Tectonics and the Lithosphere

Earth LayersThe centre of the earth is called the inner core and is solid because of the pressure exerted on it. Around this solid core is a liquid outer core. The main substances in the core are iron and nickel which give this part of the earth a density in the range of 8 g/cm3.

AsthenosphereThe mantle, a mostly solid layer is the largest of earth’s layers. The upper part of the mantle is called the asthenosphere. The asthenosphere is part solid, part liquid and is able to flow like hot asphalt or tar, especially if pressure is applied to it.

LithosphereThe lithosphere is the outer layer made up of solid rocks with an average density of 2.6 g/cm3. Parts of the mantle and asthenosphere that are liquid maintain their heat as radioactive elements decay (like in nuclear reactors), releasing their energy as heat.

Kinds of RocksThe three main kinds of rocks are igneous, sedimentary, and metamorphic.

Igneous RocksIgneous rocks form when magma from below the earth cools, either at or below the surface of the earth. If magma cools within crust rocks, it is called intrusive and if it comes out of cracks or fissures in the earth’s surface it is called extrusive. Magma flowing from fissures is referred to as lava. When magma or lava cools slowly, rocks form with larger crystals. When intrusive magma cools quickly, rocks with smaller crystals form.

Igneous RocksIntrusive rocks are sometimes called plutonic rocks. Granite is one kind of plutonic rock with large crystals while obsidian is a kind of rock with no crystals that has cooled very quickly on the earth’s surface. Metallic mineral deposits are commonly found along with or imbedded in intrusive igneous rocks. Metal veins in igneous rocks form as super-hot water within the igneous rocks cools and metallic compounds dissolved in the water settled out of the metallic brines (solutions) to form ore deposits.

Sedimentary RocksSedimentary rocks are rocks that have been made from eroded pieces of other rocks or from precipitation (settling out of solution) of materials from water. There are two main types of sedimentary rock: clastic and nonclastic.

Clastic Sedimentary RocksClastic sedimentary rocks form from eroded pieces which are cemented and pressed back together in a process called lithification. If gravels are lithified they become conglomerates, if sands are lithified they become sandstone, and muds or clays when lithified become shale.

Nonclastic Sedimentary RocksNonclastic rocks form when solids settle out from water (precipitate) or when remains of organisms like skeletons or shells gather together on the bottom and lithify there (because of pressure or heat) into sedimentary rock. Limestone rocks form when skeletons and shells of sea organisms become cemented together. The building material, cement, is made from limestone. Oil and gas form from the remains of tiny plants and animals, often marine organisms. Coal forms from plants covered up in swamps.

Fossil FuelsTo form fossil fuels, dead organisms must be covered and then be pressed together under anaerobic conditions (with no oxygen present).

How Pressure and Heat Affect Fossil FuelsThe longer time that fossil fuels are pressurized and heated, the more they are converted into light liquids or gases. The shorter the time that fossil fuels are pressurized and heated, the more tars and heavier liquids that get formed.

Where Fossil Fuels AccumulateNatural gas and oil accumulate in reservoirs called traps. A trap forms when an upper rock layer is impermeable and keeps the gas/oil from rising. The top layer of impermeable rock is called a cap and under this, the oil/gas rises above ground water because they are lighter or less dense than water. When rocks are compressed from the side, they form anticlines and synclines. It is below the impermeable caps of anticlines that oil and gases tend to accumulate.

Salt DomesOil traps can also form when a salt dome rises through sedimentary layers due to its plasticity and lower density. Salt domes begin as salts accumulate on the bottom of a shallow sea that is exposed to intense sunlight leading to great evaporation and precipitation of salt. When this is covered by sediments, the salt begins rising to form a salt dome. Salt domes have been found along the coast of the southern US, Iran and along the great lakes in Southern Ontario. Salt domes can be many km deep and wide.

Sedimentary Rock Features: Fossils and StrataSedimentary rocks often have in them the remains of once-living organisms called fossils. Sedimentary rocks typically are formed in layers called strata. The layering of sedimentary rocks sometimes gives them the property of splitting along layer lines. The sand, silt and mud that become sedimentary rocks are made through the action of rivers whose movement keeps these particles in suspension until they reach much calmer lakes and oceans where these particles settle out in layers on the bottom.

Ancient Sea in Central CanadaIn central Canada there are sediments up to 7,000 m thick which are thought to have been deposited in this region when it was an ancient sea, under water.

Continental ShelvesSediments that form around the edges of continents are called continental shelves. These shelves of sediments slope gently downward and at their edges the bottom plunges deeply. Continental slopes are regions where the sediment slope is steep and not as long as continental shelves. Continental shelves are regions of great economic importance because rivers supply them with many nutrients that foster algae growth, which in turn feeds marine animals like fish. The majority of fish and shellfish harvested on earth come from continental shelf areas.

Coral StructuresA fringing reef forms around a volcanic island or land mass. It is made by corals which are colonies of animals that make shell homes of rock-like calcium carbonate (limestone). As coral organisms cement layer upon layer of their homes on lower layers, a reef structure arises from the ocean floor and may emerge from the ocean if the ocean floor rises. If a land mass moves away from a fringing reef, a space develops between the land and the reef, and the reef is now called a barrier reef. If a volcano sinks, leaving the reef, a structure called an atoll is formed.

EvaporitesAs salty waters in very warm, sunny regions are exposed to intense solar radiation, mineral deposits settle out which are called evaporites. The minerals of evaporites are usually salts, a common one being sodium chloride (NaCl), table salt. Halite deposits are evaporites made from salts like table salt.

