Unit 2. Planet Earth: Origin and Evolution

Table of Contents

Unit 2. Planet Earth: Origin and Evolution. 1

Chapter 2.1 Topical explorations 1

2.1.1 How to make a solar system?. 1

2.1.2 What is light?. 4

2.1.3 Is Earth unique in our solar system?. 5

2.1.4 How can a solid Earth be a dynamic and evolving planet?. 6

2.1.5 What’s shakin’ on Earth?. 8

2.1.6 Is Earth molten inside?. 9

2.1.7 Is anyone safe from natural catastrophes?. 10


Our planet is an actively evolving system that creates growing pressures on the well-being of it occupants. The impact of natural and anthropogenic hazards leading to catastrophes, increasingly affect the global citizenship due to growing populations, human concentrations near danger zones, and our resource use. This unit explores the formation of our solar system and planet, Earth’s internal makeup and unique surface conditions and the role of plate tectonics.  Activities focus on today’s challenges of living on the surface of this active planet.




Chapter 2.1 Topical explorations

2.1.1 How to make a solar system?


Our universe started with a Big Bang, which was an immensely hot explosion that occurred approximately 13.7 billion years ago at a single point, or a singularity (Figure 2.1).




The Big Bang





Evidence of microwave background radiation in the universe, the red shift of moving objects, and the current temperature of the Universe offer a sound foundation for this theory of the Big Bang. Scientists determined the date of the Big Bang distance (d) of moving objects divided by their velocity (v).


Time (t) = d/v


The energy of the Big Bang explosion was so great that the universe is still continuing to expand to this day. As the temperature of the explosion dropped, matter that is familiar to us was formed after ~1 million years. As matter cooled and coalesced, the Universe’s most common atom, hydrogen (H) was formed (and some Helium, He), which clustered in cloud nebulas. Gravity slowly contracted these rotating gas clouds, which created dense central regions that become protostars. As greater quantities of atoms contracted in the center, the pressure in the protostars increased to the point in which nuclear fusion reactions could begin. Fusion reactions start when the energy provided by gravitational collapse is large enough to heat the region to ~10,000,000 K. Stars create the building blocks of our world as all elements up to Fe are formed from nuclear reactions. As a result of energy release from fusion, stars generate the light and energy that is available to planetary systems.


Average stars generate energy by fusing H to make He gas. Our Sun is an example of an average star, converting ~700,000,000 tons of H to ~695,000,000 tons of He and releasing ~5,000,000 tons of energy every second. Stars evolve over time such that, once all of the H available for fusion is depleted and provided they are sufficiently large, stars begin fusing He, which generates heavier elements like carbon (C). While He fusion occurs in the core of a star, enough heat is generated so that H fusion occurs in outer regions of the star. The combined energy that these two fusion reactions generate causes the star to expand enormously and evolve into a Red Giant star. Scientists expect our Sun to change into a Red Giant in approximately 4-5 billion years.


Heavy stars can evolve into Super Giant stars that create heavier elements from nuclear fusion reactions. The high temperatures (~1,000,000,000 K) required can only be reached in stars at are several times heavier than our Sun. When the fusing core evolves such that it is composed of heavy elements up to iron (Fe), no further nuclear fusion is possible and the star collapses under the tremendous gravitational forces. As the star's implodes, huge shock waves break the object apart, creating a supernova.


Our solar system is about 4.6 billion years old and the planets are made up of byproducts from stellar evolution. Dust particles from our Sun’s rotating cloud nebula settled onto a progressively flattening disc. Accretion, or the gravitational gathering of space dust, led to the formation of numerous small planetary objects, or planetesimals, in this disc. Collisions between these planetesimals lead to larger bodies capturing smaller ones, and changes of their orbits. Over time, the continued collisions eventually created a few larger bodies, called protoplanets, then even larger bodies, which we call planets.


As the planets in our solar system formed and orbit the Sun, uncondensed gases near the Sun were blown away by solar winds, which lead to today’s subdivision among the planets into inner, rocky or terrestrial planets and outer, gaseous (jovian) planets (Figure 2.2). The composition of each planet was determined by the type of material that could survive in the solid form given the temperature of the particular region of the Nebula during formation, which is called the condensation theory. For the hot inner planets, the planet-building dust grains were made up of silicates and iron. For the cold outer planets, the H and He were retained by a combination of the larger gravity for these massive bodies and low temperature in these outer regions that formed of ice, holding onto the H and He.



