how are the lavas that form shield volcanoes different from those that form composite cones?

Answer 1

One word: viscosity. The more mafic, lower viscosity lavas of shield volcanos run out far distances and make the gradual slope of a shield volcano. Composite volcanos have high viscosity lavas (like toothpaste) and build upwards without the ability to build outwards too far. The viscosity comes from a higher silica content and water content.

Answer 2

Lavas from shield volcanoes generally have very little viscosity, meaning that they have a smaller resistance to flow. Mainly because they are made of mafic minerals like iron and magnesium which form rocks like basalt because they are formed from mainly from hot spots (like the Hawaiian Islands). Shield volcanoes' lava can have different types of flows, all given Hawaiian names. Pahoehoe lava is generally very "flowy" and relatively smooth while Aa lava is very blocky and sharp and is incredibly painful to walk on barefoot!

Composite cones generally form at convergent boundaries. Their lava is much thicker and can have a high volatile gas content. When composite cones erupt, there is generally a very large eruption where lots of ash and pyroclastic flows come out. Pumice and other types of rhyolitic rocks are mainly formed from composite eruptions. The "super volcanoes" of the world like Cerro Galan and Yellowstone are composite volcanoes and when they erupted in the past, their ash remained in the atmosphere for years, causing a general cooling trend for several years.