After the rapid continent building,the interior of Laurentia experi-
enced extensive igneous activity from 1.6 to 1.3 billion years ago. A broad
belt of red granites and rhyolites, which are igneous rocks formed by molten
magma solidifying below ground as well as on the surface (Table 6),
extended several thousand miles across the interior of the continent from
southern California to Labrador. The Laurentian granites and rhyolites are
unique due to their sheer volume, suggesting the continent stretched and
thinned almost to the breaking point. These rocks are presently exposed in
Missouri, Oklahoma, and a few other localities. However, they are buried
under sediments up to a mile thick in the center of the continent. In addi-
tion,vast quantities of molten basalt poured from a huge tear in the crust
running from southeast Nebraska into the Lake Superior region about 1.1
billion years ago. Arcs of volcanic rock also weave through central and east-
ern Canada down into the Dakotas.
These massive outpourings of igneous rocks in the interior of the
continent suggest that Laurentia was part of a supercontinent that formed
about 1.6 billion years ago and broke up around 1.3 billion years ago, coin-
ciding with the igneous activity. The supercontinent acted like an insulat-
ing blanket over the upper mantle, allowing heat to collect underneath it.
About 1.1 billion years ago, vast quantities of molten basalt poured from a
huge tear in the crust running from southeast Nebraska into the Lake
Superior region.
SILURIAN PLANTSTABLE 6 CLASSIFICATION OF VOLCANIC ROCKSProperty Basalt Andesite Rhyolite
Silica content Lowest about 50%, Intermediate about 60% Highest more than 65%,
a basic rock an acid rock
Dark mineral content Highest Intermediate Lowest
Typical minerals Feldspar Feldspar Feldspar
Pyroxene Amphibole Quartz
Olivine Pyroxene Mica
Oxides Mica Amphibole
Density Highest Intermediate Lowest
Melting point Highest Intermediate Lowest
Molten rock viscosity Lowest Intermediate Highest
at the surface
Formation of lavas Highest Intermediate Lowest
Formation of pyroclastics Lowest Intermediate Highest