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The argument for extinction by asteroid
The Earth's crust is made up of layers of rock.
The deeper the layer, the older it was created.
Fossils of dinosaurs are only found in layers of
rock formed between about 250 and 65 million
years ago.
About 65 million years ago an asteroid collided
with the earth.
The collision caused a 180 km wide crater, near what is
now Chicxulub in Mexico.
A small layer of dust formed about 65 millions of
years ago and has been found in many places
around the earth.
We know that dust in the atmosphere can effect
the climate.
This dust contains large amounts of the element
iridium.
Iridium is usually extemely rare in the earth's crust.
Iridium is much more common in asteroids.
So...
 The asteroid must have caused the dust layer.
 This dust in the atmosphere would have altered
the Earth's climate.
 There are no dinosaur fossils in layers of rock
above the iridium layer.
 The dinosaurs must have been unable to
survive the climate change caused by the
asteroid impact.