Download Lecture 11 Review

Lecture 11 Review
1) The Earth’s internal structure is inferred from seismic studies. There are two types
of waves:
P-waves are longitudinal, compression waves like sound waves.
S-waves are transverse, shear wave like water waves.
The wave velocity is proportional to the material properties through which it travels,
elastic property
β
bulk mod ulus
=
=
inertial property
ρ
density
where density is a measure of how many atoms are packed in a given volume and bulk
modulus measures the stiffness of the material lattice, i.e.
∆p
change in pressure
stress applied
β=
=
=
∆ V V fractional compression observed strain
wave velocity =
Earth’s core was discovered in 1906 when
observers found that they received modified
seismic waves from earthquakes on the far side
of the Earth. Waves from are altered by passing
through the core, and certain types of waves
from B do not even reach the observer. The
conclusion is that rock layers at different depths
have different properties.
Seismic Data
2) Changes in velocity are an indicator of changes in density and phase. From these
data one can infer the following features of the Earth.
Core
r = 0 to 3500 km, 16% of volume
average density = 10 gm/cc
average temperature = 2700 K
Mantle
r = 3500 to 6400 km, 82% of volume
average density = 5 gm/cc
average temperature = 2000 K
Crust
r = 10 to 100 km thick
average density = 3 gm/cc where water is 1 gm/cc
average temperature = 0 to 1500 K
Chemical View - differentiation by density
A) core of metals
B) mantle of rocks
C) crust of low density rocks (slag) - same as Moon
Mechanical View - layering by rigidity
A) core of metals and mantle rock
B) plastic, molten slush, asthenosphere
C) lithosphere - light crust + upper mantle rock floating on top of
asthenosphere
3) Thermal History
The Earth is a differentiated planet, as
mentioned in the last lecture. Early
heating allowed molten iron and
nickel to fall to the center of the Earth
to form a solid inner core surrounded
by a semi-molten outer core. The
thermal history of Earth is similar to
the other terrestrial planets. For
instance, Mars formed by accretion,
then became hot enough for
differentiation to occur. Cooling
followed moderated by radiogenic
cooling. Mars is smaller than the
Earth and has lost more of its internal
heat and its primary atmosphere.
Earth is bigger than Mars and still
maintains a hot interior, its lithosphere
is thin, and volcanism is common.
Plates of the lithosphere carry the
continents with them. Plate tectonics
may be unique to Earth
4) Surface
The present surface configuration is relatively young. Two frames in a time-lapse
movie of Earth’s surface, showing the dramatic movement of continental land
masses due to plate tectonic motions in the last 4 percent of Earth’s history (170
million years). Geologists are not certain of the motions in earlier times, though
there is evidence of previous collisions and splitting.
A.
170 million years ago, Pangaea, composed of two major regions called
Laurasia and Gondwanaland, was just beginning split apart.
B.
70 million years ago, the Atlantic was widening and present land masses were
beginning to take shape. India had not yet collided with Asia to make the
Himalayas.
Pangaea, composed of Laurasia and Gandwanaland, splits into major
continents 170 million years ago
The current surface is composed of seven major “plates” riding on a semi-plastic
asthenosphere.
Evidence for changes in the magnetic field of the Earth are found at the bottom of
ocean floor rifts where outflowing magma solidifies with the current magnetic
alignment frozen in place. The origin of a banded magnetic anomaly pattern across a
ridge crest is suggested in this schematic drawing. Sea floor at the ridge crest is
polarized in the same direction as the present (normal) magnetic field. The flanking
negative anomalies are caused by reversed polarity of the remanent magnetization of
sea floor, formerly solidified at the ridge crest when the field was reversed.
5) The surface changes with time due to weather. Convection currents flow when
hot air flows to meet cold air. Differential velocities result from rotation of the
Earth. This is called Coriolis drift and produces clockwise rotation for highpressure areas and counterclockwise rotation for low-pressure areas.
6) Few features of earlier meteor impact are left. Most have either been subsumed
by plate movements or eroded by weather and water.