Why did dinosaurs go extinct?

Dinosaurs flourished and dominated the earth for about 186 million years, during the Mesozoic Era. Dinosaurs went extinct at the end of the Mesozoic era and at the beginning of the Paleogene period, the first period of the Cenozoic Era. About 65.5 million years ago, a giant asteroid hit the earth in Mexico's Yucatan Peninsula. It is estimated that the asteroid had a 10 km diameter and was about the size of a small town. The asteroid hit our planet so strong that, after the impact, which was 2 million times more powerful than the most powerful explosive ever detonated by humans, it formed the Chicxulub crater. The ambient temperature immediately rose in some areas to as high as 1,480 degrees Celsius, which led to the burning of forests and of all things unprotected.

The powerful impact caused several very large earthquakes. These caused volcanic eruptions, mostly in the area known as the Ring of Fire. Another consequence of the devastating earthquakes was the generation of a huge tsunami, which also contributed to the destruction of several species of animals and plants found in the impact zone of killer waves.

The dust that resulted from the formation of the crater and the ash that resulted from volcanic eruptions caused the formation of a cloud that surrounded the entire planet. This cloud threw Earth into a long night for several months. The immediate effect was decreased sunlight and, therefore, the disappearance of several species of plants. 

As a consequence, this affected herbivorous dinosaurs. A chain reaction ensued, with carnivorous predators also disappearing as their food supply dwindled.

Scientists consider the idea that a small percentage of dinosaurs survived this planetary cataclysm and their total disappearance was a slow process.