(Pre) History Walk

## Pre-History Walk - A Geological Timeline

Earth’s geosphere has undergone a lot of changes since its formation. Evidence of large-scale changes can be seen in the rocks that make up the crust. Earth’s geological time can be placed into four distinct eons (an eon is a period of a billion years or more).

Since its creation, the Earth has forgone 4 distinct eons. In chronological order, these are the Hadean Eon, the Archean Eon, the Proterozoic Eon, and the Phanerozoic.

### The Hadean Eon

* There were no continents.

* Ubiquitous (ever-present, ongoing) active volcanism which resulted in a worldwide magma ocean.

* The rocks created around this time were from the beginning of Earth’s formation, as a result, they were eventually destroyed or buried by layers of rock above it.

### The Archean Eon

* Formation of the first large continents. Remnants of these continents are called cratons. Evidence of these can be found in Canada, Africa, and Australia.

* Earth’s surface began to cool and heavy elements began sinking past the oceans and toward the center of the planet.

* Earth’s layers started to form, creating a distinct crust, mantle, and core. The crust started to break up because of convection currents within the mantle.

### The Proterozoic Eon

* Large-scale marine deposition of sediments as deep oceans began to form.
	+ For example, banded Iron formations are characterized by thick deposits of Iron Oxides which were precipitated from ocean water as oxygen concentration increased.
* Supercontinents such as Nuna and Rodina were formed however with the shift of convection currents the breakup of these supercontinents occurred.
* Modern plate tectonics became active, and the ancient cores of the continents moved over wide areas of the globe, accumulating smaller fragments of crust, and sometimes colliding with other large landmasses. The opposite also happened, leading to continental rifting.
* Older stable rock formations, such as the Guiana Shield, were formed in this period.

### The Phanerozoic Eon

* The continents drifted and formed the supercontinent called Pangea. Pangea eventually broke up and formed the continents we know today.

* Volcanism is now less frequent and most of the mountain ranges today were formed, including large mountain ranges such as the Himalayas.

* Changes to the geosphere are now taking place and some changes are triggered by human factors such as mining and deforestation however, these pale in comparison to more powerful events such as earthquakes and volcanic eruptions.

### Changes to the Geosphere

#### Creation of Lithosphere

Most of the large-scale changes in the geosphere are because of tectonic plate movements. Plate tectonics caused the creation of new landforms at divergent plate boundaries such as mid-oceanic ridges (at the boundary of two oceanic plates) and rift valleys (at the boundary of two continental plates). Mountain building occurs because of converging continental plates as well as prolonged volcanic activity.

Images depicting what occurs at divergent plate boundaries. On the left - continental plates and on the right - oceanic plates.

#### Destruction of Landforms

* Tectonic plate movement may also lead to the destruction of landforms; this frequently occurs at convergent plate boundaries where subduction occurs.
* Violent catastrophic events such as volcanic eruptions can also result in the destruction of landforms; this was the case in Krakatoa in the 1883 volcanic eruption.

* Krakatoa is an archipelago found in the Sunda Strait in Indonesia, situated along the Indo-Pacific convergence zone; resulting in great seismic activity and vulcanism.
* Over a 5 month period, from May-October 1883, there was a series of intensely violent, ongoing eruptions from the Krakatoa volcano with accompanying disasters such as near-100ft tsunamis that rocked even the shores of South Africa and Australia.
* The explosions resulted in much of the landmass of the island subsiding into a caldera. Geologists also theorized that the eruptions caused much of the crust to subduct into an emptying magma chamber, which resulted in the extensiveness of the explosions.
* When the eruptions diminished in August, all that remained of the island was a caldera. In December 1927, a small volcanic islet breached the sea level and was named "Anak Krakatau" (child of Krakatoa); subsequent eruptions have resulted in the growth of this volcanic cone and the island became more substantial.

#### Further Changes

* Smaller scale changes in the geosphere can occur because of its erosion and deposition in fluvial and maritime environments. Some examples of these include;
	+ V-shaped valleys are a result of erosion of land due to the paths of rivers.
	+ River deltas are formed due to the deposition of sediments as the river slows its course at the mouth.
	+ Beaches are a result of the deposition of previously eroded materials that are carried to seas and oceans by rivers.
* Humans also affect the geosphere significantly by flattening mountaintops for development, constructing dams and reservoirs which result in the obstruction of waterways, and extracting materials from within Earth's crust.