Chapter 4 - Geology of Earth: the Earth is old

We will focus not on details of the Earth's history, but the chances of finding life elsewhere. Still, there are some key points about Earth's history and geology that are very critical to understand. The text book will give you lots of details. As always, use these notes as a guide to focus your reading and study of certain topics. Also, always ask questions that you may have.

Some clicker questions to start

1. What is the age of the Earth?

a. Less than 10,000 yrs

b. 4.5 billion yrs

c. A few hundred million years

d. The Earth has been around forever.

2. Could the Earth have a very different age than the Sun? A. Yes B. No

General aspects of the question of the Earth's age:

- The Origin of Earth must be coupled to origin of Sun: it is not possible that either one is much older than the other, nor that Earth was "captured" by Sun at later time.

Why not?: all planets orbit in same plane, almost circular orbit of Earth, etc., hence capture of any planet by the Sun in the solar system not very likely because they would all be moving in different directions and not in one plane.

- The Origin of Moon is coupled to origin of the Earth: Our Moon is very peculiar in that of all terrestrial planets, only Earth has a large moon. Likely, the formation of the Moon came early in chaotic solar system due to major collision of young earth with another large body. Again, capture of Moon later on is impossible.

- Earth cannot be very young: Geological evidence shows drastic changes in Earth's surface which must have taken a long time.

E.g.: - New Mexico was ocean in the past, like many other

places on Earth too.

- Layers and layers of sedimentary rock on top of one another in many locations. Fossil evidence of past living creatures in many of these layers.

- Earth cannot be infinitely old: Basically, this is an energy problem. An infinitely old Earth would be cold. The Sun cannot shine forever, the Sun could not start as a cold sphere of gas at its present size with cold planets orbiting it that then heated up. The formation of solar system from collapsing gas cloud resulted in hot star (Sun) at the center with planets from the beginning. To stay hot, energy is required. This comes in various forms, but none of them will last forever.

Key aspects in untangling the history of the Earth:

1. Understanding the sources of change to the Earth's surface: subject of Geology

e.g. formation and destruction of mountains, volcanism

plate tectonics

oceans

ice ages

changes to Earth's atmosphere

2. Deciphering the fossil record of past life forms on Earth; it helps trace geological and climatological changes, and other catastrophic events.

3. Determine age and properties of Moon; the Moon's surface is old and cratered.
A picture of the lunar surface

3. What is the origin of the craters on the Moon?

a. Volcanic eruptions.

b. Evidence for early water puddles on the Moon.

c. Depressions caused by impacts of other objects.

d. Crust deformations as the Moon cooled down.

The craters on the Moon suggests that the early Earth must have had many craters too (fewer today, but not zero.)

4. Understand energy production in the Sun:
How constant a source of light (energy) is the Sun? How long can the Sun shine?

Some concepts and tools related to Earth's geology:

- Different types of rocks: igneous (think: volcanic), sedimentary (think: deposited slowly), and metamorphic (think: rocks changed under high pressure and heat).
- Understanding forces of change: erosion, plate tectonics, impacts, volcanic outbursts, ice ages

- Radio-metric dating of rocks
- Catastrophism and uniformitarianism theories about geological changes.

GEOLOGICAL HISTORY OF EARTH

The challenge in dating the Earth's crust (and hence the Earth itself) is that due to erosion, volcanism, and plate tectonics, the Earth's surface is continuously changing. Contrast this with the Moon, where the lack of atmosphere and liquid water implies a mostly unchanging surface once the Moon had cooled and most of the impacts, that were much more frequent in the early solar system, had occurred. Lunar rocks have been brought back by the astronauts and dated in the same way as the Earth's rocks; the oldest lunar rocks are indeed over 4 billions years old. We have also found rocks from Mars on Earth (how can that be?) and have measured their ages.

