Most of the dinosaurs went extinct long before the catastrophic mass extinction at the end of the Cretaceous period, 65 million years ago. Background extinctions and many minor extinctions accounted for the disappearance of most of the dinosaur species.

During the late Cretaceous, the number of dinosaur families was dwindling. The latter part of the Cretaceous period was a time of high tectonic activity (continental drift) and accompanying volcanic activity . The supercontinent Pangaea was splitting up and the continents were taking on their modern-day forms. The separation of Laurasia and Gondwana was complete. Many mountain ranges were formed . The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. The Earth was getting colder.

About 65 million years ago, at the end of the Cretaceous, a large fraction of plant and animal families suddenly went extinct. In this Cretaceous-Tertiary or K-T mass extinction (K is for Kreide, meaning chalk in German, which describes the chalky sediment layer from that time; T is for Tertiary, the next geologic period), all land animals over about 55 pounds went extinct, as did many smaller organisms.

The K-T mass extinction obliterated the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, some families of birds and marsupial mammals, over half the plankton groups, many families of teleost (bony) fishes, bivalves, snails, sponges, sea urchins and others.

This catastrophe eventually led to the Age of Mammals.

There are a lot of theories about why this K-T (Cretaceous-Tertiary) extinction occurred, but a widely accepted theory (proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist), is that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth about 65 million years ago. The impact would have penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires (generated by hot debris thrown from the crater), tsunamis, severe storms with high windsand highly acidic rain , seismic activity, and perhaps even volcanic activity . The impact could have caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path.

The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally.

Those organisms that could not adapt to the temperature and light changes would die out. Since plants' energy is derived from the sun, they would likely be the first to be affected by changes in climate. Many families of phytoplankton and plants would die out, and the Earth's oxygen levels may well have dramatically decreased, both on land and in the oceans, suffocating those organisms which were unable to cope with the lower oxygen levels.

Major changes in the food chain would result from all of these these environmental upheavals. The herbivores (plant eaters) who ate those plants would starve soon after the plants died. Then, at the top of the food chain, the carnivores (meat eaters), having lost their prey, would have to eat each other, and eventually die out. Their large carcasses must have provided smaller animals with food for quite a while.

There are many impact craters on Earth. A 120-mile-wide (180 km), 1-mile-deep (1600 m) impact crater, Chicxulub, is found at the tip of the Yucatán Peninsula, in the Gulf of Mexico. This crater dates back to 65 million years ago, and is probably the site of the K-T meteorite impact. Evidence of K-T period tsunamis all around the Gulf of Mexico has been found.

The Shiva crater is a another huge impact crater located under the Arabian Sea off the coast of India near Bombay. This crater also dates from the K-T boundary, 65 million years ago, when the Chicxulub crater at the tip of the Yucatán Peninsula also formed. Although it has shifted because of sea floor spreading, when pieced together it would be about 370 miles (600 km) by 280 miles (450 km) across and 7.5 miles (12 km) deep (and may be just part of a larger crater). It is estimated to have been made by a bolide (an asteroid or meteoroid) 25 miles (40 km) in diameter. This crater was named by the paleontologist Sankar Chatterjee for Shiva, the Hindu god of destruction and renewal.

In the clay layer from the Cretaceous-Tertiary (K-T) boundary, scientists have found chemical evidence that supports the Alvarez impact theory. The K-T layer consists of the sedimentary deposits that accrued from the end of the Cretaceous period to the beginning of the Tertiary period. It is divided into two layers, the Magic Layer (3 mm thick) and the Ejecta Layer (2 cm thick).

• Siderophiles - The Rare Earth Elements Os, Au, Pt, Ni, Co, Pd, and Ir, are Siderophile Elements. Their abundance in the lower K-T layer is indicative of an asteroid impact. Iridium (Ir) has been found in the K-T layer around the world. The discovery of a 100,000-years-thick layer of iridium in the K-T boundary in New Zealand, Denmark, and Italy. Iridium is rare on Earth except near the Earth's center, but relatively abundant in chondritic meteors (stony meteors with chondrules, spherical blobs of silicates which pre-date planetary formation). A meteoritic origin of this iridium layer seems likely. This layer became known as the iridium anomaly.
• Tektites - Tektites are quartz grains which are vaporized under intense heat and pressure, and cool into glass beads with no crystalline structure. Tektites were probably formed during a meteorite or comet collision. Tektites are abundant in the K-T layer.
• Shocked quartz - When quartz is put under extremely high pressure, it can cleave in parallel planes. Shocked quartz is found at nuclear bomb sites and known meteorite impact areas. Shocked quartz is abundant in the K-T layer.
• Stishovite (Silicon Dioxide) - a form of quartz created under conditions of high heat and pressure. It is used as an indicator of meteor impact. It has been found in abnormally high abundance in the K-T layer. Most likely formed during a massive collision.
• Glass beads - Kenneth Miller has discovered a two-inch layer of glass beads in the K-T layer near the Bass River in New Jersey, USA, supporting Alvarez' theory.