What happens when the Earth crosses a comet's orbit and encounters comet debris?

what would happen if a comet collided with our planet?

I can visualize it harming the atmosphere before it makes its crash,

perhaps that was the cause of extinction of life years ago,such thing may cause eruption of volcano that sends intense level of atoms in the ozone layer and stop the penetration of sun rays,as a consequence, temperature drop down ,rains will be more repeated that the level of the sea would rise,an other effect of such collision is earthquake that could eradicate ail forms of existence all these are enough reasons for scientists to search more and more how to escape such a tragedy