Introduction
Geomorphology-is the shaping of the surface of the planet by the various forces of nature i.e. agents of denudation. This is sub divided into weathering (breakdown in situ) and erosion (breakdown, transportation and deposition elsewhere i.e. ice, rivers, wind and the sea). While fluvial processes (i.e. rivers) are the most important and coastal processes are important for an island nation such as Ireland, moving ice (glaciers) produces some of the most spectacular landscapes in the mid to high latitudes.
Introduction to Ice Ages
Ice Ages, periods in Earths history when sea ice and/or land glaciers have covered a significant portion of the planet's surface and significant cooling of the atmosphere has occurred. The Earth has existed for about 4.5 billion years. During that time it has experienced several ages, each lasting tens of millions of years. The total of these episodes may account for as much as 15% to 20% of the planet's history. The icy cover has ranged from about 10% to about 30% of the entire surface of the planet.
Glaciation
The roots of glaciology began, in Europe just over two hundred years ago. The "Little Ice Age," from the end of the l5th century to the beginning of the 19th sparked an interest in the processes of the earth. Observations and questions followed about the erratic positioning of boulders that had littered the landscape for the previous thousands of years. A Swiss minister, Kuhn, first recorded written documentation in 1787. He felt that the present glaciers of the region were at one time more extensive, and in their retreat they left large rocks marking their pathways of old. Less than ten years later (1785), a Scottish scientist, Scott Hutton (father of modern geology) described how the ice of the past had moved boulders in alpine regions and he published these ideas in his book Theory of the Earth
Over this century our understanding of how glaciers shape the landscape has grown
Glacial periods
The most recent ice age, the Pleistocene Epoch (last 2 million years) lasted from about 1.6 million years to 10,000 years before the present. During that time at least 20 glaciations, or periods when the ice cover increased, occurred. Each of these periods was followed by an interglaciation, or an interglacial period when the ice cover shrank. The most recent glaciation in North America, called the Wisconsin glaciation, lasted from about 115,000 - to 10,000 years ago (approx the same as in Ireland).
The climate during that time was much different from what it is today, with temperatures on the continents as much as 15C colder. In areas that are currently occupied by subtropical deserts, cooler and wetter climates caused large lakes to form from increased rainfall and glacial runoff. The past 10,000 years have been part of a relatively warm interglacial period (time between 2 ice ages). However, the presence of massive continental ice sheets on Greenland and Antarctica, along with numerous smaller glaciers in mountainous regions throughout the world, indicates that Earth is still in the grip of an ice age. There is the debate however about the influence of global warming.
Ice Ages, these are periods in the Earth's history when a significant, extended cooling of the atmosphere and ocean took place. The Earth last entered such an ice age about 1.6 million years ago, at the beginning of the Quaternary period. Although continental ice sheets withdrew from North America and Europe about 10,000 years ago—at the end of the Pleistocene epoch—many scientists believe that the Quaternary Ice Age is not over yet. Evidence of earlier ice ages also exists.
Since the time of the earliest recorded life on Earth (about 3.6 billion years ago), the planet's average surface temperature has been about 20° C (70° F), with a range of uncertainty of about 5° C (9° C). For more than 90 per cent of that time the Earth has been free of ice ages, and no large glaciers have existed except in high mountains. Ice ages occur about every 150 million years, and last a few million years.
POSSIBLE CAUSES OF ICE AGES
Although the cause of ice ages is still a subject of controversy, an argument based on astronomical observations of the galaxy has gained credibility in recent years. The Earth and its solar system are located asymmetrically within one limb of the Milky Way galaxy. The galaxy completes one rotation about once every 300 million years, taking the solar system through denser and thinner regions of interstellar dust and through changing gravity and magnetic fields. As with tidal processes, two disturbing phases appear to exist for each full cycle—so that every 150 million years a very slight change takes place in the solar system's galactic environment, possibly altering the Earth's climate.
In addition, Earth-based processes are also involved. According to Plate Tectonics theory, because of continental drift periodic changes take place in the Earth's geography, the effects of which can be understood by considering the changes that preceded the present ice age. These changes occurred about 60 million years ago, when a warm equatorial seaway called the Tethys sea separated the northern land mass (Laurasia) from the southern one (Gondwanaland), bringing warm swirling currents to all the oceans. The old southern lands began drifting northwards, however, so that Africa, Arabia, and India successively collided with Eurasia. Finally, Australia separated from Antarctica, allowing a cold current to circle the globe. One by one the former equatorial seaways were blocked by land. Each ocean was now isolated and connected with polar latitudes by great swirls of cold current.
This, then, is the very speculative ice age scenario: a slight external cooling because of the galaxy's rotation, as well as a favourable geographic-oceanographic setting. A chain reaction of cooling may then be initiated by minor variations in the Earth's orbit.
ORBITAL EFFECTS
Within each ice age are remarkable fluctuations known as glacials and interglacials: these are cold and warm phases that correspond to a cycle of about 100,000 years. Recognition of this glacial cycle required complex mathematical calculations, first worked out by a Yugoslav scientist, Milankovitch (1879-1958), who showed that the cycle has additional modulations that make it fluctuate considerably. They correspond to three variables in the Earth's orbit.
Most important of these variations is the eccentricity cycle of 93,408 years—the variation of the orbit from its almost circular path. This affects the spin rate of the Earth-Moon system, which increases when the Earth and Moon are closer to the Sun. The slower the spin rate, the stronger is the Earth's magnetic field, which, in turn, tends to screen off the incoming particles of high energy from the Sun, thereby cooling the climate.
The second of the orbital cycles is the change in the tilt of the Earth's equatorial plane in relation to its orbital plane over a period averaging 41,000 years. About 25 per cent of the glacial and interglacial temperature differences are due to this change, which varies from about 22° to 25°. The third orbital phenomenon is the 25,920-year precession cycle, which is similar to the wobble of a spinning top. At present the Earth's axis points to the Pole Star, and the northern hemisphere is closest to the Sun in the winter, resulting in relatively mild summers and winters. About 11,000 years ago, however, the axis pointed so as to give the northern hemisphere colder winters and warmer summers. Because the winter ice in high latitudes remains far into the summer, this arrangement 11,000 years ago resulted in a secondary glacial episode, with intense droughts in the subtropics.
Another geographic element is also involved in the precession cycle. Besides the blocked seaways of the present ice age, most of the northern hemisphere is land, which generates a continental climate, whereas the southern hemisphere is encircled by a continuous seaway that provides far more maritime climates. If the land and sea were uniform in both hemispheres, the precession effect would be cancelled out.
Studies indicate that Milankovitch's cycles do not fully account for the timing of events in the recent glacial/interglacial cycle. Some researchers propose that other large-scale influences, including feedback from changes in ocean currents, are equally important.
ANCIENT ICE AGES The longest of the ancient ice ages was probably the Permo-Carboniferous, which began about 300 million years ago and affected all southern hemisphere lands. Still earlier, about 435 million years ago, another giant ice sheet extended from Brazil to North Africa and all the way across to Yemen and Saudi Arabia. Palaeomagnetic measurements indicate that the South Pole then lay in West Africa. About 600 million years ago, yet another great glacial age occurred. Evidence of these ancient ice ages is provided by layers of tillite, a rock consisting of hardened glacial drift.