Author: Włodzimierz Mizerski
Polish Geological Institute – National Research Institute
Asst. Prof. Włodzimierz Mizierski works at the Geological Museum of the Polish Geological Institute – National Research Institute, specializes in regional geology, stratigraphy, and tectonics.
The Earth is around 4.6 billion years old. Its crust would have been in place by around 4.4 billion years ago, just 200 million years after the planet’s formation. The oldest minerals discovered thus far – zircon in metamorphic rock in Jack Hills, Australia – date back around 4.36 billion years, and in fact the earliest traces of life on Earth date back to not much later than the earliest igneous rocks. Gneiss found in the Isua Greenstone Belt in Iceland, dating back around 3.7 billion years, and the similarly aged metamorphic rocks in Australia and South Africa, show the presence of fossilized cyanobacteria. The earliest stromatolites (accretionary structures formed by cyanobacteria) are 3 billion years old. This is significant because stromatolites are formed in aquatic environments, and as such they show that bodies of water existed around that time. The water in Earth’s hydrosphere most likely originated during planetary cooling, when volatile components were outgassed during intensive magmatic processes (although according to another theory, it may originate from the accretion of frozen comet nuclei which became a part of primordial Earth following collisions). Water reservoirs soon became marine and oceanic regions. There is no doubt that ocean basins existed during the Paleozoic, and likely further back during the Proterozoic, as shown by deep-sea sediments. Ophiolites (forming part of present-day continental plates) are fragments of ancient oceanic crust; they are complexes of alkaline and ultra-alkaline igneous rocks with layers of metamorphosed sediments formed in deep oceans. This means that oceans may have existed as early as during the first two billion years of Earth’s existence. So how do we reconcile this with the fact that the oldest rocks forming the ocean beds are just 200 million years old, making them twenty times younger than the oldest continental rock?
The theory of plate tectonics
The answer, according to the current theory, is that the lithosphere is divided into plates of various sizes; they are in constant motion, carried by convection currents in the Earth’s underlying mantle. They can comprise oceanic or continental crust, or a combination of both. However, in the early days of our planet’s existence, it is likely that only plates similar to the contemporary oceanic crust existed. Magmatic and metamorphic processes occurring at certain boundaries between individual plates later initiated the formation of early elements of continental crust, formed of lighter, acidic igneous rocks, which continued to gradually expand throughout Earth’s history.
There are three types of boundaries between plates, known as divergent, convergent, and transform boundaries. Divergent boundaries run alongside rifts along mid-oceanic ridges, where alkali magma arises from the asthenosphere and the Earth’s mantle, solidifying and filling the space between two plates as they drift apart, generally perpendicularly to the rift’s axis. The rift is a narrow gap filled by liquefied mantle. This theory of how oceanic crust is formed is supported by the presence of belts of magnetic anomalies, running parallel to mid-oceanic ridges, and by the fact that rocks comprising ocean beds get increasingly older with distance from the axis of the ridge.
Convergent boundaries are regions in which oceanic plates move towards each other to form a subduction zone, where one plate is pushed deeper into the Earth’s mantle. One oceanic plate moves under a continental or another oceanic plate at a steep angle, creating what is known as a Wadati-Benioff zone. Earthquake loci fall along planes dipping downwards, providing direct evidence for two parts of the lithosphere moving against one another.
Transform boundaries, also known as conservative plate boundaries, run across mid-oceanic ridges and expanses of ocean beds. Fragments of the Earth’s crust can be shifted horizontally along such faults by as much as 1000 km. The longest faults run alongside the Pacific Ocean bed; the Mendocino fracture zone is 1150 km long, while the Murray zone runs 850 km. Faults may also be present in continental crust. An example is the San Andreas fault in California, known for its numerous earthquakes, marking the boundary between the Pacific plate and the North American plate. Another example, dating back around 350 million years, is the Great Glen Fault in Scotland, with a shift between plates of around 350 km; it is the site of the Great Glen loch and the famous Loch Ness.
According to plate tectonics theory, the buildup of oceanic crust along constructive plate boundaries (in mid-oceanic ridges) is compensated for by its destruction in subduction zones. This explains the fact that contemporary ocean beds are formed of oceanic crust whose age does not exceed 200 million years, since older plates would have been destroyed in subduction zones. Small fragments of extremely old oceanic crust can be found in a few locations, not in ocean beds but in continental plates: they exist as the ophiolite complexes discussed above.
Some are present in Lower Silesia, since the Sudeten Mountains are an old mountain range formed as a result of magmatic and tectonic processes taking place above a subduction zone. Studies of the absolute age of Sudeten ophiolites in Poland show that they date back around 400 million years – the early Devonian. Ophiolites found in the Czech part of the mountains are even older, dating back as far as 500 million years. They are most likely fragments of the ancient Rheic Ocean, dating it to the early Paleozoic. Traces of metamorphism and tectonic processes, and the age of sedimentary rocks formed in the oceanic crust, reveal that the ocean was destroyed during the Variscan orogeny (between the Devonian and Carboniferous). Naturally, the ocean would have stretched some way south of the present-day Sudetes.
Today’s distribution of lithospheric plates is relatively recent, and it arose as a result of the restructuring of convection currents in the Earth’s mantle which started during the Jurassic. It led to the breaking up of the former Pangea landmass and the formation of present-day continents and oceans. However, this relatively short period (just 200 million years) was sufficient to see the destruction of the Tethys Ocean and the formation of the Indian Ocean in its place. Fragments of the seabed rocks of this former ocean and sedimentary rocks form part of the fold layers stretching across mountain ranges from the Atlas and Baetic Mountains in the west to Indonesian ranges in the east.
Continental fractures occur when branching convection currents appear underneath them. When the currents reach the lithosphere, they are diverted to the sides causing stretching of the landmass, eventually leading to its fracturing and the appearance of a new ocean crust between the receding continental blocks. This stage in the process of ocean formation is observable today; for example, in Eastern Africa, a rift system is presently formed of a system of tectonic trenches running along the meridians, with a thinned continental crust underneath. Such a rift was the starting point of the formation of the Atlantic and Indian oceans. The African continent is undoubtedly breaking up, and in a few million years the expanding gap between its eastern and western parts will fill with oceanic waters.
Today, a few massive lithospheric plates and several smaller ones interact in a number of different ways. One region where such processes are particularly notable is the Ring of Fire, otherwise known as the circum-Pacific belt. Around the outer edges of the ocean there are extensive subduction regions where fragments of oceanic crust are being destroyed. The region is unusually tectonically active; earthquakes are a daily occurrence, and it is home to over 75% of our planet’s active and dormant volcanoes. It marks the boundary between two massive magmatic provinces.
Plate tectonics is a theory which provides the fullest explanation of processes occurring within the lithosphere, today and in the past. However, this does not mean that there are no remaining questions regarding the dynamics of the lithosphere and the underlying causes. The aforementioned convection currents in the Earth’s mantle remain hypothetical; while heat convection is a proven process, we still don’t know for certain whether it is really able to cause motion within the mantle powerful enough to bring about horizontal movement of the lithosphere. The mechanism of folding that occurs in subduction zones is also not yet fully elucidated. What is certain, however, is that the Earth’s internal temperature does have a direct impact on the lithosphere, as well as on atmospheric circulation and processes taking place within the hydrosphere.
Further reading:Dadlez R., Jaroszewski W. (1994). Tektonika [Tectonics], Warsaw
zechowski L. (1994). Tektonika płyt i konwekcja w płaszczu Ziemi [Plate Tectonics and Convection in the Earth’s Mantle]. Warsaw: PWN
© Academia 3 (43) 2014