overran nearly half the continents for millions of years. Four periods of glacia-
tion occurred between 850 and 580 million years ago. Even the tropics froze
over, as indicated by the occurrence of glacial debris and unusual deposits of
iron-rich rocks on virtually every continent, indicating they formed in cold
waters. The ice age nearly snuffed out all life, which endured extremely hard
living conditions for millions of years.
A supercontinent located on the equator rifted apart, forming four or
five major continents.The largest apparently wandered into the southern polar
region and acquired a thick blanket of ice. This was perhaps the greatest and
most prolonged period of glaciation when ice encased nearly half the planet.
The ice cover was so extensive that the period has been dubbed the snowball
Earth.If not for massive volcanic activity, which restored the carbon dioxide
content of the atmosphere, the planet could still be buried under ice.
The climate was so cold that ice sheets and permafrost (permanently
frozen ground) extended into equatorial latitudes. During this time, no
plants grew on the land, and only simple plants and animals lived in the sea.
The ice age dealt a deathblow to life in the ocean, and many simple organ-
isms vanished during the world’s first mass extinction.At this point in Earth’s
development, animal life was still scarce. The extinction decimated the
ocean’s population of acritarchs, a community of planktonic algae that were
among the first organisms to develop elaborate cells with nuclei. These
extreme glaciations took place just before a rapid diversification of multi-
cellular life, culminating in an explosion of species between 575 and 525
million years ago.
The glacial periods are marked by deposits of moraines and tillites.Thick
sequences of Precambrian tillites exist on every continent (Fig. 39).Tillites are
a mixture of boulders and clay deposited by glacial ice and cemented into
solid rock. In the Lake Superior region of North America, tillites are 600 feet
thick in places and range from east to west for 1,000 miles. In northern Utah,
tillites mount up to 12,000 feet thick.The various layers of glacially deposited
sediment suggest a series of ice ages closely following each other. Similar
tillites are among Precambrian rocks in Norway, Greenland, China, India,
southwest Africa, and Australia.
Australia was practically buried by glaciers during the late Precambrian
ice age (Fig. 40). The 680-million-year-old glacial varves in lake bed deposits
north of Adelaide, South Australia, might hold evidence of a solar cycle oper-
ating in the Proterozoic.The varves consist of alternating layers of silt laid
down annually. Each summer when the glacial ice melted, sediment-laden
meltwater discharged into a lake, and sediments settled out in a stratified
deposit. During times of intense solar activity, average global temperatures
increased. As a result, the glaciers melted more rapidly, depositing thicker
varves. By counting the layers of thick and thin varves, a stratigraphic sequence
PROTEROZOIC METAZOANS