Our solar system, consisting of the inner or rocky planets (Mercury, Venus, Earth, Mars) and outer or gaseous planets (Jupiter, Saturn, Uranus, Neptune). The outermost planet, Pluto, is a special case, whose status as a planet recently led to the proposal of an additional three planets (Cerus, in the asteroid belt between Mars and Jupiter, and Charon and Xena at the outermost regions).




2.1.2 What is light?  


Visible light in our solar system is only a small part of a broad electromagnetic radiation spectrum that also includes radio waves, infra-red (heat) rays, microwaves, x-rays and gamma rays. These electromagnetic waves differ by their wavelength and frequency, with shorter wavelength (or higher frequency) radiation carrying more energy than longer wavelength (or lower frequency) radiation (Figure 2.3). Wavelength is a measurement of the distance between wave peaks and frequency is the number of peaks that pass a given point during a specific amount of time; waves with short distances between peaks have high frequency (and high energy), and waves with long distances between peaks have a low frequency (and low energy).  For example, radio waves are safe to us, as they have a low frequency and long wavelength, while high frequency waves like X-rays are dangerous because they carry a lot of energy.


Figure  The electromagnetic radiation spectrum, which can be expressed in terms of energy, wavelength, or frequency.



Several important laws govern the energy of electromagnetic radiation. Planck's Law, also called the "black-body" formula, describes wave energy as function of frequency:

E = h . f , with h = Planck's constant (6.626E-34 J.sec) and f is frequency.


Frequency is inversely proportional to wavelength, so Planck's Law can also be written as:

E = h . c/l, with c = light speed (3E8 m/sec) and l is wavelength.


The relationship between wave energy and temperature is given by Stefan-Boltzmann Law:

E = s . T4  , where s is a constant, and T is temperature (in degrees K).


Stefan-Boltzmann's Law tells us that much more energy is contained in hot objects than in cold objects, which we painfully learn from touching a hot stove instead of a cold stove. Combining these two laws shows the relationship between temperature and wave properties, wavelength and frequency, which allows us to determine the temperature of far away objects by measuring their electromagnetic radiation:

h . f =  s. T4 and h . c/l =  s. T4


Temperature is function of frequency and an inverse function of wavelength. This tells us that our hot Sun emits radiation with a higher electromagnetic frequency and shorter wavelength than cooler Earth, with a lower frequency and longer wavelength radiation.  The radiation of our Sun, with a surface temperature of ~6000K, emits most of its energy in part of the spectrum we recognize as visible light, which is the reason many life forms on Earth, including humans, capitalize on this property.  Other object radiate in other parts of the spectrum, such as Earth emitting in the red part of the spectrum, as it has a surface temperature of ~300K.



2.1.3 Is Earth unique in our solar system?


Earth, the third rock from the Sun, is a unique planet in our solar system because it contains liquid water, which is fundamental for the existence of life. In fact, water occurs on our planet in all three forms: solid (ice), liquid and gas (vapor). The distance at which Earth orbits the Sun is mostly responsible for this. The farther you are from an object emitting radiation (such as the sun) the less radiation you receive. This is determined by the R-Squared Law, telling us that a doubling of the distance away from an emitting object reduces the radiation received by a factor of four. Recall that radiation is a form of energy, and that temperature is a measure of the average speed of moving atoms. Therefore, as the speed of the atoms increase, temperature increases. The mathematical description for this relationship tells us that temperature increases as the mean atomic speed squared. The R-squared law and the relationship between radiation and temperature predict the temperature of the planets in our solar system, based on the distance in which they orbit the Sun. We predict that planets farther from the Sun are colder than those closer to the sun (Figure 2.4).



