The Pleistocene epoch is called an ”Ice Age”. The Ice Age began at least 2.5 million years ago and had a duration of a million years. The most recent retreat of ice had taken place between 10,000 and 15,000 years ago. The areas which were extensively buried by ice sheets included northern North America, Northern Europe, and Northwest Asia. In these areas, ice sheets do not exist today.
The Pleistocene epoch, however, was not a period of continuous glaciation. During this period, the continental glaciers alternately advanced and retreated. The time spans between glacial advances are called ”interglacial periods”. During these periods warm climates returned to most of the formerly glaciated lands. Some of the interglacial periods lasted longer than the glacial ones. Some of the interglacial periods lasted longer than the glacial ones. Four major periods of glacial advances and retreat have been identified in North America and up to six in Europe.
Earth has been warm enough to be ice-free for much more of the time than it has been cold enough to be glaciated. The major part of the Earth’s age comprises of the unglaciated periods, but still, the glaciation periods lasted for hundreds of thousands of years long. A glacial period is a long interval of time (comprising thousands of years) in an ice age. Glacial periods have a very long time span comprising hundreds of thousands of years. Approximately a dozen major glaciations have occurred over the past 1 million years, the largest of which peaked 650,000 years ago and lasted for 50,000 years. The most recent glaciation period, often known simply as the “Ice Age,” reached peak conditions some 18,000 years ago before giving way to the interglacial Holocene epoch 11,700 years ago. We are almost in the middle of the current glacial period. The intensity of the current glacial period at this stage very less intense, if compared to the stage belonging to 20,000 years ago. This is not the one and only glacial period on the Earth, but rather, the Earth has gone through many glacial periods before this one. Detail of the so far known glacial periods is as under;
(1). Huronian Glacial Period (2.4-2.1 billion years ago):
The Huronian glaciation is the oldest series of protracted climatic refrigeration events that extends from 2.4 billion years ago (Gya) to 2.1 Gya. This period is associated with the rise of atmospheric oxygen. During these events, glaciers covered continents, extended to low latitudes, and reached sea level. The ice ages were followed by a protracted-time interval with greenhouse (warm and humid) conditions. The name is derived from the Huronian Supergroup, a glaciomarine to the fluvio-deltaic sedimentary sequence of dolostone, siltstone, argillites, diamictites, and sandstones exposed on the north shore of Lake Huron in Ontario, Canada between Sault Ste. Marie, Sudbury, and Cobalt.
(2). Cryogenian Glacial Period (850-635 million years ago):
Cryogenian is the longest glaciation period in the history of the Earth. This period started 850 million years ago and lasted till 635 million years ago. The time span of this period is the longest among all the glaciation periods that spans over 215 million years. This period is included in the Neoprotozoic Era of the geological time scale. The Tonian Period that lasted from 1 billion years ago to 720 million years ago, follows the Cryogenian Period of Glaciation and was succeeded by the Ediacaran Period (approximately 635 million to approximately 541 million years ago). The beginning of the Cryogenian Period was defined arbitrarily: that is, it corresponded to the onset of the first glacial episode to follow the date of 750 million years ago. For some 65 million years of the period’s 85-million-year span, much if not all of Earth’s surface was covered in ice. The Cryogenian’s longest glaciation, the Sturtian, lasted for the period’s first 50–60 million years. After a brief interglacial, a second cold interval, the Marinoan glaciation, dominated the planet for most of the period’s final 15 million years. These two long glaciations are thought to have been caused by volcanic activity associated with the ongoing breakup of the Rodinia supercontinent, which started near the end of the Tonian Period. The emergence of the Laurentian basaltic province and other flood basalts produced by the massive outpouring of magma is thought to have resulted in increased weathering, a process that pulls carbon dioxide from the atmosphere. Scientists contend that enough atmospheric carbon dioxide was removed to weaken the planetary greenhouse effect; colder global climate conditions followed. The global reach of the ice sheets and glaciers during the Sturtian and Marinoan glaciations is supported by the discovery of glacial deposits and other rocks that formed in the presence of ice near the location of Earth’s Equator during the Cryogenian.
(3). Andean Saharan Glacial Period (460-430 mya):
This period of glaciation lasted for about 30 million years, during the Ordovician and Silurian periods in geological time scale. In this period the volcanic activities and weathering related to mountain building prevailed, which produced a large amount of Carbon Dioxide, which helped cool the planet. As a result of the cooling, water present in a liquid and gaseous form converted into solid ice glaciers. This period is also called the “Late Ordovician Glaciation Period”, because of its starting in the later stage of the Ordovician Period. During this period ice had covered West Africa (Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the center of glaciation moved from northern Africa to southwestern South America.
(4). Karoo Glacial Period (360-260 mya):
It is also known as the “Late Paleozoic Icehouse”, because of its happening during the Paleozoic Period in the geological time scale. large land-based ice sheets were present on Earth’s surface. It was the second major glacial period of the Phanerozoic. It is named after the tillite (Dwyka Group) found in the Karoo Basin of South Africa, where evidence for this ice age was first clearly identified in the 19th century.
The tectonic assembly of the continents of Euramerica (later with the Uralian orogeny, into Laurasia) and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, and the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea, which led to progressive cooling of summers, and the snowfields accumulating in winters, causing mountainous alpine glaciers to grow, and then spread out of highland areas, making continental glaciers which spread to cover much of Gondwana.
At least two major periods of glaciation have been discovered:
- The first glacial period was associated with the Mississippian subperiod (359.2–318.1 Mya): ice sheets expanded from a core in southern Africa and South America.
- The second glacial period was associated with the Pennsylvanian subperiod (318.1–299 Mya); ice sheets expanded from a core in Australia and India.
(5). Quaternary Glacial Period (2.6 mya-present): This is the ice age in which we live. We are almost in the middle of this glacial period. The severity of the current stage of the quaternary glacial period is not so intense as it was some 50,000 years ago.