 The Monument and Preserve encompass three major lava fields and about 400 square miles ( 1 @,@ 000 km2 ) of sagebrush steppe grasslands to cover a total area of 1 @,@ 117 square miles ( 2 @,@ 893 km2 ) . All three lava fields lie along the Great Rift of Idaho , with some of the best examples of open rift cracks in the world , including the deepest known on Earth at 800 feet ( 240 m ) . There are excellent examples of almost every variety of basaltic lava , as well as tree molds ( cavities left by lava @-@ incinerated trees ) , lava tubes ( a type of cave ) , and many other volcanic features . 
