What conditions could produce 50 km long lava tubes?
In Hawaii, the Mauna Loa lava tube from the 1859 lava flow 


  enters the ocean about 50 kilometers (31 mi) from its eruption point


And in the Undara Volcanic National Park in Queensland, Australia


  Bayliss Cave is the remains of a lava tube that was once over 100 kilometres (62 mi) in length.


So a lava tube, 50 km long on the Moon, may not be that remarkable.

Additionally, The Undara field has feature call the Wall, which is a continuous ridge 40 km long and 20 m high. 


  The Wall is believed to be the closest terrestrial analogue to the basaltic ridges seen on the moon. The longest lava flows in the Solar System are on Venus.