The role of orbital distance from the sun





The prediction of decreasing planetary temperature away from the Sun is generally correct, but Earth is warmer than we would expect.  Figure 2.5 shows predicted temperatures and measured temperatures for the planets. The presence of natural greenhouse gases, such as like CO2, methane, water vapor, nitrogen oxides and chloroflurocarbons, in the atmosphere of planets affects this relationship, resulting in some planets that are warmer than expected based on their orbital distance alone. On Venus, Earth, and a few other planets to lesser degree, the composition of the planetary atmosphere selectively absorbs longwave (or infrared) radiation, trapping radiation emitted from the planetary surface back into space. Radiation of energy from the surface back into space is key to a planet’s surface energy equilibrium, because if all radiation from the Sun stayed at the surface, planetary surface temperature would continue to increase, instead of reaching a fairly constant temperature.



The influence of the natural greenhouse effect on planetary temperature



Radiation from the Sun passes through the atmosphere and warms the surface of the planet. The surface of the planet converts this shortwave energy to longwave energy and reradiates much of this back out into space. However, the presence of greenhouse gases in the atmosphere, selectively absorb some of the longwave radiation that the planet emits back to space, and traps it in the lower atmosphere as heat.  This natural greenhouse gas condition results in an average temperature on Earth that permits water in all three forms and the sustenance of organic life. Greenhouse gas production on Venus, on the other hand, especially CO2, was not balanced by sequestration of these gases in rocks and minerals, leading to an atmosphere that became exceedingly hot, well beyond the predicted temperature for this planet (Figure 2.5). This runaway greenhouse effect of Venus is one the scenarios explored in today’s discussion on fossil fuel burning and greenhouse gas production.


Clearly, the natural greenhouse effect is beneficial and partially responsible for the existence of life on Earth. This effect should be distinguished from today’s anthropogenic greenhouse effect that is causing concern. Human activities such as fossil fuel combustion, forest burning and land use changes, are altering the balance of the natural greenhouse gases by increasing their concentration in the atmosphere. This causes more long-wave radiation to be trapped in the Earth’s atmosphere and warms the surface of the planet, shifting the planet’s equilibrium.  Whereas we are nowhere near the state of Venus, which would not permit any life on the surface, we are certain that today’s human activities cause global warming.


2.1.4 How can a solid Earth be a dynamic and evolving planet?


Earth is a dynamic planet that continues to evolve, because it is still geologically active. The Earth consists of different layers that formed during the early accretion and differentiation period in her history. Recall that accretion is a process by which an object increases in size by the gradual addition of smaller parts, whereas differentiation is the gravitational separation of materials according to their specific gravities in a mixture that was originally homogeneous.


Apart from the thin outer regions of atmosphere, ocean and crust, the Earth is composed of two main compositional layers (Figure 2.6). Earth’s innermost layer is an iron core, which formed very early in the planet's history, about 50 million years after initial accretion. The Earth's core is present in two states, an inner core of solid iron and an outer core of molten iron, which together comprises of 30% of the planet’s total mass, with a radius of ~3400 km. On top of the core lies a less dense mantle, consisting of silicates, which is about 2900 km thick and makes up around 65% of Earth's total mass. The mantle is solid rock, but over geologic time behaves like a very sticky liquid, a process called plasticity. Above the mantle, lies the Earth’s crust made of light silicate rocks and other oxides, which is ~45 km thick. The outermost few hundred km of the solid Earth represents only a minimal fraction of the Earth’s mass, but this is where much of the action is concentrated.



Layers of Earth and movement of plates




Add convection arrows in mantle and outline of today’s plates (a and b).


The outer layer of Earth consists of (lithospheric) plates that move at cm per year. There are nine major tectonic plates and several smaller crustal ones on today’s Earth. Plates are constantly being created and consumed, with their interaction generating earthquakes, volcanoes and mountain ranges. Earth’s internal heat from the original planetary creation drives plate movements over the asthenosphere (low strength portion of the upper mantle). Escaping heat creates convection that is the primary driver for plate motion (Figure 2.6). These heat cells are similar to atmospheric convective cells that circulate heat around the globe. Hot mantle material is less dense, so it rises and spreads laterally at the top, where it cools and sinks back down into the mantle, continuing the process cycle.  Plates are the cold portion of the cycle, so they sink into the mantle at subduction zones, while elsewhere the upwelling creates new plates (at ocean ridges). 


