Geologic time: Why does it matter how old the Earth is?
Throughout much of the last three centuries, scientists debated how old the Earth was, and sought evidence to support their hypotheses. Early scholars relied primarily on the Bible, which they interpreted to calculate the Earth's age as 6,000 years.
Naturalists in the 18th century, however, looked to rocks and landforms to try and understand the age of the Earth and the rate at which it changes. Perhaps the most famous of these early geologists was James Hutton, who made some observations about rocks throughout Great Britain that led him to challenge the prevailing beliefs of the day on Earth formation and processes. Many of his contemporaries either believed that God had created the world exactly as it now stood or that all of the change since creation had taken place over a very short period. Hutton observed:
1. That Hadrian's Wall, which had been built by the Romans from approximately AD 122-128, had eroded only a little in the intervening 1,600 years, while many of the other rocks he observed had eroded many times more.
Naturalists in the 18th century, however, looked to rocks and landforms to try and understand the age of the Earth and the rate at which it changes. Perhaps the most famous of these early geologists was James Hutton, who made some observations about rocks throughout Great Britain that led him to challenge the prevailing beliefs of the day on Earth formation and processes. Many of his contemporaries either believed that God had created the world exactly as it now stood or that all of the change since creation had taken place over a very short period. Hutton observed:
1. That Hadrian's Wall, which had been built by the Romans from approximately AD 122-128, had eroded only a little in the intervening 1,600 years, while many of the other rocks he observed had eroded many times more.
2. That there were places where rocks that had obviously been laid down in layers were tipped at two different angles right next to each other (a geologic feature called an angular unconformity. In the example below, the light gray rocks are tilted at about a 45 degree angle, while the brown rocks are nearly vertical, suggesting that they were laid down as sediments at different times, eons apart).
These observations led Hutton to develop his theory of uniformitarianism: the concept that the same Earth processes have been happening over the entire span of Earth's history at the same rates. He argued that if rocks always form and erode at approximately the same rate, it would be impossible for the Earth to be only 6,000 years old and have the types of geologic features that it does. Over the last two centuries, of course, much more scientific evidence has been found to support this theory and more accurately date the Earth to 4.6 billion years.
Why does it matter for understanding change today?
While Hutton's concept of uniformitarianism was initially applied specifically to geology, it applies to all naturally-occurring Earth processes (as opposed to anthropogenic processes that are initiated and carried out by humans). This means that as we begin to explore Our Changing Earth, we can apply uniformitarianism to all Earth processes, including those in the atmosphere, the biosphere, and the hydrosphere, as well as the lithosphere.
To better understand how the Earth is changing, and the role humans have played in that change, we can look at the Earth systems and processes that have been going on since the formation of our planet and ask two fundamental geographical questions:
1) What is the speed or rate of change over the last 4.6 billion years? Has the process always occurred very slowly (like plate movements), very rapidly (like earthquakes or volcanic eruptions), or at variable rates?
2) What is the spatial extent of change? Has the process always occurred across a global scale (like the Jet Stream), is it very localized (like a glacier carving out a valley), or has it always varied?
We begin by looking at processes that have occurred globally across very long timescales and usually relatively slowly: the building of landforms in the lithosphere. Specifically, Module 2 explores how the Earth has formed over the past 4.6 billion years, and how the global processes associated with landform formation also impacted the formation of our atmosphere over time.
If you had a time machine and went back 2 billion years in time, could you even survive on the Earth's surface? Were the atmospheric and surface conditions acceptable for humans to survive?
To better understand how the Earth is changing, and the role humans have played in that change, we can look at the Earth systems and processes that have been going on since the formation of our planet and ask two fundamental geographical questions:
1) What is the speed or rate of change over the last 4.6 billion years? Has the process always occurred very slowly (like plate movements), very rapidly (like earthquakes or volcanic eruptions), or at variable rates?
2) What is the spatial extent of change? Has the process always occurred across a global scale (like the Jet Stream), is it very localized (like a glacier carving out a valley), or has it always varied?
We begin by looking at processes that have occurred globally across very long timescales and usually relatively slowly: the building of landforms in the lithosphere. Specifically, Module 2 explores how the Earth has formed over the past 4.6 billion years, and how the global processes associated with landform formation also impacted the formation of our atmosphere over time.
If you had a time machine and went back 2 billion years in time, could you even survive on the Earth's surface? Were the atmospheric and surface conditions acceptable for humans to survive?