Why did american cities experience explosive growth in the late 19th century?

"There were 2 main reasons: 

The Second Industrial Revolution created the need for a workforce in new factories,so many people moved from the countryside to the cities to find jobs in these factories.The effects of the depression of 1873-1896,which spread to America,further encouraged people to go to the cities to find work. 

There was a long economic depression in Europe from 1873-1896,which encouraged many Europeans to emigrate to America.Most of these arrived at major port cities,and found jobs in factories so stayed there.

Social darwinism refers to the the theory that people who succeed in our economic system will one day out compete those who fail. one day the rich will out succeed the poor so we'll that there will eventually be no more poor