Download U 8 Synopsis

Unit 8:
How did our earth take shape?
Synopsis:
Learn how our earth was formed and how it has changed over 4.5 billion years to become as it is
today.
The early earth: We now discuss topics normally studied within the discipline of geology. We
saw that our earth formed by the violent process of accretion, as a result of chemical reactions,
electrical attraction and violent collisions between the particles of matter orbiting the young
sun. Not surprisingly, the young earth was extremely hot, so hot, in fact that it melted, perhaps
10 million years after it began to form. The heat came from three main sources: 1) from the
large numbers of collisions with other bodies orbiting the Sun; 2) from the presence of large
amounts of radioactive material (formed by a nearby supernova explosion, whose chemical
traces can be detected today); and 3) from increasing pressure as the earth itself grew in size.
Differentiation: the earth forms layers: In the molten infant earth, different types of matter
were sorted into layers in a process called ‘differentiation’. The heaviest materials, such as iron
and nickel, sank to the centre to form the earth’s largely metallic core, the source of its
magnetic field. Lighter materials remained in the middle, a region known as the ‘mantle’, where
they were kept hot and semi-molten. An egg-shell thin crust formed from lighter materials such
as granites, which solidified at the surface. And gassy materials, such as carbon dioxide,
bubbled up through cracks in the surface to form the early atmosphere. (We know about the
earth’s structure by studying how seismic [earthquake] waves travel through the earth.)
Eons of the Earth’s history: Geologists divide the earth’s history into four main “eons” or
periods. The “Hadean” or “hellish” eon lasted from the earth’s formation until about 4 billion
years ago. The “Achaean” eon lasted from 4 billion to c. 2.5 billion years ago. During this
period, oxygen began to build up in the atmosphere as a result of the activities of bacteria
capable of “photosynthesis” (using sunlight to store energy, producing oxygen as a by-product).
The Proterozoic eon lasted from c. 2.5 to c. 540 million years ago. The Phanerozoic or “eon of
large organisms”, lasted until the present day. It began with the so-called ‘Cambrian’ explosion:
the sudden appearance from about 600 million years ago, of the first multi-cellular organisms.
Plate tectonics: Since the 1960s we have learnt that the surface of the earth has constantly
changed. The earth’s crust, like a cracked egg-shell, is divided into a number of distinct plates.
These move around, carried by the movement of molten material inside the earth. Plates can
collide to form mountains; ocean bed plates can dive under continental plates; and, as at the
San Andreas Fault in California, plates can also grind past each other. Most earthquakes and
volcanoes can be found at the cracks between plates.
The earth’s changing surface: For the last 600 million years we can roughly trace the movement
of crustal plates. We see that plates periodically gathered together to form huge
supercontinents, then broke apart to form a world like today’s in which land masses are more
dispersed. These changes affected global climates and the distribution of living species.
A place fit for life? Of all the planets in the solar system, ours turned out to be the best place for
complex chemical reactions to take place. That is why it was here, on our earth, that life first
appeared.
The ‘chronometric revolution’: dating the history of the earth: Our understanding of the
history of the earth has been revolutionized in the last 50 years, in part because of the
‘chronometric revolution’, a series of new technologies that allowed us to accurately date
events in the distant past. Dates are vital for any study of the past because they place different
events in chronological order so we can discuss relationships between them. Relative dates tell
us the order in which things happened, but not when they happened. In the nineteenth
century, geologists knew that dinosaurs existed before humans, but they had no way of knowing
when dinosaurs existed. Absolute dates tell us when something happened. We now know that
the dinosaurs existed until about 65 million years ago, whereas humans appeared only about
200,000 years ago. Until the 1950s, we could only assign absolute dates to an event if it was
recorded in written documents. That meant that only events in human history after the
invention of writing (about 5,000 years ago) could have absolute dates; and only a small number
of them. Then, in the 1950s, a whole series of new techniques were developed for assigning
absolute dates to events before writing. These mostly depended on the use of radiometric
materials such as Carbon 14 (an isotope of carbon with 8 neutrons). Radioactive materials break
down at a consistent rate so if you take a lump of matter containing Carbon 14, for example,
you can measure how much has broken down and estimate when that material was formed.
Different materials allow you to date different sorts of events. There also emerged other
methods of assigning absolute dates, including the counting of tree rings in wood. These
revolutionary new dating techniques lie behind most of the dates used in this course.