Complex sentence: Mars has shield volcanoes and volcanic plains formed by flood lavas, similar to the Columbia Plateau in the state of Washington, but Yogi is the first Mars rock confirmed to have a basaltic composition.
Split into simpler sentences: Mars has shield volcanoes and volcanic plains formed by flood lavas, similar to the Columbia Plateau in the state of Washington. But Yogi is the first Mars rock confirmed to have a basaltic composition.