become extinct before this event. In fact, the dinosaurs had been steadily declining for tens of thousands of years before the Chicxulub asteroid hit.

There have been a number of mass extinction events in Earth’s history and a number of large asteroid impacts. However, these impacts have never been the cause of mass extinctions. Some climate-sensitive species, such as frogs, are also known to have survived the Cretaceous extinction event, calling into question the true long-term effects of the asteroid on the environment.

Despite these problems with the asteroid theory, it remains the strongest explanation for the extinction of the dinosaurs. Perhaps the answer does not lie in one explanation for the extinction of the dinosaurs, but in a combination of factors, such as the impact of the asteroid at a time of increased volcanic activity. In the final analysis, perhaps the extinction of the dinosaurs was a case of accumulation of untimely events that very few species were able to cope with.