Global Warming Causes

Glacials interglacials and geologic time

As well as classifying rocks, geologists were also dividing the history of the Earth into episodes they called eras, periods, and epochs. The table "Geologic Time Scale" below lists the present divisions of geologic time—the history of the Earth—with the approximate dates when they began.

The Great Ice Age occurred recently, so the period of time that encompassed it was called the "most new" epoch, but using the Greek words pleistos (most) and kainos (new) to make the word Pleistocene. The Great Ice Age has ended, however, so the time since then required a different name. It is called the "entirely new," or Holocene epoch—holos means

*Ma means millions of years ago.

(The Proterozoic, Archean, and Priscoan eons are sometimes known informally as the precambrian.)

*Ma means millions of years ago.

(The Proterozoic, Archean, and Priscoan eons are sometimes known informally as the precambrian.)

"whole" or "entire." The Holocene is also known as the Recent and the post-glacial epoch. As the ice sheets retreated at the end of the ice age, melting ice caused sea levels to rise. Evidence of this rise was first found in the late 19th century along the coasts of New England and Belgium. The rise was called the Flandrian transgression and when the Holocene is studied as an interglacial episode it is often known as the Flandrian, a name that is mainly used in Europe. So the time in which we are now living can be called the Holocene, Recent, post-glacial, or Flandrian.

An interglacial is a time of warmer climates that falls between two glacials, or ice ages. Louis Agassiz was correct in his interpretation of the erratic rocks. They were pushed to their present locations by glaciers. He was mistaken, however, in supposing there was a single ice age. The truth was even more dramatic. For about the last 3 million years, ice ages have been occurring at intervals of approximately 100,000 years and lasting from 50,000 years to 250,000 years. They began in the Pliocene epoch, before the commencement of the Pleistocene, and there have been at least seven; some scientists suggest there were 20. Between these glacial periods

Laurentide ice sheet. The line shows the boundaries of the Laurentide ice sheet at the time of the last glacial maximum, about 20,000 years ago.

The sea was frozen to the north and northeast of the ice sheet.

Laurentide ice sheet. The line shows the boundaries of the Laurentide ice sheet at the time of the last glacial maximum, about 20,000 years ago.

The sea was frozen to the north and northeast of the ice sheet.

Fennoscandian ice sheet. The ice sheet that covered northern Europe from the North Cape to the Dnieper during the last ice age.

there have been warmer interglacials, each of which has lasted between 8,000 and 12,000 years. In other words, the global climate has swung repeatedly between glacial periods, when ice sheets covered a substantial part of the world, and interglacials, when the ice retreated. The maps show the extent of the Laurentide ice sheet over North America and the Fennoscandian ice sheet over Europe around 20,000 years ago during the coldest part—known as the last glacial maximum—of the most recent (Wis-consinian) glacial.

Each glacial and interglacial is known from evidence found in North America, Great Britain, Northwest Europe, and also the European Alps, eastern Europe, and Russia. The table "Pleistocene glacials and interglacials" on page 66 lists them, with their approximate dates and their North American, British, and northwest European names. The dates are derived mainly from studies of oxygen isotopes (see the section "Revealing the past" on page 37), but only the more recent ones can be dated by this method. Consequently, with increasing distance in time the dates become less precise.

PLEISTOCENE GLACIALS AND INTERGLACIALS

Approximate date

('000 years BP) N. America Great Britain N.W. Europe

Approximate date

('000 years BP) N. America Great Britain N.W. Europe

BP means "before present" (present is taken to be 1950). Names in italic refer to interglacials. Other names refer to glacials (ice ages). Dates become increasingly uncertain for the older glacials and interglacials, and prior to about 2 million years ago evidence for these episodes has not been found in North America; in the case of the Thurnian glacial and Ludhamian interglacial the only evidence is from a borehole at Ludham, in eastern England.

BP means "before present" (present is taken to be 1950). Names in italic refer to interglacials. Other names refer to glacials (ice ages). Dates become increasingly uncertain for the older glacials and interglacials, and prior to about 2 million years ago evidence for these episodes has not been found in North America; in the case of the Thurnian glacial and Ludhamian interglacial the only evidence is from a borehole at Ludham, in eastern England.

As their investigations continued and geologists became more familiar with the subtle, telltale signs of glacial action on rocks, earlier ice ages were discovered. Those of the Pleistocene are only the most recent. Ice ages were less frequent in earlier times, but several have been identified. The first occurred about 2.5 billion years ago, near the start of the Pro-terozoic eon. There were several more ice ages starting around 850 million years ago, in the late Proterozoic; one that lasted up to 20-25 million years in the Ordovician period; and another about 300 million years ago around the transition from the Carboniferous to the Permian periods. A more recent ice age began about 100 million years ago, during the Cretaceous period. Antarctica began to be covered in ice about 25 million years ago. The ice expanded around 12 million years ago and that is also when ice first appeared at the North Pole. The Antarctic ice reached its present extent about 5 million years ago, although dwarf trees were still growing in Antarctica as recently as 4 million years ago.

Snowball Earth

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