This scenario is the theory of plate tectonics, which revolutionized the Earth sciences by offering a comprehensive explanation for many observations and processes, such as past faunal distributions and glaciations, and earthquakes and volcanoes that take place on our planet today.  Plate tectonics allowed reconstructions of past plate reorganizations, such as the large landmass Pangea that formed ~300 million years ago, connecting North and South America with Europe and Africa, and other exotic reorganizations, that explaining links among rocks and life on today’s disparate continents.

2.1.5 What’s shakin’ on Earth?


A remarkable picture emerges when plotting the surface location, or epicenters, of earthquakes on a map of Earth.  The majority of earthquakes are confined to narrow, linear zones that represent today’s tectonic plate boundaries. The type of fault motion associated with earthquakes defines the displacement at the plate boundary. The interiors of plates are notably free of earthquakes, with only a few exceptions to this rule (Figure 2.7). Earthquakes are the product of large releases of built up energy along slowly moving plate boundaries. The Earth’s plates can accumulate only a small amount of energy called the elastic limit. When energetic pressures exceed this elastic limit, the rock breaks and releases the energy as earthquakes.  Because plate motion occurs over millions of years, earthquakes are a steady and lasting feature at these locations.  Earthquake energy is released along faults, and this can occur gradually, with little or no seismic activity (called creep) or with potentially cataclysmic results. The greater the amount of energy released, the larger the earthquake magnitude, and the higher the impact and destructive potential.


One type of seismic zone follows a near-continuous line of ocean ridges. The plate is thin, hot and weak at these boundaries, so elastic strain cannot build up enough to cause large earthquakes. Volcanic activity is also common for this type of seismic zone, called a divergent boundary; for example, Iceland, Azores and Tristan da Cunha. 


The second type of earthquake is related to regions of convergence between plates. One plate is thrust or subducted under the other plate, where often a deep ocean trench is produced. This seismic region is called a convergent boundary. In many areas, ocean trenches are associated with volcanic island arcs, for example the Japan trench. Elsewhere, along the Peru-Chile trench, the Pacific plate is being subducted under the South American plate, forming volcanism that created the Andes Mountains. Convergent zone earthquakes can be shallow, intermediate, or as deep as 680km, according to their location near the downgoing lithospheric slab. These inclined planes of earthquakes are also known as Benioff-Wadati zones.


The third type of earthquake zone is shallow-focus event that are mostly unaccompanied by volcanic activity. The San Andreas fault is a good example of this, as is the Anatolian fault in Northern Turkey and the alpine fault in New Zealand. Along these faults, two plates are scraping past one another. The frictional resistance between the plates builds relatively large elastic strains before major earthquakes periodically relieve the energy.  Activity does not normally occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault, while major earthquakes along the North Anatolian Fault have been steadily moving west.


Using minimally three seismograph stations, scientists determine the location and magnitude of an earthquake from the difference between primary (faster) and secondary (slower) wave travel times, and the magnitude from the signal strength generated by the release of earthquake energy.  The relatively slow waves traveling along the Earth’s surface are responsible for much of the damage associated with earthquakes.  Large earthquakes can cause catastrophic loss of life and massive amounts of structural damage from direct ground shaking, and indirect effects, such as fires, landslides, and tsunamis.



Use EQ and volcano distribution map (insert hazard map in activities?)



2.1.6 Is Earth molten inside?


Except for the outer core, the Earth is a solid rock. However, expressions of localized melting are apparent in volcanoes, because the solid mantle is very near the melting point of rock. Small changes in temperature, pressure or water content create local melting in magma chambers that erupts as lava at the surface, forming volcanoes.


As you move toward the center of the Earth, the ambient temperature increases, which is called the geothermal gradient. Temperature increases changes the state of matter from solid to liquid to gas. Pressure also increases with depth, but opposes this phase pattern, such that increasing pressure will tend to keep matter in a more dense form. In Earth, the balance between gradual increase in temperature and pressure results in a solid mantle, but with temperatures that would produce melts at the surface. So, melting occurs when we increase temperature or when we reduce pressure (Figure 2.8). In terms of our planet, this means an added source of heat will change solid mantle to liquid, where the material is already close to the melting curve. Similarly, bringing relatively hot mantle rock to the surface (reducing the pressure) will result in melting without requiring additional heat. These two conditions explain the occurrence of volcanism around local mantle hotspots, such as Hawaii and Yellowstone, and volcanism at divergent plate boundaries. 


