Over the past half-billion years, about a dozen individual continental plates
came together to form Eurasia. It is the youngest and largest modern conti-
nent and is still being pieced together with chunks of crust arriving from the
south, riding on highly mobile tectonic plates.
Rodinia rifted apart and the separated continents opened a proto–Atlantic
Ocean called the Iapetus. The rifting process formed extensive inland seas,
which submerged most of Laurentia some 540 million years ago, as evidenced
by the presence of Cambrian seashores in such places as Wisconsin. It also
flooded the ancient European continent called Baltica. The Iapetus was similar
in location and size as the North Atlantic and was dotted with volcanic islands,
resembling the present-day southwestern Pacific Ocean.
When the continents reached their maximum dispersal roughly 480
million years ago, subduction of the ocean floor beneath the North Amer-
ican plate initiated a period of volcanic activity and mountain building.
From the early Silurian into the Devonian, about 420 million to 380 mil-
lion years ago, Laurentia collided with Baltica, the ancient European land-
mass,and closed off the Iapetus.The collision fused the two continents into
the supercontinent Laurasia (Fig. 87), named for the Laurentian province of
Canada and the Eurasian continent. These Paleozoic continental collisions
raised huge masses of rocks into several mountain belts throughout the
world. The sutures joining the landmasses are preserved as eroded cores of
ancient mountains.
Figure 87Distribution
of the continents 400
million years ago with the
formative Laurasia in the
Northern Hemisphere and
Gondwana in the
Southern Hemisphere.SILURIAN PLANTS