What caused the Twin Towers to collapse?

As you know the Twin Towers in New York were destroyed when two hijacked planes crashed into them on 11th September 2001. You're probably wondering what caused them to collapse by 10:29AM Eastern time.

Well, the planes crashed into the towers at speeds of 550mph. The first plane crashed between the 93rd and 99th floors of the North Tower while the second plane crashed between the 77th and 85th floors of the South Tower.

The impact of the planes blasted the fireproofing off the buildings' steel core columns which now made them vulnerable to fires like the ones started from the collision.

As the fires reached temperatures of 1,500° Fahrenheit (815.556° Celsius), the integrity of the steel is melted away.

At 9:59AM Eastern time, the South tower is the first of the two to collapse when the core columns buckled which caused the floors to sag. This was too much pressure for the floors below the damaged area which had to hold up the top floors and thus, it collapsed with each floor pancaking on top of each other.

Similarly, the North Tower collapsed at 10:28AM and as it fell, the top floors collapsed into the rubble and were destroyed by the fall. There were some 1.8 million tons of debris left from the collapse of the towers. A lot of it was sent to Fresh Kills landfill while several other chunks have been used to create 9/11 memorials in all 50 US States. Today, an elegant memorial stands in the towers' place.