Dating Earth

Respond to this Friday Faithfuls challenge by writing anything about how old the Earth is, or you could write about whatever else you think might fit.  The age of the Earth is a topic that leads back to the issue of Darwinism.  For nearly 150 years debates concerning the meaning and consequences of the theory of evolution as proposed by Charles Darwin have continued in various circles, particularly in the church.  Darwinism is a term representing the theory of evolution in combination with particular meaning and consequences attached to it.  Before Darwin the age of the Earth was determined by what was written in the Bible, where many people believed that Earth was only about 6,000 years old, contradicting Darwin’s theory of evolution, thus not nearly being old enough for countless species to have evolved.  At a broad level, a Creationist is someone who believes in a god who is the absolute creator of heaven and earth, which was made out of nothing.  ‘Young Earth Creationists’ think that Irish archbishop James Ussher’s seventeenth-century calculation of about 6000 years is a good estimate for how old the Earth is.  This age was arrived at using the chronology evidence given in the Bible, where the creation story in Genesis is taken literally.

Sometime around the year 425 B.C., Herodotus a Greek historian, dated the age of the Earth using the rate of growth he observed in the Nile Delta and he determined that Earth was several thousand years old.  He observed that the Nile River Delta was in fact a series of sediment deposits built up in successive floods.  By noting that individual floods deposit only thin layers of sediment, he was able to conclude that the Nile Delta had taken many thousands of years to build up.  The notion that one could estimate ages of geologic features by determining rates of the processes responsible for such features, and then assuming the rates to be roughly constant over time was an important step in Geology.  Georges-Louis Leclerc, Comte de Buffon built his reputation upon a lifelong scientific odyssey – a 36-volume exploration of the natural world, his Histoire Naturelle.  George du Buffon concluded that Earth had a beginning, and will have an end, determined by the cooling of the Earth from a molten ball torn from the Sun, to an ultimate death by freezing sometime in the future.  In 1780, he calculated a span to the present day experimentally, from the cooling of iron spheres taken from a furnace, and published an age of the Earth of 75,000 years.  His experiment involved melting rocks and then timing how long the rocks took to cool and solidify.

In 1785, the Scottish geologist James Hutton proposed that different layers of rock came from specific periods of geologic time.  Hutton claimed that the Earth went through a series of cyclic depositions and uplifts and so had no beginning or end; thus, it was infinitely old.  Hutton also came up with the idea that present is the key to the past, meaning that geologic manifestations present today can tell scientists about how they were formed in the past.  In 1788, English Geologist, Charles Lyell was instrumental in discovering geological time by estimating the time necessary to bring about the change of species during Tertiary time (20 million years) and then, dividing the history since Cambrian time paleontologically into twelve equal parts.  Lyell estimated that life had been on earth for about 240,000,000 years.

Charles Darwin an unpaid naturalist who signed up for a 5-yr expedition around the world aboard the H.M.S. Beagle and he took Lyell’s volume one of Principles of Geology with him on the trip.  He realized that in spite of all species reproducing, no one species overwhelmed the Earth, concluding that not all individuals produced in a generation survive.  He also found that individuals of the same kind differ from one another and concluded that those with the most favorable variations would have the best chance of surviving to create the next generation.  Based on geological evidence of observed rates of erosion and sedimentation, and the gradual processes of natural selection that he observed in organisms, Darwin estimated the age of Earth to be at least 300 million years.

In 1822, Jean-Baptiste Joseph Fourier, French mathematician and physicist did temperature studies on the soil of Earth for his mathematical theory of heat conduction, that was based his reasoning on Newton’s law of cooling, with the flow of heat between two adjacent particles being proportional to the difference in temperature.  Fourier studied the diurnal and annual variations of temperature underground and terrestrial temperatures.  He realized that the surface temperatures on Earth changed drastically, but temperatures of the ground below the Earth remained more constant.  At a certain depth below the surface of the Earth, the temperature at a given place experiences no annual variation and this permanent underground temperature becomes less and less according as the location of this place is more and more distant from the equator.  Fourier calculated that a planetary object the size of Earth should not be as warm as it is at its distance from the Sun.  Therefore, he reasoned, there must be something else apart from incoming solar radiation, some other factor that keeps the planet warmer.

