The Earth

Earth as a Planet among other Planets

• Introduction
• Earth's Atmosphere
• Plate Techtonics
• Discussion Questions
• Optional Website Reading

Introduction

Earth is the planet we know best, so we begin with a set of planetary features we can use for comparison: the atmosphere, surface, plate tectonics, and core structure.

Earth's Atmosphere

Earth is surrounded by circulating air. Driven by energy that it receives from sunlight, air is warmed at the equator, rises and flows away to the north and south, cools, and eventually falls back toward the surface at around 30° north (or south) latitude. Air at around 60° rises and falls back near the poles. These giant vertical circulation cells drive the east-west winds like the trade winds and jet streams.

Air rising over the ocean tends to carry moisture, which condenses over land and provides rain and wet climates along the west coasts of land masses in the northern hemisphere, or the east coasts of land masses in the southern hemisphere.

Local weather phenomena can become violent storms. When heat causes air to rise very quickly, the resulting change in pressure and partial draws in air from the surrounding atmosphere at high speeds, creating hurricanes over the ocean, and tornadoes over land or shallow water. The circulation of air in storms helps us understand similar phenomena in the atmospheres of the gas giant planets like Jupiter and Neptune.

Plate Techtonics

A short history of geological theories, swiped almost entire from my Natural Science site.

Steno (1631-1686)

Steno was a Danish naturalist who worked for the Grand Duke of Tuscany in Italy. He published a study of the rocks and minerals in the mountains near Florence, and produced three guiding principles for the study of the earth:

• Superposition: As sediments settle in the sea or other water areas, they produce a sequence of strata: the upper layers are more recent than the deeper layers.
• Initial horizontality: Sedimentary strata are originally produced in flat layers parallel to the surface of the sea. Inclined layers show that the area has undergone geological forces which have bent it.
• Lateral extension: Strata layers are continuous within their bounds, so anything which cuts the layer into sections (like a river eroding it) must be younger than the strata.

Steno became a Catholic and left geology to serve as Apostolic Vicar of Northern Germany; in his later life he devoted his time to theology.He believed that the world was relatively young, and that fossil-bearing strata had been deposited by the Biblical Flood.

The Flood figured prominently in most theories throughout the eighteenth century. Georges Buffon (1707-1788) was among the first to propose an expanded timescale, a seven epoch division over about 75,000 years, instead of theBiblical 6 days of Creation. He based his time scale on calculations made by Newton, and determined the time necessary for an earth-size sphere of iron to cool from red hot to its current average temperature. Buffon's epochs were

Earth forms out of matter ejected by comet collision with sun (hence the molten starting point).

Crust cools, forms mountains.

Vapors condense, cause flood.

Further cooling causes cracks; water sinks into earth's interior and dryl ands appear.

Land animals appear.

Continents diverge.

Humans appear.

Controversies over mechanisms of formation

Buffon had no evidence to back his theory, but it stimulated interest among a number of naturalists, who began making field observations. The growing prosperity of the middle class and the rapid growth of trade made it possible for many to voyage across the seas and even around the world.

Two major groups developed, with the "Neptunists" promoting water as the primary force of geological formation and the "Vulcanists" promoting heat. In the eighteenth century, French naturalists realized that basalt flows were the result of volcanic activity, and linked many mountains and basalt outcroppings to ancient volcanoes. Basalt often appears as giant crystals, similar to those formed during slow cooling of similar rocks.

At the same time, the German naturalists studied the sedimentation of rocks,and realized that many currently dry areas would once have been covered by seas. Abraham Werner (1749-1817) proposed that rock strata formed from a universal primeval ocean. The process produced four kinds of rock: primitive rocks crystalized out of the ocean (without fossils), transitional rocks resulting from chemical precipitation, sedimentary rocks (rich in fossils),and derivative rocks such as sand and clay, which formed from weathering of the other three kinds.

Another theory developed which stressed the geological activity of the internal heat of the earth (in contrast to the surface heat favored by theVulcanists).The "Plutonists" believed that geological forces operated at the same rate and in the same way throughout the world for its entire existence. Studying the current processes was equivalent to studying the general mode of past processes. The foremost Plutonist was the Scottish geologist James Hutton (1726-87). Hutton believed that the interior of the world was molten rock which pushed into cracks in the crust, forcing sedimentary strata up (and tilting it in the process). This harder rock survived as the core of the mountain when the sedimentary exterior while eroded by wind and water.

