Transcript Earth`s Origin & Early Evolution

Evolution of the
Earth
Seventh Edition
Prothero • Dott
Chapter 6
Origin and Early Evolution of the Earth
Fig. 6.1
Footprints on the moon.
Some people believe this
was a Hollywood stunt.
Fig. 6.2
Artistic rendition of
Buffon’s hypothesis of
planetary origin by
passage of a comet near
the sun.
Fig. 6.3
3 Stages in Planetary Evolution
1. Planetesimals
… small bodies formed
from dust and gas
eddies
2. Protoplanets
9 or 10 formed from
planetesimals
3. Planets
formed as protoplanets
swept up matter by
gravitational attraction.
Broadly, four stages can be identified in the process of planetary formation.
1.
The gravitational collapse of a star leads to the formation of a core to the gas cloud and the formation of a
huge rotating disc of gas and dust, which develops around the gas core. A star such as Beta Pictoris shows a
central core of this type, with a disc of matter rotating around the core. Beta Pictoris is thought to be a
young star showing the early stages of planetary formation.
2.
The condensation of the gas cloud and the formation of chondrules. Chondrules are small rounded
objects found in some meteorites.. The presence of chondrules gives rise to a special class of meteorites
known as chondrites. For example, the Allende meteorite is chondrule-rich and contains minerals rich in the
elements Ca and Al, and Ti and Al, minerals which are unlike terrestrial minerals. It also include metallic
blobs of Os, Re, Zr. The chemistry of these unusual minerals suggest that they are early solar system
condensates.
3.
The accretion of gas and dust to form small bodies between 1-10 km in diameter. These bodies are known
as planetesimals. They form initially from small fragments of solar dust and chondrules by the processes of
cohesion (sticking together by weak electrostatic forces) and by gravitational instability. Cohesion forms
fragments up to about 1 cm in diameter. Larger bodies form by collisions at low speed which cause the
material to stick together by gravitational attraction. Support for this view of the process of accretion comes
from a region on the edge of the solar system known as the Kuiper Belt, where it is thought that the
accretionary 'mopping up' has failed to take place.
4.
More violent and rapid impact accretion. The final stage of accretion has been described as 'runaway
accretion'. Planetesimals are swept up into well defined zones around the sun which approximate to the
present orbits of the terrestrial planets. The process leads eventually to a small number of large planetary
bodies. Evidence for this impacting process can be seen in the early impact craters found on planetary
surfaces An explanation of the type given above for the origin of the planets in the solar system is supported
by mathematical simulations which show how accretion works by the progressive gathering together of
smaller particles into large. It also provides an explanation of the differences between planetary bodies in
the solar system and explains the differences between the heavier terrestrial planets close to the sun, and the
lighter, more gaseous planets situated at a greater distance.
http://www2.glos.ac.uk/gdn/origins/earth/ch3_2.htm
Fig. 6.4
Stages in formation of early earth. From (A) a homogeneous, lowdensity protoplanet to (B) a dense, differentiated planet
Fig. 6.5
Cross section through a spinning disk-shaped nebular cloud
illustrating formation of planets by condensation of planetesimals.
Temperatures refer to conditions at initial condensation. Note the
“ice” line between the asteroid belt and Jupiter.
Horsehead Nebula
• The famous
“Horsehead
Nebula” in the
constellation
Orion. This gas
and dust cloud is
thought to be the
birthplace of
suns (stars) and
possibly planets.
Another view of the Horsehead Nebula. The dimly glowing stars may be
new stars just starting to shine. Some may even have planets starting to
form, though this is hard to confirm.
Fig. 6.6
Planet Jupiter showing
moons Io (crossing at
equator) and Europa.
Fig. 6.7
The earth’s interior.
1.
2.
3.
4.
Crust
Mantle
Outer core (liquid)
Inner core (solid)
Note density
discontinuity at coremantle boundary
Divisions of the Earth's interior
A more detailed
view of the
Earth’s interior
showing the “D”
layer between the
inner core and the
lower mantle.
This region is
thought to give
rise to mantle
plumes and play
a role in the
Earth’s magnetic
field.
3-D image of the crust
3-D image of the crust beneath the
San Francisco Bay area developed
from monitoring the paths that
earthquake waves pass through it.
