Geosphere

The geosphere consists of the solid proportion of the Earth and extends from the inner core to the crust, yet is predominantly classified as the lithosphere; the upper mantle and crust. The lithosphere is the area that affects Earth systems the most, as it is the rock forming portion of the Earth and therefore dictates the landmasses upon which we live.

Figure 10 cross section of Earth (Retrieved from: http://media.diercke.net/omeda/800/10155E_8_Erde_Schalenbau.jpg Date 20/9/2015)

The lithosphere formed shortly after the Earth formation as it underwent cooling, solidifying the molten rock to form the crust. From this the oldest rocks on the planet, the Acasta Gneiss, in Canada have given an age of 4.06 billion years. Yet the age of the Earth is given to be 4.56 billion years old, an age which has been determined by radiometric dating of meteorites. From this we know that the meteorites formed 4.56 billion years ago it can be inferred that the Earth formed in the same system due to similar compositions (meteorites made of rocks found on Earth) and so shares the same age. Zircon minerals found within the sedimentary rocks of Jacks Hill, have given an age of 4.4 billion years.

So why are there three different ages?

The oldest age of 4.56 Billion years is from a logical assumption and gives the earliest date at which the earth would’ve formed. At which it would still be molten until water and release of internal radiation, would cool the earth enough for the formation of minerals to take place and so the formation of zircon minerals.

This gives an absolute physical age for the Earth as 4.4 billion years old which still leaves a difference of 340 million years to the oldest dated rock. There is a difference in mineral age and rock age as minerals are what rocks are comprised of, therefore the zircon minerals dated would have formed been included within a rock. The mineral itself is very small and robust, it doesn’t breakdown easily and is very resistant to temperature and pressure.

Therefore over 340 million years rocks were formed and altered to become new rocks, a process which requires mineral alteration within them. This means that every rock on the planet (in the crust) has at some point in the Earth’s history undergone change to become a new rock by igneous, metamorphic and/or sedimentary processes. This is especially interesting when you think about large continents and every rock in them has been subducted, weathered, compressed, sheared, melted, etc. to become the rock it is today.

(Retrieved from: http://pubs.usgs.gov/gip/geotime/age.html Date: 11/9/2015).

These conditions are a result of igneous, sedimentary and metamorphic processes all of which happen due to plate tectonics (PT). Plate tectonics is the movement of large crustal bodies that are defined by plate boundaries. An age for the formation of plate tectonics has yet to be determined as early stages of the Earth had rocks form there is no evidence of PT, a reason for this is that the mantle was too hot for plates to form and made the crust to buoyant for any movement (Retrieved from: https://geosociety.wordpress.com/2013/04/28/when-did-plate-tectonics-begin-on-earth-and-what-came-before/ Date 11/9/2015).

Plate Tetonics (Retrieved from: http://pubs.usgs.gov/gip/dynamic/slabs.html Date 15/9/2015).

Eventually tectonic plates formed and started to move. This movement is driven by two factors:

1.    Slab push and pull: slab push and pull is formed from subduction and rift zones where the rift zone pushes the plate apart and the subduction zone pulls the subducting plate into it.

2.    Mantle convection currents: Convection currents in the mantle move harder crustal plates, due to friction between the two layers.

These two movement processes allow the plates to move apart and collide, creating three different boundaries; convergent, divergent and transform.

Plate Boudaries and with associated  properties (Retrieved from: http://media-2.web.britannica.com/eb-media/53/4953-004-C647C3B2.jpg Date 15/9/2015).

Its at theses boundaries where most of the planets rocks are formed.

·         Convergent plate boundaries can have metamorphic, igneous or both processes occur at the same time, as both orogenous and subduction zones can occur.

·         Divergent boundaries form igneous rocks as the crust is pulled apart allowing mantle to rise up and form new rocks.

·         Transform boundaries produce new rocks from shearing.

·         Sedimentary processes are a result of erosion and burial of pre-existing rocks and can occur anywhere.

So PT is especially important in regards to the geosphere as it creates the rocks we see and live on. This makes it especially important to us as we are dependent on rocks to grow our crops and materials to create things.

So the geosphere is an important sphere as it controls so many factors and variables that underlie all the other spheres and how they interact with one another. It is also an important driver of the Earth system and the how it is dynamic.