Volcanism and Igneous Rocks

Magma and Igneous Rocks Igneous Rocks are  formed by crystallization from a liquid, or magma. They include two types Volcanic or extrusive  igneous rocks form when the magma cools and crystallizes on the surface of the Earth Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth. Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and  properties of a liquid. Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called aplutonic or intrusive igneous rock.

Types of Magma Chemical composition of magma is controlled by the abundance of elements in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO2 is the most abundant oxide. Mafic or Basaltic--  SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na  Intermediate or Andesitic--  SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K  Felsic or Rhyolitic--  SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na. Gases - At depth in the Earth nearly all magmas contain gas.  Gas gives magmas their explosive character, because the gas expands as pressure is reduced. Mostly H2O with some CO2  Minor amounts of Sulfur, Cl , and F  Felsic magmas usually have higher gas contents than mafic magmas. Temperature of Magmas Mafic/Basaltic - 1000-1200oC  Intermediate/Andesitic -  800-1000oC  Felsic/Rhyolitic -  650-800oC. Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content.   Higher SiO2 content magmas have higher viscosity than lower SiO2 content magmas  Lower Temperature magmas have higher viscosity than higher temperature magmas.

Summary Table
Magma Type	Solidified Volcanic Rock	Solidified Plutonic Rock	Chemical Composition	Temperature	Viscosity	Gas Content
Mafic or Basaltic	Basalt	Gabbro	45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na	1000 - 1200 oC	Low	Low
Intermediate or Andesitic	Andesite	Diorite	55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K	800 - 1000 oC	Intermediate	Intermediate
Felsic or Rhyolitic	Rhyolite	Granite	65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na	650 - 800 oC	High	High

Origin of Magma As we have seen the only part of the earth that is liquid is the outer core.  But the core is not likely to be the source of magmas because it does not have the right chemical composition.  The outer core is mostly Iron, but magmas are silicate liquids.  Thus magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH.  Thus, since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present. We know that temperature increases with depth in the earth along thegeothermal gradient.  The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth.  Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid.  Thus, magmas form only under special circumstances.  To understand this we must first look at how rocks and mineral melt. As pressure increases in the Earth, the melting temperature changes as well.  For pure minerals, there are two general cases.

For a pure dry (no H2O or CO2present) mineral, the melting temperate increases with increasing pressure.

For a mineral with H2O or CO2present, the  melting temperature first decreases with increasing pressure

Since rocks mixtures of minerals, they behave somewhat differently.  Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures.  Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma.  The two general cases are:

Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.  The degree of partial melting can range from 0 to 100%

Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.

Three ways to Generate MagmasFrom the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way. The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material (decompression melting) or by intrusion of magma (heat transfer). Lowering the melting temperature can be achieved by adding water or Carbon Dioxide (flux melting).

Decompression Melting  - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle.  Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient.  Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it.

If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form.  Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids.  Basaltic magmas appear to originate in this way. Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys.  Thus, generation of magma in these three environments is likely caused by decompression melting.

Transfer of Heat-  When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat.  Upon solidification they lose this heat and transfer it to the surrounding crust.   Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas.

Transfer of heat by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments. Flux Melting - As we saw above, if water or carbon dioxide are added to rock, the melting temperature is lowered.   If the addition of water or carbon dioxide takes place deep in the earth where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma.  One place where water could be introduced is at subduction zones. Here, water present in the pore spaces of the subducting sea floor or water present in minerals like hornblende, biotite, or clay minerals would be released by the rising temperature and then move in to the overlying mantle.   Introduction of this water in the mantle would then lower the melting temperature of the mantle to generate partial melts, which could then separate from the solid mantle and rise toward the surface.

Chemical Variability of Magmas The chemical composition of magma can vary depending on the rock that initially melts (the source rock), and process that occur during partial melting and transport. Initial Composition of Magma The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting.   In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas.  Melting of crustal sources yields more siliceous magmas. In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma. Magmatic Differentiation But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.   These processes are referred to asmagmatic differentiation and include assimilation, mixing, and fractional crystallization. Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous. Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks. Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. The processes is called magmatic differentiation by Fractional Crystallization. Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica. Thus crystal fractionation can change a mafic magma into a felsic magma. Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case. Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO2. Initially the composition of this magma is expressed as 50% SiO2 and 50% MgO. i.e. Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma. Now what are the percentages of each molecule in the liquid?   If we continue the process one more time by removing one more MgO molecule Thus, composition of liquid can be changed.