Colors correspond with different
chunks of the Earth's crust that have
been pushed together along the San
Andreas and Hayward faults.
Earthquakes are shown as yellow
dots.
Divisions of the Earth's interior
The question of how
oceanic crust forms
came up in class. One
thought is that
thermal plumes rising
off the “D” layer
travel across the
mantle and breach the
crust. This gives rise
to “mid-ocean ridges”
or incipient oceans.
The surface
expression of amidocean ridge is shown
in the next slide.
The East African Rift – Surface Expression of a Mantle Hot Spot
ETOPO 30
DEM Model
Divisions of the Earth's interior
The upper 300 km of
the Earth, essentially
the crust and very
upper mantle, are
know in better detail,
as shown in the next
slide.
Note the probable
source of basaltic
magma between 400
and 650 km.
What is basalt? Why
is it important?
Fig. 6.8
Structure of upper 300 km of Earth. The moho (M) was previously
taken to be the boundary between the crust and upper mantle. It is
basically a seismic anomaly, but it is not as profound as the seismic
low-velocity zone. The zones shown here are based on analysis of
seismic velocities from earthquakes.
Fig. 6.10
Change in the Earth’s heat
flux through time.
Although the diagram looks
complicated, there are only
4 radioactive isotopes that
heat the planet and 2 are
uranium. The other 2 are
Th (thorium) and K-40
(potassium 40).
Note that the Earth's
present-day heat flux is
only about 20% of what it
was originally.
Differentiation of Chemical Elements in Earth
Present distribution of major elements and U, Th, He and Ar
in the Earth’s atmosphere, crust and in seawater. (Elements
listed in order of abundance.
Fig. 6.12b
The Acasta Gneiss. Great Slave Province, NW
Territories, Canada. One of the oldest (4.03 Bya) dated
rocks on Earth.
Fig. 6.12a
Etched zircon from the Acasta Gneiss showing the growth
rings (bands). The 4.04 Billion year age of the Acasta comes
from age dating zircons like these.
Fig. 6.13
Atmospheric stratification and
important types of radiation
and radiation shields.
Note the density stratification
with regard to the gases
(lightest farthest out, heaviest
closer to Earth surface.
Fig. 6.14
Evolution of Earth’s atmosphere from early Achaean (5 Bya) to present. Note the
changes from Stage I to Stage II, particularly the evolution of nitrogen (N), the
virtual disappearance of hydrogen (H) and methane (CH4).
The important change from Stage II to Stage III was of course the rise of oxygen
(due to evolution of photosynthetic algae). Note the presence of the noble gases,
Ar, Ne, He and Kr. Why?
Fig. 6.15
The Global Chemostat.
This diagram shows the important flows for two elements, O and C (though not
reduced C). Other important elements, such as N, P, S, Na, Ca, and K follow
similar cycles. (Chemostat = hold chemistry constant or change slowly).
Start analyzing the cycle with the algae (as prime movers) and follow the chain.
Algae actually started the chemostat over 4 Bya. This chemostat is one of the
hallmarks of a planet with advanced life forms and it is probably very rare in
the universe.
Fig. 6.16
The global thermostat. Shallow water is heated by the sun to form the Earth’s
most important heat reservoir. The photic zone above the thermocline is the
habitat of algae and phytoplankton which from the base of the aquatic food
chain.
Below the thermocline the water is cooler and less agitated, hence less
oxygenated. These waters may even become stagnant and reducing. When they
do they constitute the first step in the preservation of organic matter, which
eventually leads to gas and oil deposits.
Fig. 6.9
Schematic diagram
illustrating Elsassar’s model
for the Earth’s magnetic
field. The solid mantle rotates
at a different rate from the
liquid outer core, which is
molten Fe and Ni sulfides.
The magnetic field is
important for the evolution of
complex life on Earth since it
shields organisms from
cosmic radiation (the same
high-energy particles that
form C-14 in the upper
atmosphere.
This slide shows
the interaction
between the earth’s
magnetosphere and
the solar wind.
Early in the Earth’s
formation the solar
wind blew the light
gaes, H an He to
the farther reaches
of the solar system.