( ( Composite volcanoes ) ( usually ( ( have ( ( a magma ) ( with ( a ( ( very low ) viscosity ) ) ) ) ) . ) ) )
(ROOT (S (NP (NNP Composite) (NNS volcanoes)) (ADVP (RB usually)) (VP (VBP have) (NP (NP (DT a) (NN magma)) (PP (IN with) (NP (DT a) (ADJP (RB very) (JJ low)) (NN viscosity))))) (. .)))
Composite volcanoes usually have a magma with a very low viscosity.
(ROOT (S (NP (DT A) (NN crater)) (VP (MD can) (ADVP (RB usually)) (VP (VB be) (VP (VBN found) (PP (IN on) (NP (NP (DT the) (NN top)) (PP (IN of) (NP (JJ composite) (NNS volcanoes)))))))) (. .)))