Geology 111G
Lecture 1.
Introduction to Earth; Interior Structure
Scientific Method
Origin of Solar System,
Earth
Basic Earth Structure
Interior
Structure of the Earth
Layers
based on Rock Composition
Layers
based on Rock Physics
I. Scientific Method: Procedure by which science
progresses. Involves
testing of ideas, or hypotheses, by means of experiment or observation. Ideas not rejected are retained for
additional testing. Hypotheses that
survive repeated testing are elevated to the status of theory. An overarching theory is commonly
referred to as a paradigm.
In most cases, these tentative ideas are not proven, but nevertheless
explain the set of observations that they address (or vice versa). This is the nature of science, which
makes many people uncomfortable, but science nevertheless makes valuable
predictions about natural phenomena.
II. Big Bang Theory:
Main theory for origin of universe, holds that it all began with an enormous
explosion at 13.5-12 Ga that dispersed all matter and energy.
A. Gas subsequently contracted
gravitationally into rotating nebulae (nebular hypothesis of Immanuel
Kant, 1755). From the nebula that
became our solar system, the sun and planets differentiated. The planets formed by accretion of
planetesimals, km-scale chunks of condensing material that ultimately became
rock.
B. Ages of some old rocks.
1. Oldest meteorites: 4.56
2. Oldest moon rocks: 4.46
3. Oldest mineral grains on earth: 4.3 Ga,
detrital zircons recycled into sedimentary rocks in
3. To explain this discrepancy, hypothesis
is that a Mars-sized asteroid hit the Earth about 4.5 billion years ago,
ejecting a large molten blob that became the moon, and releasing so much energy
that the earth was melted. Note that melting resets rock systems dated in
geology.
C. Significance of 4.56 melting event is
that it homogenized the earth by melting most of it, knocked its rotation axis
off vertical to 23¼ and apparently increased rate of rotation.
III. Differentiation of
Earth into a Layered Sphere: The
newly molten Earth was segregated into layers of different density by
gravitational differentiation; the denser material sank, whereas the lighter
material rose. This leads to
concentric density structure.
A. Interior structure of the earth: To understand plate tectonics and
mechanisms that cause the crust to change, we must understand two schemes by
which geologists subdivide the structure of the earth. There are two schemes:
1. Subdivision based upon internal layers
of differing composition
a. Crust: the
outer layer or shell of the Earth, which varies dramatically in thickness
1) Continental crust: ranges from 30 to 70 km thick (average
40 km), has a density of 2.7 g/cm3, underlies the continents.
2) Oceanic crust: Averages 7 km thick, has a density of
3.0 g/cm3 (basaltic), and underlies the ocean basins.
b. Mantle: a dense
rocky shell that underlies the crust.
Has a greater density than the overlying crust, 3.3 g/cm3 at top, 5.5
g/cm3 at bottom. Extends from 40 to 2900 km.
c. Core: a
spherical body that forms the interior part of the earth and consists of iron,
some nickel, sulfur, and a little silicon. Extends from
2900-6400 km. Outer core: 10-12 g/cm3. Inner core: 13 g/cm3.
2. Subdivision based upon the physical
properties of the layers, particularly strength, which is determined by a
combination of composition, confining pressure, and temperature.
a. Lithosphere: outer
100 km of rock, which is relatively cool, strong, and behaves as a rigid, if
segmented shell.
b. Asthenosphere: weak,
plastic, even partly melted rock between 100 and 350 km depth. The comparative weakness of the
asthenosphere is due to the elevated temperature of rock in this zone. How hot is it down there? Introduces the concept
of geothermal gradient.
c. Mesosphere: high temperature and high strength layer between
350 km and the core. Here, the high
strength is due to high confining pressures at depth.
d. Core: again
the central sphere, here divided into a liquid out core and a solid inner core.