Shield volcanoes on Earth include the well-known Islands of Hawai'i. The kind of lava that is fluid enough to make shield volcanoes is called basalt. These are Hawai'ian names. The surfaces of pahoehoe lava flows are generally very smooth and billowy. Sometimes they have a ropy texture like melted taffy or caramel.
Pahoehoe flows very commonly contain lava tubes.
Both would indicate that the lavas on Ascraeus Mons, at least at this location, are probably composed of basalt. This plume casts a shadow that can be seen below it. Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission.