Perpetual change has been the normal state of our planet since its formation more than 4.5 billion years ago. 4500 million years are an enormous amount of time in which the planet continuously and profoundly changed its appearance again and again and is continuing to do so. Global ice ages alternated with hot greenhouse worlds, continents formed and have been wandering restlessly across the surface of the planet ever since, colliding with each other just to separate again. Between them entire oceans emerged and disappeared again. Time is a central concept in the Earth system. To imagine the time scales involved in such profound changes is beyond our imagination. With our concept of time of years and decades, the planet appears to us to be unchangeable because we perceive it geologically as a snapshot in time. The continuous movement of continents and oceans can be measured, but not experienced. However, one significant change of our planet is becoming increasingly apparent: climate change. Here the Earth system reacts on a time scale that fits into our world of experience.
How can climate change be so threatening when constant change in the Earth system is the normal state?
It is the sheer speed at which we have triggered climate change that makes it so dangerous. Natural change is happening at a slow pace and it will take millennia or even millions of years for it to become noticeable. That is enough time for the planet's ecosystems and their inhabitants to adapt. However, mankind fundamentally changed the planet within only 170 years with the onset of the industrial revolution. However, for the Earth system 170 years is less than a blink of an eye. Our activities are happening abruptly for the planet and its inhabitants. An adaptation of the ecosystems is simply no longer possible at such rapid changes. The comparison is profoundly shocking: For the Earth ecosystem, the cosmic asteroid impact 65 million years ago happened quite as abruptly as the climate change we have initiated nowadays. We are all aware of what happened to dinosaurs and their ecosystems as a consequence. They disappeared forever and were replaced by completely new ecosystems. Presently we are already amidst such a fundamental change. But why is our urgently needed action still omitted? It is just because we do not understand our home planet!
How does our planet work?
Understanding the complex Earth system is definitely among the greatest challenges of our time. How the Earth has been changing through the eons of time, sometimes slow and steady, sometimes abrupt and catastrophic, and how it will continue to change is one of the most exciting adventures of our time. The planet consists of various system components that we are all familiar with: The atmosphere and the hydrosphere with its oceans, the polar ice caps and the mountain glaciers of the cryosphere, the solid ground at our feet, the lithosphere, as well as all living beings and their ecosystems, the biosphere, together control the complex Earth and climate system of our planet. But that list is far from complete. Since its formation, the planet holds a fiery furnace in its interior. This heat and its striving for equilibrium are the driving forces of vertical and horizontal movements in the Earth's interior. Consequently, these forces rule what is happening on Earth's surface. In addition, the Earth receives its energy from the Sun. Cosmic material permanently precipitates to the Earth's surface as dust and small meteors that burn up in the atmosphere. Rarely, large cosmic bodies in the form of asteroids and comets are on a collision course with Earth. Each of these system components has its own scientific field with researchers working at the leading edge of science. Here, the great challenge we are facing is becoming obvious: All of these system components of our planet interact with each other with a bewildering complexity. As if this were not enough, they do so on completely different time scales and space scales. As a consequence, all these scientific disciplines need to work closely together to understand the Earth system as a whole.
Some of these time and space scales are familiar to us, others are beyond our experience. The atmosphere with its water cycle has a short memory of days to weeks. In time-lapse we would see the weather hectically fidgeting over the planet. The cryosphere removes water from the atmospheric water cycle in the form of snow and stores it for years to millennia as ice in glaciers and the polar ice caps. During the ice ages, this caused sea-level decreases of more than 100 m, turning many shallow seas on the continental shelfs into dry land. The oceans react much more slowly on the short-lived weather of the atmosphere. They store the sun's energy and release it back into the atmosphere with a seasonal shift. In doing so, they are dampening the contrasts of the seasons. The ocean currents circulate on time scales of 1000 years in a closed looping system around the planet, sometimes in the deep sea, sometimes at the surface. The ocean currents act like a conveyor belt transporting heat, nutrients, salt and gases around the globe. The earth's solid crust, the lithosphere, however, changes its appearance noticeably over the course of much longer time scales. It already requires millions of years to watch mountains grow and disintegrate, to see the continents slowly change position among each other, or to experience how a new ocean is born while another is closing. Depending on depth, the rock inside the Earth is solid, ductile or even molten. On incredibly long time scales, this rock creeps up and down in the Earth's mantle and thus controls what happens on the surface of our planet. Earthquakes, volcanic eruptions, mountain building and continental drift are expressions of this movement. Rarely, but devastating, large comets and asteroids appearing from deep space are on collision course with Earth. Between all these system components, the biosphere has been interacting successfully and continuously with its living creatures for almost four billion years, and this again with sometimes drastic effects on the planet itself. For example, the abundant oxygen that we breathe so naturally is not an original constituent of our atmosphere, but rather an invention of life itself. The first oxygen released by bacteria more than 3500 million years ago was actually a devastating environmental poison that caused a profound change of the planet. Today we benefit from this catastrophe at the beginning of time. All this is just a first glimpse at a fascinating network of complex interacting cycles, each of which strives for a natural state of equilibrium. If one or more of these system components are triggered too vigorously, the system will tip into a new state of equilibrium, often irreversibly. In the history of Earth this often had dramatic effects on the living world. The asteroid impact that wiped the dinosaurs out of existence is perhaps the best known event, although it was neither the only one, nor the most fatal of such events. However, the new state of equilibrium of the living world was the rise of mammals. What will happen if mankind irreversibly pushes the climate system beyond such a critical tipping point is yet unclear and a dangerous experiment with unknown consequences. What is clear, however, is that we will not like the consequences. It is time to stop this experiment with our planet that is nothing but a leap in the dark. Though it is literally five to midnight, our chance is to rethink, to rediscover and appreciate the unique beauty of our planet. Those who develop interest in how our unique planet works will rethink. This is mankind's greatest opportunity to embark together on a path towards a sustainable interaction with our wonderful home planet Earth.
The restless planet
The Earth's crust is fragmented into several tectonic plates driven by vertical and horizontal movements in the Earth's interior. These plates restlessly relocate among each other and wander across the globe. We notice these movements as sporadic earthquakes and volcanic eruptions. The much feared tsunamis are triggered when earthquakes suddenly displace the entire water column of the ocean. However, the movement of the earth's crust not only moves ancient continents, but also opens and closes entire ocean basins when tectonic plates drift apart or diverge. The Atlantic, Pacific and Indian Oceans are very familiar to us. However, a few hundred million years ago, the volcanic ocean floor material they are made of did not yet exist. Instead, there were similarly large oceans with illustrious names such as Tethys, Panthalassa and Iapetus. These associated ocean sea floors have long since vanished back into the Earth's interior. In fact, all ocean floors do not grow older than 190 million years and they are by no means to be considered as flooded continents. The continents so familiar to us have been forming continuously for more than four billion years and are made of a completely different rock material called granite. This continental granite has a different chemical composition compared to the heavier volcanic ocean floor basalt and therefore exhibits a slightly lower density. These subtle differences in density ensure that the continents are on average located 700 m above sea-level and are not destroyed by recycling back into the Earth's interior. The same principle causes ice cubes to float on water and to protrude out of the water. Because of this slightly higher density, the volcanic ocean floor, which is continuously produced at the spreading zones of the mid-ocean ridges, rests on average 3500 m below the sea-level. It is therefore recycled back into the Earth's interior at the so-called subduction zones. The Ring of Fire around the Pacific Ocean is the most famous example and notorious for its earthquakes, tsunamis and explosive volcanoes. The subduction of the ocean floor is necessary for continuity reasons, because otherwise the surface of the planet would constantly increase. Continents and ocean floors form temporary rigid connections with their deep-rooted bedrock. When the heavy ocean plates between two continents have finally disappeared in the subduction zones, the continents are on a collision course. These continent-continent crashes at snail's pace result in massive crustal thickening. At this stage the subducted oceanic plate has long disappeared from Earth's surface. However, it still exerts an enormous pull on the lighter continental plate. Finally, these forces grow so enormously that the oceanic plate is breaking off the continental plate at depth. As a result, the oceanic plate sinks into the depth at accelerated speed. In turn, the continental plate, which had been dragged down, now experiences a strong uplift. Freed from its traction force it reacts like a ball that is pressed under water and then suddenly released. Trailing fragments of the submerged ocean floor rises rapidly together with the buoyant continental crust. This rapid ascent creates mighty mountain belts, such as the Alps or the Himalayas. Parts of the crust of both continents are stacked on top of each other, with the remnants of the volcanic ocean crust folded into the center of the mountain chain. The summit of the Matterhorn in Switzerland consists of grey granite originating from the African continent. In contrast, the base of the mountain consists out of greyish granite from the Eurasian continent. However, between these granites greenish, exotic rocks are located at an altitude of over 2000 m. They are the strongly deformed remnants of the former volcanic ocean floor.
In the course of many hundred million years such continent-continent collisions lead to the unification of all continents into one supercontinent. Along the collision sutures, where the oceans once were located, endless lofty mountain ranges arise. Supercontinents evolve just to disintegrate again. The plural is justified. Restlessly driven by the heat beneath our feet, this has happened more than once in Earth history. Pangaea, the "All-Earth" supercontinent, which had agglomerated around 300 million years ago, began to disintegrate again 240 million years ago. Pangea was the stage for the rise of the dinosaurs. Two large fragments of Pangaea, the northern continent Laurasia and the southern continent Gondwana formed with the ever growing Tethys Ocean in between them. These landscapes set the stage for the reign of the dinosaurs during the Triassic, Jurassic and Cretaceous periods. Yet, earlier supercontinents had no less enchanting names and stories to tell. Rodinia, Nuna and Kenorland take us so deeply back into the past of our planet that there was no life other than bacteria. These enormous cycles unfold about every 500 million years and involve powerful tectonic tensions that manifest themselves in earthquakes and volcanic eruptions, the violence of which is beyond our imagination.
The water cycle is superimposed on these enormous rock cycles. It not only provides our daily weather and fills the ocean basins with water. The amount of water available to a region in the form of rain and snow, and consequently ice, determines the erosion rate. Thus the dry climate of the highlands of Tibet remained as a plateau with minor relief differences, while the intensive water cycle of the Indian monsoon shaped the Himalayas from the crustal thickening that resulted from the continent collision between India and Asia. The weathering of the water cycle is capable to erode entire mountain belts. Creeks, rivers and seams transport the rock debris, rubble, sand and rock flour into the oceans, where they sediment and impose a heavy burden on the ocean floor.
Enormous cycles emerge with durations of many hundred million years, driven by plate tectonics and their propulsion deep in the interior of the planet. This is where the ocean plates subduct to form veritable plate graveyards at the border between the Earth's mantle and its core at a depth of 2900 km. This anti-plate tectonic accumulation causes a cooler region at depth, compared to where the hot mantle material rises to the surface of the planet. This resulting global cold-warm pattern at a depth of 2900 km controls what and especially where tectonic motion occurs at the Earth's surface. In the warmer regions, so-called plumes rise. They transport ductile rock material towards the surface and determine where the continents will go in the future and where new ocean basins will form and where old ones will close.
All these processes are written into the various rocks that lie at our feet. Picking up a rock from the ground and deciphering its eventful geological history captivates the mind and causes a deep awe. The adventures preserved in the rock, from its creation and journey to the present day, narrate the most exciting thriller of all times.
Understand your planet
This is the launch of a new series of texts, dedicated to convey how the Earth system works.
Additional chapters will highlight the geology behind the of this website. These texts bridge between the detailed geological stories behind each photo in the galleries and the broader context of the formation of entire regions. The long-term goal is to combine these chapters into a book that brings the fascinating geological history of our planet to life.
Table of contents of upcoming articles
The origin of atoms and their diversity
Atoms connect to form a planet
Early Earth's appearance
A journey into Earth's interior
The exotic molecule called water
Of basalt and granite
Ancient continents and young ocean basins
The rise and fall of the mountain ranges
Four-dimensional plate and anti-plate tectonics
Rock cycles and plate graveyards
Who painted the sky blue
Hot house - Ice house
About life and Gaia