Typically 'screen tearing' is caused by a game updating the video/graphics buffer that is rendered on the screen before the previous buffer was rendered on the screen. A basic lesson, the graphics for your screen are stored in memory and then rendered on the screen. Sometimes games will change the buffered image before the previous cycle completed. If the game pushes a frame to the buffer before the monitors refresh rate hits you will get tearing.
There are two solutions to this problem, the first comes from the fact that the update to the buffer should (typically) occur when the monitor is restarting it's rendering cycle (known as V-SYNC). If you have the option in your settings try turning on V-SYNC. If V-SYNC is on when having issues you might try turning it off instead. Generally you always want V-SYNC on unless you want tearing or unexpected graphical artifacts.
The other option is based on a typical solution that most games use to prevent the problem. They use multiple buffers (2 or 3 typically) so while one buffer is being rendered, the other is being updated. Some games have an option to enable double or triple buffering, if so, enable it.
Souce: Game Programming Algorithms and Techniques: A Platform-Agnostic Approach