Bowen's Reaction Series Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature.  As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first.  Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase.  But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO2 rich.  If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature

Igneous Environments and Igneous Rocks The environment in which magma completely solidifies to form a rock determines: The type of rock The appearance of the rock as seen in its texture The type of rock body. In general there are two environments to consider: The intrusive or plutonic environment is below the surface of the earth. This environment is characterized by higher temperatures which result in slow cooling of the magma.  Intrusive or plutonic igneous rocks form here. Where magma erupts on the surface of the earth, temperatures are lower and cooling of the magma takes place much more rapidly.  This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks. Extrusive Environments When magmas reach the surface of the Earth they erupt from a vent called a volcano.  They may erupt explosively or non-explosively. Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas and sometimes rhyolitic magma). Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface Produce Pillow lavas if erupted beneath water

Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas). Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface. Bursting of bubbles fragments the magma into pyroclasts and tephra (ash). Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

Tephra that falls from the eruption column produces a tephra fall deposit.

If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed.  This is the most dangerous type of volcanic eruption.  The deposits that are produced are called ignimbrites.

Intrusive Environments Magma that cools at depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons, from Greek god of the underworld - Pluto. When magma intrudes it usually affects the surrounding rock and is also affected by the surrounding rock.  It may metamorphose the surrounding rocks or cause hydrothermal alteration. The magma itself may also cool rapidly near the contact with the surrounding rock and thus show a chilled margin next to the contact.

It may also incorporate pieces of the surrounding rocks without melting them.  These incorporated pieces are called xenoliths (foreign rocks). Magma intrudes by injection into fractures in the rock and expanding the fractures.   The may also move by a process called stoping, wherein bocks are loosened by magma at the top of the magma body with these blocks then sinking through the magma to accumulate on the floor of the magma body.

In relatively shallow environments intrusions are usually tabular bodies like dikes and sills or domed roof bodies called laccoliths.
Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude.  Discordant means that they cut across preexisting structures.  They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth.
Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude.  Sills usually are fed by dikes, but these may not be exposed in the field.
Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion.  They are also concordant types of intrusions.

Deeper in the earth intrusion of magma can form bulbous bodies called plutons and the coalescence of many plutons can form much larger bodies called batholiths. Plutons are large intrusive bodies, of any shape that intrude in replace rocks in an irregular fashion.  Stocks are smaller bodies that are likely fed from deeper level batholiths.  Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.

If multiple intrusive events occur in the same part of the crust, the body that forms is called abatholith.  Several large batholiths occur in the western U.S. - The Sierra Nevada Batholith, the Coast Range Batholith, and the Idaho Batholith, for example (See figure 6.10d in your text).

During a magmatic event there is usually a close relationship between intrusive activity and extrusive activity, but one cannot directly observe the intrusive activity.   Only after erosion of the extrusive rocks and other rock above the intrusions has exposed the intrusions do they become visible at the earth's surface (see figure 6.10a in your text).

The rate of cooling of magma depends largely on the environment in which the magma cools.   Rapid cooling takes place on the Earth's surface where there is a large temperature contrast between the atmosphere/ground surface and the magma.  Cooling time for material erupted into air and water can be as short as several seconds.   For lava flows cooling times are on the order of days to weeks.   Shallow intrusions cool in months to years and large deep intrusions may take millions of years to cool.

Because cooling of the magma takes place at a different rate, the crystals that form and their interrelationship (texture) exhibit different properties. Fast cooling on the surface results in many small crystals or quenching to a glass. Gives rise to aphanitic texture (crystals cannot be distinguished with the naked eye), or obsidian (volcanic glass).

Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture.

Porphyritic texture develops when slow cooling is followed by rapid cooling. Phenocrysts = larger crystals, matrix orgroundmass = smaller crystals.

Classification of Igneous Rocks  Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization.   You will explore the classification of igneous rocks in the laboratory portion of this course.

Extrusive/Volcanic Rocks Basalts, Andesites, and Rhyolites are all types of volcanic rock distinguished on the basis of their mineral assemblage and chemical compostion (see figure 6.13 in your text).  These rocks tend to be fine grained to glassy or porphyritic.  Depending on conditions present during eruption and cooling, any of these rock types may form one of the following types of volcanic rocks. Obsidian - dark colored volcanic glass showing concoidal fracture and few to no crystals. Usually rhyolitic . Pumice - light colored and light weight rock consisting of mostly holes (vesicles) that were once occupied by gas, Usually rhyolitic or andesitic. Vesicular rock - rock filled with holes (like Swiss cheese) or vesicles that were once occupied by gas. Usually basaltic and andesitic. If vesicles in a vesicular basalt are later filled by precipitation of calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdularl basalt.

