Historical Geology Understanding Our Planet\'s Past

The second half of the Paleozoic followed a Silurian ice age, when Gond-

wana wandered into the southern polar region around 400 million years ago

and acquired a thick sheet of ice. Gondwana, to this point located in the Antarc-

tic, now shifted its position. Its location can be shown by paleomagnetic data,

which indicate the locations of continents relative to the magnetic poles by ana-

lyzing the magnetic orientations of ancient iron-rich lavas.The south magnetic

pole drifted from present South Africa in the Devonian, ran across Antarctica in

the Carboniferous, and ended up in southern Australia in the Permian.

The location of the southern pole is also indicated by widespread glacial

deposits and erosional features on the continents that comprised Gondwana

during the late Paleozoic. The mass extinctions of the late Ordovician 440

million years ago and the middle Devonian 365 million years ago coincided

with glacial periods that followed long intervals of ice-free conditions.

Gondwana in the Southern Hemisphere and Laurasia in the Northern

Hemisphere were separated by the Tethys Sea (Fig. 106). Into this seaway

flowed thick deposits of sediments washed off the surrounding continents.

Their accumulated weight formed a long, deep depression in the ocean crust,

called a geosyncline, which later uplifted into folded mountain belts when

Gondwana and Laurasia collided.

A warm climate and desert conditions over large areas are indicated by

the widespread distribution of evaporite deposits in the Northern Hemi-

sphere, coal deposits in the Canadian Arctic, and carbonate reefs. Warm tem-

peratures of the past are generally recognized by abundant marine limestones,

dolomite, and calcareous shales. A coal belt, extending from northeastern

Alaska across the Canadian archipelago to northernmost Russia, suggests that

vast swamps were prevalent in these regions.

Figure 106Around 400
million years ago, all
continents surrounded an
ancient sea called the
Tethys.

Historical Geology

EURAMERICA

SIBERIA

GONDWANA

NORTH

CHINA

SOUTH

CHINA

Rheic

Ocean

Paleo-

tethys

Ocean

Paleo-

tethys

Ocean

Panthalassic

Ocean

equator