Here is a discussion of the history of dating the age of the Earth:
Changing Views of the History of the Earth

There are many places on Earth where the effects of plate tectonics and erosion have exposed the older rock layers, showing a gradual build-up of layer upon layer of sedimentary rock, with the oldest layers at the bottom. The Grand Canyon is a famous example, but we can see similar features in many mountain chains. Note that the age of the rocks is usually much larger than the age of the mountain! Such geological features have been used in the past by e.g. James Hutton to argue for an old Earth. Likewise, Charles Lyell has been advocating an old Earth. The key point is that within many of these layers, that cannot have been deposited very quickly, we find fossil evidence showing drastic changes in past life forms. This in itself, even without accurately dating the layers, implies that the Earth must be very old. It is simply not possible to deposit thick layers all over the earth through sedimentation over brief periods of time.

The age of rocks is determined by radio-metric dating.

The principle behind this is that rocks contain radio-active elements that over time decay into other elements. A radio-active element is a particular form of certain atoms that are not stable; the nucleus of the atom disintegrates into other nuclei. Isotopes are often unstable nuclei. An isotope is a form of the nucleus of an element that has a different number of neutrons in it compared to the normal form of that element. E.g.  ¹⁴C(arbon) has 6 protons and 8 neutrons in the nucleus, while the stable form ¹²C(carbon) has 6 protons and 6 neutrons.  By studying the relative amounts of the radio-active materials and their end products, we can find the time the rock was formed (although Carbon is not a good element to date rocks, its half-life is much too short, see below). Rocks form through three processes: igneous rock is formed from molten materials that cools down (think of volcanoes), sedimentary rocks are formed by gradual deposition of materials on the surface, metamorphic rocks are structurally transformed by high heat and pressure (so they do not cool down from molton lava, but are transformed deeper in the crust due to the conditions there). Once a rock is formed, its composition is fixed and so then the radio-active clock starts ticking and causes the changes in composition over time.

See information in book on radio-metric dating methods, different types of rocks, plate tectonics, etc.

Some key points on radio metric dating:

- radiometric (sometimes called radioactive) dating is the key point in dating rocks. Make sure you understand the basic principle, and the concept of "half-life".

- There are many radio-active elements with different half lives. For dating "young events" we have e.g. the ¹⁴C isotope with a half-life of 5730 years. This decays into ¹⁴N (nitrogen) + electron.

Then there is ²⁶Aluminum with a half-life of 700,000 yrs, and ²³⁸Uranium with 4.47 billion years. The ²³⁸Uranium decays into ²⁰⁸Pb (lead). The book gives more examples.

Question: How much of a radio active substance is left after 3 half lives have past?

a. 0% b. 12.5% c. 50% d.75%

Question: If the half-life of ¹⁴C is only 5730 yrs, and carbon formed in stars, how can we still have this isotope on Earth?

- radiometric dating is complex and not all rocks can be dated individually. However, the overall picture that emerges is very conclusive:

The oldest rocks on earth are about 4 billion years old. (That does NOT mean the Earth is 4 billion years old though; its age is 4.5 billion years, and is known to about 0.02 billion years accuracy). Meteorites from space are 4.5 billion years old, giving very consistent numbers. Very detailed physics models of the sun confirm that the sun is 4.5 billion years old (we know more about the sun's interior than the earth's, from studies of solar oscillations, observations of solar neutrinos, and chemical analysis of the outer solar atmosphere).

- The early thoughts about geological changes centered on CATASTROPHISM. This was in the Western world inspired by biblical story of Noah's flood and other myths, and also based on experiencing major volcanic eruptions or earth quakes that had large effects. However, much of geological evolution happens slowly, a view advocated by Hutton, Lyle, and others, which is called UNIFORMATARIANISM.

Catastrophic changes can occur in short time, e.g volcanic eruptions, local floods, etc. It is e.g. possible that in historic times a major flood did occur in the Middle East. However, the occurrence of catastrophic local floods is very different than a global world-wide flood that encompassed the world and happened over a very short time scale. Note that sea levels, as a result of the melting of the ice after the end of the last ice age did lead to a major rise in sea level (about 400 ft!). But that likely happened over a long time span, in a time from before we have written records, and over a time span that likely was sufficient that affected populations could move up to higher ground as the sea level rose.

