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. Protoplanets9 or 10 formed from planetesimals

3. Planetsformed 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, low-density 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 showingmoons Io (crossing at equator) and Europa.

Fig. 6.7

The earth’s interior.

1. Crust2. Mantle3. Outer core (liquid)4. Inner core (solid)

Note density discontinuity at core-mantle 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 amid-ocean 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.