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The K-T boundary is the point in Earth history that separates the Mesozoic Era (popularly known as the Age of Dinosaurs) from the Cenozoic Era (popularly known as the Age of Mammals).   As the cartoons below illustrate, the world underwent some dramatic changes.

Prior to the K-T boundary impact, the climate was sunny and warm. The seas were also higher than they are today. The Gulf of Mexico, for example, covered the southern portion of Florida and the northern part of the Yucatan Peninsula. An inland sea flooded the interior of North America, covering parts of central Texas, Oklahoma, Colorado, Wyoming, and Montana. It was a good time for cold-blooded animals such as dinosaurs.
The K-T boundary mass extinction has been linked to the impact of an asteroid that may have been 10 kilometers in diameter. When the asteroid hit the Earth, it was vaporized and spewed its chemical contents around the world. One of those chemical elements was iridium, which scientists detected around the world within a distinct layer deposited at the same time dinosaurs and most other plants and animals in the world went extinct.
The K-T boundary impact occurred on what is today the northern part of the Yucatan Peninsula in Mexico.. At the time of the impact, that portion of the peninsula was covered by a shallow sea. Subsequent seafloor sedimentation filled in the crater and buried it. The crater is not visible at the surface of the Earth today.
The impact event generated a vapor-rich plume of ejecta that rose through the atmosphere, into space, before reentering the Earth's atmosphere. When that debris reentered the Earth's atmosphere, it heated the atmosphere and caused a series of chemical reactions that may have produced nitric acid rain. In addition, the impact event occurred in a region of Earth partially covered with a sulfur-rich rock called anhydrite. When that rock was vaporized by the impact, sulfur reacted with water in the atmosphere to produce sulfuric acid rain. The acid rain was not sufficient to acidify oceans, but it was able to affect shallow water systems, such as rivers, lakes, and estuaries. The acid rain also would have helped defoliate any surviving plants. Both of those processes would have attacked that base of the food chain, affecting herbivores and then carnivores.
The rocky and vaporous debris ejected from the Chicxulub crater choked the atmosphere with dust. In addition, the impact may have ignited forest fires that added smoky soot to the atmosphere. The combination of those materials prevented sunlight from reaching the surface of the Earth. Scientists still debate how dark it may have been, but it seems clear that photosynthesis was interrupted. That means photosynthetic plankton in the seas died, undermining the base of the food chain in oceans. In addition, plants on land would have also perished, undermining the base of the food chain on the continents.
The impact vaporized seawater and a portion of the rock in the Earth's crust. That material was ejected from the crater and injected into the atmosphere. In addition to the sulfur that caused acid rain (see cartoon above), the impact also produced carbon monoxide, carbon dioxide, and methane, all of which are important greenhouse warming gases.
Some of the gases injected into the atmosphere by the impact destroyed the ozone layer. Huge amounts of ozone-consuming chlorine and bromine, for example, were produced by vaporizing seawater, underlying sediments, and the crust of the Earth. With the ozone shield gone, harmful ultraviolet light may have reached the ground once the impact-generated soot and dust settled to the Earth's surface.
Dinosaurs, being land-roving animals, were not well preserved in the geological record except in very rare environments such as bogs and tar pits. They also occurred in small numbers compared to smaller marine organisms whose demise is so clearly captured in the KT rock record. So, the details of the dinosaurs' extinction remain undocumented and somewhat controversial. However, as the preceding cartoons show, there is abundant circumstantial evidence that these once-abundant land dwellers fell victim to the same calamity that wiped out the majority of marine organisms.
Unlike dinosaurs, we have telescopes to detect asteroids that may hit the Earth in the future. At the moment, we do not know of any asteroids that will hit the Earth, but astronomers are searching the skies to identify them. If we discover asteroids several years before they will hit the Earth, then we will have an opportunity to deflect them out of harms way.

The educational cartoons above were created in 1988 to depict the theoretical post-impact effects of a Chicxulub-sized asteroid upon Earth’s biosphere, dinosaurs in particular. They were conceptualized by Buck Sharpton (Senior Staff Scientist) and drawn by Shirley Brune, a graphic artist with the LPI at the time. 
LPI holds the copyright to these cartoons.