Mountains are made over long times, by uplifting and collisions of the major plates on earth, or occasional volcanic eruptions. Accurate position measurements across the continents can now measure how much mountains rise every year, e.g. in the Himalaya's, due to plate tectonics.

Most canyons, like the Grand Canyon have been created over millions of years as the river cuts through the various rock layers. The oldest rocks at the bottom layers of the canyon are of course much older than the canyon itself; it didn't take the Colorado River billions of years to carve the canyon. But it did expose the oldest rock layers which are about 2 billion years old.

Here is a great summary of the geology and age of the Grand Canyon:
The geology of the Grand Canyon (with pictures!)

PLATE TECTONICS

It was Alfred Wegener who proposed in the 1912 that the Earth's continents are drifting apart, based on the similarity in coast lines between Africa and South-America, both in shape and geological features found there. This idea, which was not accepted for many years, is now solidly confirmed through observations of the actual movements of the plates. It accounts for the occurrence of earth quakes and volcanoes, the rise and subduction of plates, the presence of earth encompassing long zones of volcanic or earth quake activity ("ring of fire"), the creation of mountain chains, etc. Read the web based materials and text book to familiarize yourself with this radical idea.

Plate tectonics, volcanoes, earth quakes, and the ring of fire.

The ultimate "engine" which provides the energy to move continents is the interior heat of the earth, combined with the Earth's rotation, which create convective motions that bring hot material up from the interior to the mantle, and which drive motions of the mantle. Make sure you understand what convection is:

Click on: What drives plate tectonic motion?

Some aspects critical to developing and sustaining life on Earth:

• Earth's location in the solar system: just the right temperature and pressure to have liquid water in abundance.

• Geological changes. Obviously, changing one region from ocean to desert (as New Mexico) has large consequences for life, it will not merely move life from one place to another but also change life as it evolves to new climate conditions.

• Atmosphere and climate. Earth's atmosphere did not start out oxygen rich; the oxygen disappears from the atmosphere in chemical reactions and must be replenished; plants do this through photosynthesis.

An atmosphere can form around a planet due to "outgassing"; originally gasses such as methane and CO₂ trapped in rocks at time of formation are released when the rocks are heated or melt inside Earth. Volcanoes are critical. Early bacteria and eventual evolution of plant life led to the creation of an oxygen rich atmosphere which is now critical for life as we know it.

Can a planet maintain an atmosphere? The air can escape into space if the planet's gravity is not strong enough and/or the air's temperature is too high. E.g. light elements such as He can escape the Earth's gravitational field at prevailing temperatures since they move faster at a given temperature than heavier elements such as oxygen molecules. Mercury is an example of a small planet (hence low gravity, and low escape velocity) close to the Sun (hence hot, so atmospheric particles would move very fast) which does not have an atmosphere to speak of.
 




GREENHOUSE EFFECT - REGULATING A PLANET'S TEMPERATURE

Question. What do you know about the greenhouse effect on Earth?

a. The greenhouse effect is due to the atmosphere trapping visible light from the Sun.

b. The greenhouse effect is bad for the Earth's climate.

c. The greenhouse effect is also referred to as the "ozone hole problem"

d. All of the above.

e. None of the above.

The "greenhouse effect" is critical to regulate the difference between day/night temperature on Earth and to maintain a higher temperature than the Earth would have without an atmosphere. The greenhouse effect is the property of the atmosphere that makes it act like a greenhouse (or a parked car): light can get in, heat generated inside cannot get out and so the interior warms up. The atmosphere contains "greenhouse gasses" such as carbon-dioxide (CO₂) and water (H₂O) that help it act as the glass walls of a greenhouse, in that these molecules do not transmit the infrared light (heat) emitted by the Earth's surface so it cannot escape back into space. The result is a warmer planet. This is a good thing.

Excess emission of "greenhouse gasses", which comes from e.g. burning of fossil fuels, can lead to global warming, which is a global increase in the average temperature on Earth. This may well lead to serious consequences; we will find out in the next 50 to 100 yrs. Remember, fossil fuels: oil, gas, coal, are all dead plant life and in the burning process we release large amounts of carbon-dioxide into the atmosphere. The chance that we would turn into Venus is very small at this point, but the chance for drastic climate change should certainly be taken seriously.