At the end of the nineteenth century, one of the most apparently serious problems faced by Darwinian evolution was that the evolutionary process was so slow that the Earth had not been around for long enough for the observed diversity of life to have arisen.  William Thomson had had an interest in geology and wondered if physics could illuminate its mysteries.  The main proponent of Darwin’s old Earth argument was William Thompson aka Lord Kelvin who knew the temperature at which rock melts (typically in the range 1000 ± 300°C for most types of rock) and it seemed reasonable to assume that life could not have been present at the time when the whole surface of the Earth was molten magma.  In 1862, William Thompson estimated the Earth to be from 20-40 million years old.  Thompson knew from Fourier’s results of temperature of the soil at certain depths that, while the temperature of the surface of the Earth undergoes wide daily and annual variations, the Sun’s influence diminishes rapidly once one heads more than a few hundred meters down into the ground.

Thompson felt justified in ignoring the Sun’s influence and took as a boundary condition that the temperature at the surface of the Earth was zero, appropriate for outer space.  If he could determine how long it would take a sphere, initially heated uniformly to around 1000°C, to cool into outer space so as to form a temperature gradient near its surface of the size then observed, then this time would set an upper limit on the time available for evolution.  Charles Robert Darwin an English naturalist and geologist was tremendously worried by Kelvin’s conclusions, more than by any other argument proposed against his Theory of Evolution.  Kelvin’s mathematics were impeccable, so what’s going on?  We now know that radioactivity, a source of energy unknown to Kelvin, primarily from Uranium deposits deep under the Earth’s mantle is responsible for heating the Earth from within.

Thomson’s friend the Scottish physicist J.D. Forbes, took measurements in and around Edinburgh, and estimated that the earth’s temperature rose by one degree Fahrenheit for every 50 feet of descent finding this to be consistent with depth.  This geothermal gradient indicates that heat is flowing from the earth’s hot interior to the cool surface, where it escapes.  The heat loss can thus be measured, and Kelvin reasoned that, if in losing heat the earth is becoming progressively cooler, then in times past it must have been warmer.  The farther back in time, the warmer it must have been.  Kelvin regarded this phenomenon as dissipation of heat from an originally molten condition, and he showed that it could not have been very long ago, in terms of geologic time, that the earth was molten.  Lack of details on melting points of rocks and their thermal conductivity at high pressures and temperatures prohibited exact calculations of the time since the crust solidified.

These assertions had ominous overtones for evolution, not only because of the short length of geologic time they allowed, but also because, old or young, the earth’s crust had been considerably hotter in the geologic past.  If this were true, then the surface could not have maintained the kind of long-term stability that was necessary for the gradual evolution of life.  Late in the nineteenth century most geologists had simply adjusted their ideas of the duration of geologic events to accommodate Kelvin’s foreshortened time framework.  Paleontologists who insisted that the earth must have endured far longer than this could offer only qualitative arguments that, in the face of the physicists’ impressive numerical data, were largely ignored.  In the long run, however, the paleontologists seemingly vague hunches prevailed.  At the turn of the twentieth century Kelvin’s rigid mathematics on the consequences of a rapidly cooling earth were rendered meaningless by the discovery of radioactivity.

In 1896 Henri Becquerel and Marie Curie discovered that certain isotopes undergo spontaneous radioactive decay, transforming into new isotopes.  Atoms of a parent radioactive isotope randomly decay into a daughter isotope.  Over time the number of parent atoms decreases and the number of daughter atoms increases.  After radioactivity was discovered, Ernest Rutherford applied this knowledge and in 1903 he determined that the Earth’s age could not be determined by measuring cooling rates, and that the Earth could be, and probably was, billions of years old.

In 1946, a geologist named Arthur Holmes used radiometric dating to determine an age for Earth rocks by measuring the decay rate of uranium into lead to calculate the age of rocks and he found the Earth to be at least three billion years old.  Radiometric dating involves the decay, or breakdown, of radioactive elements and it requires an understanding of isotopes.  The decay rate is measured in terms of half-life.  Half-life is defined as the length of time it takes half of the remaining atoms of a radioactive parent element to decay.  Since geologists know how long these isotopes take to decay, they can determine a rock’s age by looking at the ratio of parent (pre-decay) and daughter (post-decay) isotopes in a sample.  Using radioactive dating, scientists have determined that the Earth is about 4.5 billion years old, ancient enough for all species to have been formed through evolution.  In 1956, the American geologist Clair Patterson announced that the Earth was 4.5 billion years old.