Avolcano is a very large, gently sloping mound composed mainly of basaltic lava flows. composite stratospheric cinder cone shield

A shield volcano is a very large, gently 'sloping mound' composed mainly of 'basaltic lava flows'.  

Explanation:

The volcanoes are broadly classified into four major types, viz. Cinder Volcanoes, Composite Volcanoes, Lava Domes, and Shield Volcanoes. Out of these four, the Shield Volcanoes are the largest in size and span. The low viscosity of the lava that comes out of Shield Volcanoes is due to the presence of a high amount of molten basalt in the lava.

Answer volcanic outgassing;