Since Wegener's day, scientists have mapped and explored the great system of oceanic ridges, the sites of frequent earthquakes, where molten rock rises from below the crust and hardens into new crust.
(scientists; have mapped; the sites of frequent earthquakes; T:Since Wegener's day) (scientists; explored; the great system of oceanic ridges; T:Since Wegener's day) (molten rock; rises; from below the crust) (molten rock; hardens; into new crust)
scientists scientists molten rock molten rock
have mapped explored rises hardens
the sites of frequent earthquakes T:Since Wegener's day
the great system of oceanic ridges T:Since Wegener's day
from below the crust
into new crust