Pyroclasts = hot, broken fragments. Result from explosively ripping apart of magma. Loose assemblages of pyroclasts called tephra. Depending on size, tephra can be classified as bombs. lapilli, or ash. Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff. Welding, compactioncause tephra (loose material) to be converted in pyroclastic rock.

Intrusive/Plutonic Igneous Rocks Shallow intrusions like dikes and sills are usually fine grained and sometimes porphritic because cooling rates are similar to those of extrusive rocks.   Classification is similar to the classification for volcanic/extrusive rocks.  Coarse grained rocks, formed at deeper levels in the earth include gabbros, diorites, and granites.  Note that these are chemically equivalent to basalts, andesites, and rhyolites, but may have different minerals or different proportions of mineral because their crystallization history is not interrupted as it might be for extrusive rocks (see figure 6.13 in your text). Pegmatites are very coarse grained igneous rocks consisting mostly of quartz and feldspar as well as some more exotic minerals like tourmaline, lepidolite, muscovite.  These usually form dikes related to granitic plutons. Distribution of Igneous Activity Igneous activity is currently taking place as it has in the past in various tectonic settings.   These include diverging and converging plate boundaries, hot spots, and rift valleys. Divergent Plate Boundaries At oceanic ridges, igneous activity involves eruption of basaltic lava flows that form pillow lavas at the oceanic ridges and intrusion of dikes and plutons beneath the ridges.   The lava flows and dikes are basaltic and the plutons mainly gabbros.   These processes form the bulk of the oceanic crust as a result of sea floor spreading.  Magmas are generated by decompression melting as hot solid asthenosphere rises and partially melts. Convergent Plate Boundaries Subduction at convergent plate boundaries introduces water into the mantle above the subduction and causes flux melting of the mantle to produce basaltic magmas.  These rise toward the surface differentiating by assimilation and crystal fractionation to produce andesitic and rhyolitic magmas.  The magmas that reach the surface build island arcs and continental margin volcanic arcs built of basalt, andesite, and rhyolite lava flows and pyroclastic material.  The magmas that intrude beneath these arcs can cause crustal melting and form plutons and batholiths of diorite and granite Hot Spots As discussed previously, hot spots are places are places where hot mantle ascends toward the surface as plumes of hot rock.  Decompression melting in these rising plumes results in the production of magmas which erupt to form a volcano on the surface or sea floor, eventually building a volcanic island.  As the overriding plate moves over the hot spot, the volcano moves off of the hot spot and a new volcano forms over the hot spot.  This produces a hot spot track consisting of lines of extinct volcanoes leading to the active volcano at the hot spot.  A hot spot located beneath a continent can result in heat transfer melting of the continental crust to produce large rhyolitic volcanic centers and plutonic granitic plutons below.   A good example of a continental hot spot is at Yellowstone in the western U.S.  Occasionally a hot spot is coincident with an oceanic ridge.  In such a case, the hot spot produces larger volumes of magma than normally occur at ridge and thus build a volcanic island on the ridge.  Such is the case for Iceland which sits atop the Mid-Atlantic Ridge. Rift Valleys Rising mantle beneath a continent can result in extensional fractures in the continental crust to form a rift valley.  As the mantle rises it undergoes partial melting by decompression, resulting in the production of basaltic magmas which may erupt as flood basalts on the surface.   Melts that get trapped in the crust can release heat resulting in melting of the crust to form rhyolitic magmas that can also erupt at the surface in the rift valley.  An excellent example of a continental rift valley is the East African Rift.  Large Igneous Provinces In the past, large volumes of mostly basaltic magma have erupted on the sea floor to form large volcanic plateaus, such as the Ontong Java Plateau in the eastern Pacific.   Such large volume eruptions can have affects on the oceans because they change the shape of ocean floor and cause a rise in sea level, that sometimes floods the continents.   The plateaus form obstructions which can drastically change ocean currents. These changes in the ocean along with massive amounts of gas released by the magmas can alter climate and have drastic effects on life on the planet.

Abyssal plain

Abyssal Plain

Abyssal plain is a submerged plain on the profound sea depths, generally found at profundities somewhere around 3000 and 6000 m. Lying for the most part between the foot of a mainland rise and a mid-sea edge, deep fields cover more than half of the Earth's surface. They are among the flattest, smoothest and slightest investigated areas on Earth. Deep fields are key geologic components of maritime bowls (alternate components being a lifted mid-sea edge and flanking deep slopes). Notwithstanding these components, dynamic maritime bowls (those that are connected with a moving plate tectonic limit) likewise commonly incorporate a maritime trench and a subduction zone. 