The earth has a CO₂ - Cycle that regulates the climate, as the book discusses. We have sustained very large temperature variations in the past, yet life is still here. (Note that CO₂ is not the only "greenhouse gas"). BUT:

The problems for us in terms of global warming are:

1. The CO₂-cycle likely operates far too slow to accommodate rapid change;

2. People cannot tolerate large changes at all: life may have survived large climate changes in the past, but now we have a planet with 7+ billion people that demand food and water every day. Even small changes in climate can drastically affect food production. Likewise, a 3ft rise in sea-water level would have large consequences for many people on Earth.

3. Global warming or climate change driven by fossil fuels is one large experiment for which we have no reliable idea of the outcome: how bad will it get, and can we take the risk? The permafrost regions would release a lot of methane gas if they melt as the Earth warms. Perhaps better to avoid this "experiment" on global scales with possible disastrous consequences if we can!

Clicker questions: Are you concerned about climate change, resulting from global warming?

a. Yes

b. Maybe

c. No

Are you convinced by the scientific evidence that the current period of climate change is driven by human activities, in particular energy consumption:

a. Yes

b. No

c. Maybe

Biggest risk: a run-away greenhouse effect, whereby some warming of Earth leads to emission of more water vapor, carbon-dioxide, and methane into the atmosphere, which leads to a yet warmer Earth, which leads to more water, carbon-dioxide and methane into the atmosphere, etc. Note that the oceans and carbonate rocks (e.g. lime stone) contain far more carbon-dioxide now than our atmosphere; we don't want all that released into the atmosphere. The oceans act like a sort of thermostat in possibly helping to regulate carbon-dioxide content of the atmosphere. We don't understand this balance very well. And it likely acts too slowly to solve our present issues.

A key point: global warming is not the same as the "ozone hole problem".

Another important factor in evolution of life on Earth: Mass extinctions and Impacts:

• Mass extinctions. This is discussed more in next section, although only briefly. One of the key mechanisms for mass extinctions seems to be impacts by either comets or asteroids. Other causes may include dramatic climate change possibly caused by periods of dramatically increased volcanic activity. Note that a large impact will also lead to drastic climate change over a very brief period which would make many species vulnerable to extinction even if the climate eventually recovers.

• Asteroids and comets can both cause destructive impacts. Asteroids are the "analogs" of the terrestrial planets, in terms of being rocky in character and coming mostly from the inner solar system (out to the orbit of Jupiter). Asteroids range in size from small rocks to big 1000 mile size objects. Comets are smaller generally, consist mostly of ices and dust bound loosely together in a core of about 10 miles across. The comets come from the far outer solar system. When they approach the sun they lose some of their ice and dust causing the brilliant tails. In a collision, the impact energy equals the kinetic energy of the impacting object, which is equal to 0.5 x mass x (velocity)².

• The Earth's atmosphere stops small impacting objects (they disintegrate due to friction and heat or may skip off when hitting at a glancing angle) but does not offer any protection to large impacting objects, and those are the most dangerous.

Some more clicker questions

1. We can get an idea what typical collision speeds between asteroids or comets and the Earth would be from the speeds with which objects are orbiting the Sun. How fast would you guess the Earth is going around the Sun?

a. 200 miles/hr b. 2000 miles/hr \

c. 10,000 miles/hr d. 70,000 miles/hr

2. What would be worse in terms of impact energy in a collision with Earth? In all cases, the asteroid has the same density.

a. An asteroid that is 10 miles across colliding with us at 5 miles per second.

b. An asteroid that is 5 miles across colliding with us at 10 miles per second.

c. An asteroid that is 2 miles across colliding with us at 50 miles per second.

Some information on asteroids and comets:

Most asteroids come from the Asteroid Belt between Mars and Jupiter

Information on Asteroids and near Earth asteroids

Images and more information on asteroids

General information on comets

The 1994 Comet impact on Jupiter