The other side of the controversy holds that the ultimate cause of the extinction of the dinosaurs was:

Intrinsic: meaning of an Earthly nature and Gradual: meaning taking some time to occur (several million years)

Two main hypotheses exist today:

1.Volcanism: Scientists are quite certain that around 65 million years ago there was increased volcanic activity.

Arguments in Favor of the Volcanism Hypothesis:

ARGUMENT #1: Over a period of several million years, increased volcanism could have created enough dust and soot to block out sunlight; producing the climatic change.

ARGUMENT #2: In India during that time, huge volcanic eruptions were spewing forth floods of lava which can be seen today at the 65 million year old geologic boundary. Since the Earth's molten mantle is relatively rich in iridium, this would explain the iridium layer.

2.Plate Tectonics: Scientists have found evidence that major changes in the organization of the continental plates (continental drift) were occurring at the 65 million year old geologic boundary.

Arguments in Favor of the Plate Tectonics Hypothesis:

ARGUMENT #1: The oceans (especially the Interior Seaway in North America) were experiencing a regression owing to the continental drift that caused them to recede from the land. A less mild climate would have been the result, and this would have taken a long time.

Note that these two above hypotheses are inextricably tied together; volcanism cannot occur without the action of plate tectonics, and vice versa. If the extinction was intrinsic and gradual, both processes probably played a role.