Deep fields were not perceived as unmistakable physiographic components of the ocean depths until the late 1940s and, until as of late, none had been concentrated on an efficient premise. They are inadequately safeguarded in the sedimentary record, on the grounds that they have a tendency to be devoured by the subduction process. The making of the deep plain is the deciding consequence of spreading of the ocean bottom (plate tectonics) and softening of the lower maritime outside layer. Magma ascends from over the asthenosphere (an upper's layer mantle) and as this basaltic material achieves the surface at mid-sea edges it frames new maritime outside layer. This is always pulled sideways by spreading of the ocean bottom. Deep fields result from the covering of an initially uneven surface of maritime outside by fine-grained residue, for the most part earth and sediment. A lot of this silt is stored by turbidity streams that have been diverted from the mainland edges along submarine gorge down into more profound water. The rest of the dregs is made essentially out of pelagic residue. Metallic knobs are regular in a few regions of the fields, with shifting groupings of metals, including manganese, iron, nickel, cobalt, and copper. These knobs may give a critical asset to future mining endeavors. 

Owing to some extent to their incomprehensible size, deep fields are as of now accepted to be a noteworthy supply of biodiversity. The void additionally applies noteworthy impact upon sea carbon cycling, disintegration of calcium carbonate, and environmental CO2 fixations over timescales of 100–1000 years. The structure and capacity of deep biological systems are emphatically impacted by the rate of flux of nourishment to the ocean bottom and the material's creation that settles. Components, for example, environmental change, angling practices, and sea treatment are relied upon to have a generous impact on examples of essential creation in the euphotic zone. This will without a doubt affect the flux of natural material to the void in a comparative way and along these lines have a significant impact on the structure, capacity and assorted qualities of deep biological system.

Oceanic zones

The sea can be conceptualized as being partitioned into different zones, contingent upon profundity, and vicinity or nonattendance of daylight. About all life frames in the sea rely on upon the photosynthetic exercises of phytoplankton and other marine plants to change over carbon dioxide into natural carbon, which is the fundamental building piece of natural matter. Photosynthesis thusly obliges vitality from daylight to drive the substance responses that deliver natural carbon. 

The water's stratum segment closest the sea's surface (ocean level) is alluded to as the photic zone. The photic zone can be subdivided into two diverse vertical areas. The highest part of the photic zone, where there is satisfactory light to bolster photosynthesis by phytoplankton and plants, is alluded to as the euphotic zone (likewise alluded to as the epipelagic zone, or surface zone). The lower segment of the photic zone, where the light force is inadequate for photosynthesis, is known as the dysphotic zone (dysphotic signifies "dim" in Greek). The dysphotic zone is likewise alluded to as the mesopelagic zone, or a twilight zone. Its lowermost limit is at a thermocline of 12 °C (54 °F), which, in the tropics by and large lies somewhere around 200 and 1000 meters. 

The euphotic zone is fairly self-assertively characterized as stretching out from the surface to the profundity where the light force is more or less 0.1–1% of surface daylight irradiance, contingent upon season, scope and level of water turbidity. In the clearest sea water, the euphotic zone may stretch out to a profundity of around 150 meters, or once in a while, up to 200 meters. Broken up substances and strong particles ingest and scramble light, and in beach front locales the high convergence of these substances makes light be lessened quickly with profundity. In such zones the euphotic zone may be just a couple of many meters profound or less. The dysphotic zone, where light power is extensively under 1% of surface irradiance, reaches out from the base of the euphotic zone to around 1000 meters. Stretching out from the base of the photic zone down to the seabed is the aphotic zone, a locale of interminable dimness. 

Since the normal profundity of the sea speaks the truth 4300 meters, the photic zone speaks to just a modest portion of the sea's aggregate volume. Then again, because of its ability for photosynthesis, the photic zone has the best biodiversity and biomass of every single maritime zone. Almost all essential creation in the sea happens here. Life shapes which occupy the aphotic zone are frequently fit for development upwards through the water section into the photic zone for sustaining. Else, they must depend on material sinking from above, or discover another wellspring of vitality and nourishment, for example, happens in chemosynthetic archaea found close aqueous vents and frosty leaks. 

The aphotic zone can be subdivided into three distinctive vertical areas, in light of profundity and temperature. In the first place is the bathyal zone, reaching out from a profundity of 1000 meters down to 3000 meters, with water temperature diminishing from 12 °C (54 °F) to 4 °C (39 °F) as profundity increments. Next is the deep zone, reaching out from a profundity of 3000 meters down to 6000 meters. The last zone incorporates the profound maritime trenches, and is known as the hadal zone. This, the most profound maritime zone, reaches out from a profundity of 6000 meters down to give or take 11000 meters. Deep fields are normally situated in the deep zone, at profundities running from 3000 to 6000.