Metamorphic RocksWhen igneous or sedimentary rocks are exposed to heat and/or pressure they change into metamorphic rocks. Metamorphic rocks are of two types: foliated and non-foliated. Foliated metamorphic rocks have repeating layers while non-foliated metamorphic rocks have no repeating layers. Gneiss is a foliated metamorphic rock made from granite while quartzite is a non-foliated metamorphic rock made from sandstone. Marble is a non-foliated metamorphic rock made from limestone. Shale (Sed.) changes to slate (Metam.). Many metamorphic rocks are used as building materials.

The Rock CycleThe rock cycle shows how rocks on earth are changed from one kind to another.

Plate TectonicsPlate Tectonics is the modern theory that the earth’s crust is made up of separate, solid plates which move because of mantle convection currents, causing them in some places to collide, in other places to slip past each other in opposite direction and in yet other places to rift or move apart.

Continental DriftAlfred Wegner in 1912 proposed the theory of continental drift, that the continents were at one time together in one land mass which split up, leading to the continents moving apart to where they are today. His idea was interesting since the continents can be fitted together but there was no explanation for how the continent could have moved. Today’s tectonic theory explains how continental drift occurred.

Continental Drift AnimationIn the past, continents have separated, come together and split apart again.

Results of Moving Earth PlatesThe moving plates of the earth’s crust produce folding, faulting, earthquakes and volcanism. Folding is the process that bends and twists rocks.

Results of Moving Earth PlatesFaulting is the process which causes rocks to break and slide past each other along a fault or crack.

Results of Moving Earth PlatesVulcanism is the process by which magma from the asthenosphere flows upward through volcanos or through fissures (cracks).

Results of Moving Earth PlatesEarthquakes occur when crust movements produce shock waves. If an earthquake happens in an ocean, very large waves called tsunamis form. Large earthquakes happen where earth plates move together or slip past each other sideways. Small earthquakes occur where plates move apart.

Mountain FormationYoung mountains are formed where plates are colliding together, usually at the edges of one or both colliding plates. Older mountains like the Appalachians of the Eastern US or the Urals of Russia were formed many millions of years ago when plates collided and then welded together. Over time these older mountains came to be inland and to be greatly eroded, reducing their height and rounding their features.

Mountains Formed by Hot SpotsSome mountains form from volcanic activity, not along plate edges. These mountains form as strong convection currents of magma push up and melt a hole somewhere in the interior of a plate.

Mountains Formed by Hot SpotsThe Hawaiian Islands in the Pacific Ocean are an example of a mantle hot spot which burned through the over-riding Pacific ocean plate as it moved in a northwestery direction.

Types of Plate BoundariesA converging plate boundary occurs where two plates move towards each other. Converging boundaries can be where a more dense oceanic plate slides under a lighter continental, where two continental plates collide, or where two oceanic plates collide.

Diverging PlatesDiverging plate boundaries occur where two plates move away from each other. An upwelling of magma which diverges where it meets the plates causes the plates to rift or move apart.

Examples of Diverging PlatesDiverging plates may be two continental plates (ex: the Rift Valley in Africa) or two oceanic plates (ex: The Mid Atlantic Ridge).

Transform BoundaryTransform boundaries are where plates slip sideways past each other in opposite directions along a fault line. The San Andreas fault line is a well-known transform boundary along which many smaller and occasional larger earthquakes occur.

An Example of a Transform BoundaryThe Great Glen fault of Scotland is another example of a transform boundary.

Transform Faults Along the Mid Atlantic RidgeThe Mid Atlantic Ridge has many transform faults cutting across it because parts of the plates travels faster closer to the equator than parts of plates more northery (Less distance [circumference] per same rotation time means a slower speed). The difference in speeds causestransform faults to develop.

Features of Plate Boundaries – Oceanic RidgesOcean ridges are formed by diverging oceanic plates because the magma rising against the plates pushes them up on either side of the rift to form mountain ridges.

Features of Plate Boundaries – Subduction Zones A subduction zone is a region where a denser oceanic plate converges with less dense continental plate, subducting or sliding beneath the continental plate. Friction between the subducting plate and the continental plate causes part of the edge of the continental plate to be drawn downward, creating a trench where the subduction is occurring. Erosion from the continental plate may fill in or obscure this trench.

Features of Plate Boundaries – Deep Oceanic Trenches Deep oceanic trenches are formed where one ocean plate subducts underneath another ocean plate. The friction between the subducting plate and the over-riding plate causes the over-riding plate to move some what with the subducting plate which causes it to bend down, forming a deep trench .

Features of Plate Boundaries – Mountain FormationThe Himalayan mountains, North of India, were formed by the convergence of the Indian plate with the Eurasian plate. Likewise the Alps of Europe formed when the African plate converged with the Eurasian plate.

Features of Plate Boundaries – Degree of Continental ShelfAt continent margins where subduction is occurring, there is very little continental shelf because it gets carried down underneath the continental plate by the subducting ocean plate. Where there is extensive continental shelves, subduction zones are far from the continent’s edges or margins.

Features of Tectonic Plates – Differing Ages in a PlateSea floor rocks nearest diverging plate boundaries are younger while sea floor rocks that are progressively farther from the boundary are progressively older. Since oceanic plates always subduct when they collide with a less dense continental plate, they are constantly made at diverging boundaries and are melted and remade into magma at subduction zones. Thus continental plates are older than oceanic plates, the oldest continental rocks dating over 2 billion years while the oldest oceanic rocks date just hundreds of millions of years.

The Oldest Earth RocksThe oldest earth rocks that are exposed are found in the Canadian Shield and date over 3.96 billion years old.

Corresponding Magnetic StripsUpwelling liquid magma contains magnetic materials that line up with the earth’s magnetic field and become fixed in this alignment when the magma solidifies. As the earth’s magnetic field changes, a series of strips form on either side of the diverging boundary. The existence of these corresponding parallel strips of different magnetic orientation is evidence how plates form at divergent boundaries.

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