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Under the pressure of the layers of ice and snow above it, this granular snow fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. [25] [26] [27] Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles.
An artist's impression of the last glacial period at glacial maximum [4]. The LGP is often colloquially referred to as the "last ice age", though the term ice age is not strictly defined, and on a longer geological perspective, the last few million years could be termed a single ice age given the continual presence of ice sheets near both poles.
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate between glacial periods. The Last Glacial Period ended about 15,000 years ago. [1]
The Late Cenozoic Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation, in progress since 2.58 million years ago. Within ice ages, there exist periods of more severe glacial conditions and more temperate conditions, referred to as glacial periods and interglacial periods, respectively.
A glacier is a persistent body of dense ice, a form of rock [2]) formed from snow falling and accumulating over a long period of time; glaciers move very slowly, either descending from high mountains, as in valley glaciers, or moving outward from centers of accumulation, as in continental glaciers.
Glaciologists subdivide glaciers into glacier accumulation zones, based on the melting and refreezing occurring. [1] [2] These zones include the dry snow zone, in which the ice entirely retains subfreezing temperatures and no melting occurs. Dry snow zones only occur within the interior regions of the Greenland and Antarctica ice sheets.
Very large lakes were formed along the glacial margins. The ice on both North America and Europe was about 3,000 m (10,000 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Ice weight caused crustal subsidence, which was greatest beneath the thickest accumulation of ice.
Penultimate Glacial Period, the glacial period that occurred before the Last Glacial Period Late Cenozoic Ice Age , the geologic period of the last 33.9 million years Little Ice Age , a period of relative cold in certain regions from roughly 1450–1480