We all know the sin of Sodom. But my question is, why was Gomorrah destroyed? Please give references to your answer.
God destroyed Sodom and Gomorrah because of their grievous sins along with other towns in the plain of the Jordan, except Zoar.
Genesis 18:20-21 (NIV) Then the Lord said, “The outcry against Sodom and Gomorrah is so great and their sin so grievous that I will go down and see if what they have done is as bad as the outcry that has reached me. If not, I will know.”
There were five cities in the plain of the Jordan - Sodom, Gomorrah, Admah, Zeboyim and Zoar.
Genesis 14:8 (NIV) Then the king of Sodom, the king of Gomorrah, the king of Admah, the king of Zeboyim and the king of Bela (that is, Zoar) marched out and drew up their battle lines in the Valley of Siddim.
After Lot and Abram were separated, Lot chose to live in the plain of Jordan.
Genesis 13:11-13 (NIV) So Lot chose for himself the whole plain of the Jordan and set out toward the east. The two men parted company: Abram lived in the land of Canaan, while Lot lived among the cities of the plain and pitched his tents near Sodom. Now the people of Sodom were wicked and were sinning greatly against the Lord.
The four cities - Sodom, Gomorrah, Admah, Zeboyim were destroyed but Zoar was spared because Lot pleaded the two angels to spare the town of Zoar as he wanted to run there for safety.
Genesis 19:23-24 (NIV) By the time Lot reached Zoar, the sun had risen over the land. Then the Lord rained down burning sulfur on Sodom and Gomorrah—from the Lord out of the heavens.
Genesis 19:29 (NIV) So when God destroyed the cities of the plain, he remembered Abraham, and he brought Lot out of the catastrophe that overthrew the cities where Lot had lived.
The city of Sodom was known for sodomy and that is exactly how we get this English word. The sins of Sodom and Gomorrah as recorded by prophet Ezekiel, ~1500 years later -
Ezekiel 16:49-50 (NIV) “‘Now this was the sin of your sister Sodom: She and her daughters were arrogant, overfed and unconcerned; they did not help the poor and needy. They were haughty and did detestable things before me. Therefore I did away with them as you have seen.
The sins of Sodom and Gomorrah as recorded by apostle Jude in the New Testament, ~2000 years later -
Jude 1:7 (NIV) In a similar way, Sodom and Gomorrah and the surrounding towns gave themselves up to sexual immorality and perversion. They serve as an example of those who suffer the punishment of eternal fire.