Cretaceous–Paleogene boundary

A long time ago, the world changed.

A very large rock hit the Earth. This happened 66 million years ago. This rock hit a place in Mexico. It made a huge hole in the ground.

This hit caused many animals to die. Most dinosaurs went away after this. Some birds lived to stay. Scientists found a special layer of rock. It has much metal in it. This layer shows when the big rock hit. It helps us learn about the past.

About 66 million years ago, the Earth changed forever. This change marks the K-Pg boundary. This is a thin layer of rock found in many places.

Scientists found this layer has much iridium in it. Iridium is a metal that is rare on Earth.

A team led by Luis and Walter Alvarez studied this. They thought the metal came from a large space rock. This rock hit a place in Mexico. It made the Chicxulub crater.

The impact caused many problems for life on Earth. It likely caused a mass extinction. This means many types of plants and animals died out. Most dinosaurs disappeared, but some birds lived. The hit may have caused giant waves called tsunamis. It also sent dust and sulfur into the air. This blocked the sun and caused acid rain. Some scientists also think big volcanoes played a part. These volcanoes are called the Deccan Traps. They were in India. These eruptions could have changed the air and heat too. Many things might have worked together to change the world.

The K-Pg boundary is a special layer of rock found all over our world.

This thin band marks a massive change in Earth's history. It shows when the Cretaceous Period ended and the Paleogene Period began. This change happened about 66 million years ago. Most scientists believe this boundary marks a mass extinction event. This means a huge number of living things died out at once. It changed the path of life on our planet forever.

Scientists can see this change by looking at the chemicals in the rock. The boundary layer has a lot of iridium, which is a metal. Iridium is very rare on the surface of the Earth.

This high amount suggests a large space rock hit our world. The impact likely sent huge clouds of dust into the sky. This dust could block the sun and change the climate. It might have also caused acid rain to fall from the clouds. These changes made it very hard for many species to survive.

In 1980, a team of researchers made a big discovery. Luis Alvarez was a physicist who won a Nobel Prize. His son, Walter Alvarez, is a geologist. They worked with chemists Frank Asaro and Helen Vaughn Michel. They found that the iridium levels were hundreds of times higher than normal.

This became known as the Alvarez hypothesis. It helped explain why so many species disappeared at that time. Their work changed how we understand the history of life.

We now know the impact happened at the Chicxulub crater in Mexico. This crater is buried under the Yucatán Peninsula. It was found by geophysicists Antonio Camargo and Glen Penfield. They were actually looking for oil in the late 1970s. The crater is huge and was made by a large asteroid or comet. The impact was so strong it likely caused giant tsunamis. These waves would have rushed across the coasts of the Caribbean.

Many different things might have worked together to change the Earth. Some scientists study the Deccan Traps in India. These are massive volcanic eruptions that happened around the same time. The volcanoes released dust and gases into the air. This could have also changed how much heat the Earth held. Other things, like falling sea levels, might have changed habitats too. It is likely that the impact and the volcanoes both played a part.

The Cretaceous–Paleogene boundary, often called the K–Pg boundary, is a critical geological marker. It represents the transition between the Cretaceous Period and the Paleogene Period. This boundary occurred approximately 66 million years ago.

It marks the end of the Mesozoic Era and the start of the Cenozoic Era. Most importantly, this boundary is associated with a mass extinction event. This event destroyed roughly 75% of all plant and animal species on Earth. This included all non-avian dinosaurs, which are the dinosaurs that did not evolve into birds.

Scientists identify this boundary by looking for a specific signature in rock layers. This signature is usually a thin band of clay containing high levels of iridium. Iridium is a metal that is very rare in the Earth's crust.

The presence of so much iridium suggests that material from space arrived at this time. In 1980, a research team led by physicist Luis Alvarez and geologist Walter Alvarez discovered this phenomenon. They worked with chemists Frank Asaro and Helen Vaughn Michel. Their findings led to the Alvarez hypothesis. This theory suggests that a massive impact event triggered global climate disruption.

The most widely accepted cause for this extinction is the Chicxulub impact. A large asteroid or comet, known as the Chicxulub impactor, struck the Earth. The impact site is a massive crater buried under the Yucatán Peninsula in Mexico. Geophysicists Antonio Camargo and Glen Penfield discovered the crater while searching for petroleum in the late 1970s. The crater is enormous in both diameter and depth. It is the second-largest confirmed impact structure on Earth. Scientists have even drilled into its peak ring to collect rock samples. These samples help confirm the devastating effects of the collision.

The impact caused several immediate and deadly environmental changes. Because the asteroid hit a coastal area, it likely triggered gigantic tsunamis. Evidence of these waves exists in marine sediments across the Caribbean and the eastern United States. Furthermore, the impactor landed in a bed of anhydrite or gypsum. These minerals contain sulfur. The impact ejected large amounts of sulfur trioxide into the atmosphere. This combined with water to create a sulfuric acid aerosol. This resulted in planet-wide acid rain. This acid rain killed many organisms, including plants and plankton. It also harmed creatures that build shells from calcium carbonate, such as molluscs.

Other geological factors may have contributed to the extinction. One major factor involves the Deccan Traps in India. These are massive flood basalt eruptions. Some evidence suggests these eruptions were very active around the time of the impact. They may have released dust and sulfur into the air, blocking sunlight. This would have reduced photosynthesis in plants. They also released carbon dioxide, which increases the greenhouse effect. Some scientists believe the seismic waves from the Chicxulub impact might have even triggered more rapid eruptions in the Deccan Traps. This suggests that multiple catastrophes may have worked together.

Sea level changes also played a role during the late Cretaceous. This process is known as a marine regression, where sea levels fall or sediment builds out. During the Maastrichtian stage, sea levels fell more than at any other time in the Mesozoic Era. This reduced the area of the continental shelf. The continental shelf is the most species-rich part of the ocean. Losing this habitat likely caused a marine mass extinction. This change also altered ocean currents and winds. It also expanded freshwater environments, which changed the balance of life between land and sea.

Scientists continue to investigate other possibilities and mysteries. Some researchers have looked at the possibility of multiple impacts occurring at once. They point to other craters, like the Boltysh crater in Ukraine, formed around the same time. There is also the potential Shiva crater near India, though it is not yet universally accepted as an impact site. Other theories, such as a nearby supernova or the "Verneshot" volcanic hypothesis, have been proposed. However, many of these lack the strong evidence found for the Chicxulub impact. The K–Pg boundary remains a central focus for understanding how massive changes shape life on Earth.