Phase diagram showing melting curves as a function of pressure, temperature (a) and water content (b).



A third process that induces melting is the addition of water that weakens the atomic structure of solids (Figure 2.8). The melting curve for dry rocks lies at higher temperatures than that for wet rocks, so the initial solid may become a liquid by the addition of water. This condition occurs at convergent plate boundaries, where water-producing mineral reactions (called dehydration reactions) release water that promotes melting in nearby solid rock. Volcanoes of the Pacific Rim are mostly formed in this manner. Collectively, these three processes, which are closely linked with plate tectonics, explain the occurrence of liquid rocks and volcanoes, but do not require that the deep Earth is molten.


Earth has a long history of volcanism including large-scale planetary outgassing in her earliest years that created an early atmosphere. Today the impact of volcanoes is mostly measured as a societal hazard. They generate a variety of phenomena that affect the Earth's surface and short-term atmospheric patterns, endangering people and property, but also create fertile agricultural areas. 


2.1.7 Is anyone safe from natural catastrophes?


Geologic disasters can result from a wide variety of hazards such as earthquakes, floods, volcanic eruptions, tsunamis or landslides. In addition, atmospheric hazards like hurricanes, tornadoes and severe storms can cause severe damage and destruction to populations, property and habitats.  Anthropogenic (human-induced) hazards like toxic spills, waste disposal and fires also run the risk of causing disasters for people and natural habitats. Whether populations and communities are impacted by disasters depends on their capacity to avoid and respond to an event. This capacity in turn depends on a number of social, cultural, economic, and environmental factors that are unevenly distributed in today’s society.


Population’s vulnerability to a disaster event strongly depends on social factors such as the population’s proximity to the event, population density, and cultural factors like legends and beliefs. Additionally, a population’s vulnerability to a disaster event depends on the population’s scientific understanding of the hazard, which relies on governmental investment in public education. The strength of social infrastructure, such as investment in awareness programs, disaster preparedness and response, and early warning systems are key factors in determining a population’s vulnerability to a disaster event.  Additionally, governmental investment in appropriate infrastructure and sustainable building strongly impact a population’s ability to react to a disaster event.


Often investment in education and social programs for disaster preparedness depend on the regional economy and the financial capacity of the community to prepare for and respond to an event. Poverty is therefore a key factor in vulnerability to disasters, as it leads to poor infrastructure, increased population density, and development and habitation on susceptible land. Developing countries are more at risk for human hazards than are developed countries, due to lack of understanding, education, infrastructure and economic development (Figure 2.9). Countries with greater economic development often have a greater proportion of dispensable income that can be allocated to improve social programs, education, as well as infrastructure and disaster response programs. Globally, the sharp growth in the human population has greatly contributed to the number of people that are vulnerable to natural disasters, and this is especially the case in the developing world.



Disaster induced human death by income




However, relatively rich, developed nations face new challenges from the sheer cost and abundance of natural disasters. The number of disasters that impact countries and the expense of these events have increased exponentially in the last fifty years (Figure 2.10), which means that important choices about rebuilding and improving infrastructure will have to be made. For example, 2005 hurricane Katrina destroyed much of New Orleans and rebuilding will cost trillions of dollars, but took only ~1000 lives. In contrast, the 2004 Sumatra earthquake cost is estimated at ~$10 billion, but was responsible for >250,000 deaths. Some of these costly disasters (both socially and financially) might have been minimized if ecological considerations and sustainable development programs were utilized before the event. For example, the removal of mangroves in the coastal regions of many Indonesian islands greatly contributed to the damage and loss of life from the 2005 tsunami, demonstrated by areas where mangroves remained intact that received substantially less damage. Likewise in New Orleans, the removal of coastal wetlands, which buffer hurricane impacts, contributed to the massive damage that resulted from the storm.



Number of disasters and economic losses over time




Today’s society is mostly reactive to natural catastrophes; however, the wellbeing of ecosystems and the world’s citizens need a more proactive stand, both in wealthy and in poor nations alike.


Last updated: 9/10/2006 10:35 AM