A Geologic Journey: Exploring the Age of Mountains Essay

Mountains feel like the most permanent objects imaginable; they are enormous, made of solid rock, and seemingly unchanging over human lifetimes. It feels like much of their majesty is tied to how ancient and immobile they are compared to our relatively short human lifespans. But how old are they? The answer is that mountains are 10s to 100s of millions of years old, but those numbers don’t mean much to most people. In fact, I would argue that the history of the question is more interesting than the answer itself.

The age of mountains has been one of the most fundamental scientific questions since antiquity. Humans have always had a close relationship with mountains, a fact reflected in our deification of them. Humans frequently associate mountains with gods and prophets, from the gods of Greek mythology who lived on mount Olympus, to Moses receiving the 10 commandments on mount Sinai, or Noah landing his ark on mount Ararat, to name a few. The tendency of ancient people to assign god-like characteristics to mountains makes sense; they lived and died by the behavior of the mountain ranges they lived on or near. Life giving water flows from mountains in rivers and springs, and ancient scholars noted that rain preferentially falls on mountain-fronts than on flat plains, making the fertile foothills of mountains the best arable land to grow crops on. Ancient people also learned to fear mountains for their deadly natural disasters. Landslides, earthquakes, and volcanic eruptions all originated from mountains. A definite historical parallelism exists between mountains and deities: both can give and take life at will, and both demand the respect of anyone who lives within their domain.

The close relationship between early civilizations and mountains motivated early attempts to understand how mountains formed. As early as the middle ages, natural philosophers from many cultures independently wrote about possible natural causes to explain the existence of mountain ranges. In the mid-11th century, the Chinese polymath Shen Kuo posited that landscapes were shaped by readily identifiable processes like uplift, erosion, and deposition of sediments. In doing so, Kuo effectively founded the modern science of geomorphology (i.e., studying the Earth’s surface to understand underlying processes acting to shape it). Geomorphology remains a thriving discipline within modern geology, and is one tool modern scientists use to understand the growth and decay of mountain ranges.

At the same time that Kuo was imagining towering mountains growing and eroding into silt, the Persian philosopher Ibn Sina wrote in The Book of Healing the following extraordinary paragraph about the origin of mountains: “Either they are the effects of upheavals of the crust of the earth, such as might occur during a violent earthquake, or they are the effect of water, which, cutting itself a new route, has denuded the valleys, the strata being of different kinds, some soft, some hard ... It would require a long period of time for all such changes to be accomplished…” - Ibn Sina, 1027 Sina’s understanding of the formation of mountains is remarkably correct. Today, geologists understand that mountain ranges are the byproducts of uplift (upheaval of the crust) and erosion (the effect of water removing sediments). Modern geological research is simply continuing a long thread of academic thought that began almost a millennium ago with Sina and his contemporaries.

At the end of the quote, Sina notes that these mountainforming processes (uplift and erosion) require a great deal of time because they happen so slowly. The question of how much time is required to build a mountain remained basically unstudied for the following 800 years until the scientific revolution brought the question of age of the Earth to the forefront of geological science. In the mid-17th century, scientists, scholars, and philosophers invigorated by progress during the scientific revolution began wondering how old the Earth was. In 1625 scholar and church leader James Ussher made the first attempt to assign a number to the question, counting generations in the bible to estimate that god created the Earth on October 22nd, 4004 BC, around 6pm. This age was popular among natural philosophers for hundreds of years. Finally, in the early 19th century, scientists started using math and science to estimate the age of the Earth. The most famous age calculation was performed by William Thomson, better known as Lord Kelvin, in 1864.

His method treated the Earth like a cooling potato, assuming that the Earth had been hot and molten at formation, then gradually cooled following the laws of thermodynamics (which Kelvin wrote) to the comfortable temperatures we have today. Based on best estimates for how rocks conduct heat, he estimated the Earth was 20-40 million years old, a major increase from the 6,000 year age calculated by Ussher. This age was generally accepted by the scientific community, and provided geologists a much more reasonable amount of time for the slow mountain-building processes of uplift and erosion to create the towering mountain ranges we see across the globe. Many nineteenth century geologists assumed mountains were the most ancient and longlived features of the landscape, and therefore hypothesized that the age of mountains provides a minimum for the age of the Earth. If you could determine the rate at which erosion occurred, you could calculate the time required to erode mountains and valleys from a flat landscape, and in turn the age of the Earth.

William Thomson, also known as Lord Kelvin One prominent scientist who used this logic was none other than Charles Darwin. His foundational work On the Origin of Species had an entire chapter dedicated to determining the age of the Earth. Darwin was concerned with having enough time for the very gradual evolution of species to produce the great diversity we see in living things. The 20- 40 million year age proposed by Lord Kelvin seemed far too short, much less Ussher’s 6,000 year age. Darwin decided to perform his own calculation of the age of the Earth based on a simple “back of the envelope” calculation of the time it would take to erode the sediments that originally filled the Weald valley in southern England. Darwin claimed that at an erosion rate of one inch per century, “the denudation of the Weald must have required 306,662,400 years; or say three hundred million years”. Darwin’s estimate was quickly attacked by the scientific community, especially Lord Kelvin, who steadfastly held that the age of the Earth could not be more than 100 million years.

Little progress was made on the question of the age of the Earth until the mid-20th century, when radioactive dating changed everything. All of the previous estimates of the age for the Earth were made obsolete by the realization that the radioactive decay of unstable atoms could be harnessed as a clock to tell how old a given rock or mineral is. In 1955, scientists first published a radiometric (i.e., radioactively measured) age of the Earth: 4.5 billion years (that is 4,500,000,000 years, or 4,500 million years, or 55 million human lifetimes). This number would be made slightly more precise over the following decades, but not change significantly.

Today the best estimate is 4.54 billion years, only slightly more precise than the 1955 estimate. The incredibly old age of the Earth surprised geologists, and changed their perspective on how landscapes evolve. No longer were mountains as old as the Earth itself: even the longest lived mountains were as transient as a sandcastles on a beach. The paradigm of how geologists viewed mountains shifted, and geologists began studying mountains as objects with a distinct life-cycle: from violent birth, active youth, and proud middle age, to eventual erosion and death over the course of 10s to 100s of millions of years.