How was the planet formed? What is its relationship to other matter
in the universe? A popular hypothesis for the formation of the earth is
the nebular hypothesis. This idea dates back to the philosopher
Immanuel Kant (1755) and Laplace (1796) and has been modified as empirical
evidence and theory mount. Recent incarnations (chemical -condensation-sequence
model) start with the solar system forming from a rotating, diffuse cloud
of dust and gasses (a nebula). As the nebula cooled the matter condensed
into "planetesimals", near the sun where temperatures were highest
elements with the highest melting points (metals and heavy minerals) condensed
first. Lower melting temperature elements and compounds (water, methane,
ammonia) condensed more readily in the cooler areas further from the sun.
This helps to explain the density gradient in the solar system, the closest
planets to the sun are terrestrial while those further away are gaseous.
How did the earth form in the condensing nebula? The earth may have
formed through the accretion of many planetesimals and as the mass increased
through gravitational attraction and compression (overhead). The earth
was probably initially a homogeneous ball that heated from three sources:
1) energy of planetesimal impacts, 2) gravitational compression lowered
potential energy releasing heat, and 3) heat from radioactive disintegration
(20 cals is released for 1 cm3 of granite over 500
million years). As the earth heated it began to differentiate into various
zones of matter with different properties (overhead). Differentiation
was possible because molten material could rise or sink depending on density,
be moved by convective currents, and localize due to chemical zonation
(overhead). As the earth cooled outgassing of the mantle released compounds
(water vapor, carbon dioxide, hydrogen, nitrogen) into a primitive atmosphere.
Early geologists tried to determine how old the earth was from observations
about the features of the earth. Age = Thickness of sedimentary rock/rate
of sedimentation. Old (<1.5 billion years) but not old enough. Age =
salinity of sea/rate of salt deposition in seas. Again old, but not old
enough. Lord Kelvin (of absolute zero fame) calculated the age of earth
from its temperature, assuming it was molten at its formation. Gave 100
million years (and gave Darwin a bit of a problem: was this enough time??
Radioisotopes cleared things up (see below)
We can divide the processes that alter the earth's surface into two
categories: 1) igneous processes (volcanism and mountain
building) construct features by increasing the average elevation of the
land, 2) Sedimentary and erosive processes (deposition and weathering)
act as forces wearing down features created by volcanoes and creating new
horizontal features (e.g. river delta). The theory of Plate tectonics
provides a synthetic model for understanding how the dynamics of the earth
work. The plates move around, collide, move over or under one another.
Divergent boundaries are where plates move apart, convergent
boundaries are where plates move toward one another, transform boundaries
(e.g. San Andreas fault) are where plates move by each other. The continental
plates (lithosphere) float on molten inner layer (asthenosphere). Where
plates meet there can be uplifting or subduction. Uplifting results
in mountain building through igneous activity and at the boundaries between
plates and actual scraping off of material from the subducted plate. Subduction
results in plates being forced downward and is seen is formations such
as ocean trenches.
The rock material of continental plates can be viewed as going through
a rock cycle that can be related to plate tectonics. Magma
(molten rock) e.g., released from volcanoes, crystallizes and forms igneous
rocks ("fire formed rocks"). Through weathering and
transport sediment is formed which by lithification become sedimentary
rock. Through exposure to high temperatures and pressure, sedimentary
rock (or any rock) can be changed into metamorphic rocks. If
this rock is exposed to extreme temperatures it can become molten again
and form magma, and if released through volcanic activity be reintroduced
as igneous rock.
In what kind of rock would we expect to find fossils? Sedimentary rocks. Their structure can tell us a lot about earth history. Laid down in strata of sedimentary layers. Bedding planes generally mark the boundary between the end of one sediment and the beginning of another.
Several logical rules can be used to determine the sequence of events:
Relative dating. generally one follows several principles: superposition
the older rock is below and the younger rock is above; original horizontality:
the strata are laid down originally in a horizontal position (gravity is
what lays them down). Thus nonhorizontality must have occurred after
the deposition. The cross cutting relationship states that the cut formation
is older that the formation doing the cutting.
Another prominent feature is an unconformity which occurs when
the rate of deposition has been interrupted, the sediments eroded and deposition
renewed. A clear break in the sequence of events is apparent. One type
of unconformity is an angular unconformity where strata with originally
horizontal bedding planes now have bedding planes that intersect. Significant
because it reflects a major episode of geologic change.
All well and good for a given formation, but one would like to be able
to make general statements about larger regions. This can be done by correlation
of strata from different formations separated by some distance. Stratum
"X" may lie near the top of one formation and many miles away,
X may be found near the bottom of a new formation, at the top of which
is a different layer "Y". Several miles further on, "Y"
may lie at the bottom of a third formation, and in this way one can link
or correlate the different strata.
This may work for a large region but one would like to do this for the
entire earth. It turns out that there are diagnostic fossils found in different
formations around the world. These Index fossils help correlate
different formations on each of the major land masses. This was recognized
by William Smith (see lecture 2). The phenomenon is more pronounced than
an occasional fossil here and there: entire biotas go through successive
changes in sequential strata, illustrating the principle of faunal (biotic)
succession. We thus have the "age of trilobites" seen early
in the fossil record. Later the age of fishes, age of reptiles, age of
mammals are clear in formations around the world indicating the comparable
ages of formations separated on different continents.
These fossil beds lead to the formation of the Geologic time scale, the names of each period deriving from the locality where the characteristic formation was found. The major divisions (eons, eras) are defined by the presence or absence of fossils: proterozoic, phanerozoic (visible life or animals). Geological dating is often problematic because geologists use fossils to date rocks and biologists use rocks to date fossils. A measure independent of stratigraphy and fossil remains is necessary. With the discovery of radioactive decay it became apparent that one could use the ratio between the parent isotope and the daughter product (e.g., U238 decays through several steps to Pb206). By measuring the amount of isotope and daughter product and knowing the half life of the isotope one can estimate the absolute age of a rock formation. Problems: when the daughter material escapes and hence produces an inaccurate estimate. Additional tests with different isotopes can corroborate one another.