The chief opponent to Hutton's "uniformitarian principle", as the theory of constant geological processes was called, was the Frenchman Georges Cuvier (1769-1832), who countered it with a theory of "catastrophism"based on discontinuities in the fossil record. Cuvier thought that a series of floods had occurred: a few significant single events, rather than one long, continuous process. The most recent of Cuvier's flood would coincide with the one recorded in the Bible. Because of its alignment with the Biblical Flood stories, Cuvier's theory was well-received.

Fossils and Geology

By the beginning of the nineteenth century, geologists were a recognized group of specialist scientists. They had developed a methodology for studying and identifying different strata layers in many parts of Europe. The English surveyor William Smith published his classification of rocks in 1799, alongwith a detailed description of the area around Bath, in England. The Cambridge professor of geology, Adam Sedgwick, studied the rocks in Wales, and Sedgwick's friend Roderick Murchison studied the rocks in Devon. The Welsh sediment series(called Cambrian from the ancient name for Wales) appeared to be much older than the Devonian sediments, which themselves contained fossils of many fish. Both names are preserved in the current geological time scale.

Charles Lyell (1797-1875)

Lyell revived Hutton's theory, and in 1833, published The Principles of Geology, in which he tried to explain the processes of the past interms of those he could observe. Lyell insisted that geological forces had been constant throughout the history of the earth. He reclassified a serios of fossils into the Eocene, Miocene, and Pliocene series, all within the Tertiary series identified earlier by students of Steno. Lyell's efforts resulted in the classification of geological strata into the Pre-Cambrian, Paleozoic, Mesozoic, and Cenozoic eras which is still in use (see Science lecture).

Louis Agassiz (1807-73)

The Swiss-American Agassiz was fascinated by glaciers. He studied the glacialaction of ice in the Alps, and proposed that a glacier must once have stretched across the whole of central Europe. He performed similar researches after he became a professor at Harvard University in Boston, and showed that the same evidence existed for glacial action across most of North America. From the rates of motion of the current glaciers, Agassiz determined that at leasts ome geological processes were sporadic — occurring for brief (geologically speaking) periods of time, then quiet for long periods of time. Agassiz's theories countered the uniformitarianism of Lyell and softened the catastrophism of Cuvier.

In the midst of the growing debate over the age of the earth, based on different models of geological processes, Darwin wrote The Origin of the Species. We will talk about this more in the next few weeks, but Darwin's theories challenged the geologist to come up with a method of accounting for the isolation of one species or form of life (such as marsupials inAustralia), while accounting for the spread of others across continents now separated by wide oceans.

Alfred Wegener (1880-1930)

In response to this challenge, Alfred Wegener in 1912 came up with a model called plate tectonics. He suggested that the continents floated on the deeper layers of rock in the earth, and moved together and apart, driven by slow currents. At some time in the past (about 200-300 million yearsago), all the continents formed a single land mass called Pangaea, and animals and plants migrated freely from one area to another. According to his proposed theory, the continents separated and any new species which developed would do soin isolation from the other continents. His theory also accounted for the similarity in coast lines between South America and Africa. Because Wegener could not show how the continents could move, his theory was not well received until exploration of the ocean floor and satellite measurements during the1960s showed that the continents were actually moving as he had proposed.

There are number of short videos on different aspects of plate tectonics at the USGS Dynamic earth site.

Discussion Questions

• What kinds of evidence do geologists use to determine the interior structure of the earth?
• What observations are available from Satellite information that was not available to Wegener and his predecessors? How has this information changed our perception of the earth's structure and possible physical theories about its formation?

Optional Web Reading

• The real-time Wind Map offers a dynamic display of dominant air flows across the US at the Earth's surface.
• The Jet Stream Map shows the wind directions in the upper atmosphere (10km).
• NOAA's Perpetual Ocean project displays currents in the Earth's Oceans during 2005-2007, and is available in a three minute and a ten minute version on YouTube.

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