Formation

Maritime outside, which frames the bedrock of deep fields, is consistently being made at mid-sea edges (a sort of dissimilar limit) by a procedure known as decompression softening. Tuft related decompression liquefying of strong mantle is in charge of making sea islands like the Hawaiian islands, and in addition the sea outside at mid-sea edges. This marvel is likewise the most widely recognized clarification for surge basalts and maritime levels (two sorts of vast volcanic areas). Decompression liquefying happens when the upper mantle is somewhat softened into magma as it moves upwards under mid-sea edges. This up welling magma then cools and hardens by conduction and convection of warmth to frame new maritime outside. Growth happens as mantle is added to the developing edges of a tectonic plate, normally connected with ocean bottom spreading. The period of maritime outside layer is in this way an element of separation from the mid-sea edge. The most youthful maritime covering is at the mid-sea edges, and it turns out to be continuously more established, cooler and denser as it moves outwards from the mid-sea edges as a major aspect of the procedure called mantle convection. 

The lithosphere, which rides on the asthenosphere, is partitioned into various tectonic plates that are ceaselessly being made and devoured at their inverse plate limits. Maritime covering and tectonic plates are shaped and move separated at mid-sea edges. Deep slopes are shaped by extending of the maritime lithosphere. Utilization or demolition of the maritime lithosphere happens at maritime trenches (a kind of joined limit, otherwise called a dangerous plate limit) by a procedure known as subduction. Maritime trenches are found at spots where the maritime lithospheric pieces of two unique plates meet, and the denser (more established) section starts to dive once more into the mantle. At the utilization edge of the plate (the maritime trench), the maritime lithosphere has thermally contracted to end up entirely thick, and it sinks under its own particular weight during the time spent subduction. The subduction procedure devours more established maritime lithosphere, so maritime hull is from time to time more than 200 million years of age. The general procedure of rehashed cycles of creation and decimation of maritime hull is known as the Supercontinent cycle, initially proposed by Canadian geophysicist and geologist John Tuzo Wilson. 

New maritime outside layer, nearest to the mid-maritime edges, is for the most part basalt at shallow levels and has a tough geology. The unpleasantness of this geology is a rate's component at which the mid-sea edge is spreading (the spreading rate). Sizes of spreading rates differ fundamentally. Commonplace qualities for quick spreading edges are more prominent than 100 mm/yr, while moderate spreading edges are ordinarily under 20 mm/yr. Studies have demonstrated that the slower the spreading rate, the rougher the new maritime hull will be, and the other way around. It is thought this marvel is because of blaming at the mid-sea edge when the new maritime covering was framed. These issues plaguing the maritime covering, alongside their bouncing deep slopes, are the most widely recognized tectonic and topographic components on the Earth's surface. The procedure of ocean bottom spreading serves to clarify the idea of mainland float in the hypothesis of plate tectonics. 

The level appearance of adult deep fields results from the covering of this initially uneven surface of maritime outside by fine-grained residue, basically dirt and sediment. Quite a bit of this residue is stored from turbidity streams that have been diverted from the mainland edges along submarine gorge down into more profound water. The rest of the silt involves mainly tidy (mud particles) extinguished to ocean from area, and the remaining parts of little marine plants and creatures which sink from the upper layer of the sea, known as pelagic residue. The aggregate residue affidavit rate in remote regions is assessed at a few centimetres for every thousand years. Silt secured deep fields are less regular in the Pacific Ocean than in other significant sea bowls on the grounds that residue from turbidity streams are caught in maritime trenches that fringe the Pacific Ocean. 

Deep fields are typically secured by remote ocean, yet amid parts of the Messinian saltiness emergency a great part of the Mediterranean Sea's deep plain was presented to air as an unfilled hot dry salt-stunned sink.

Earth's interior and plate tectonics

Plate Tectonics Concepts

                     PLATE TECTONICS CONCEPTS

Introduction

Plate tectonics- The idea that Earth’s surface is divided into large plates that move slowly and change in size over time.

This idea provides a model for understanding many geologic features: faults, folds, volcanoes, earthquakes, and mountain belts.

Plate Tectonics is the culmination of two pre-existing ideas:

1) Continental Drift- The idea that continents move freely over Earth’s surface, changing their position relative to each other.  This concept was proposed by German meteorologist Alfred Wegener in the early 1900’s.

2) Seafloor Spreading- The concept that new seafloor forms at mid-ocean ridges, and then moves horizontally away from the ridge toward an ocean trench/subduction zone (the seafloor is a conveyor belt).  This idea was proposed by Princeton geologist Harry Hess in 1962.

The Early Evidence for Continental Drift

1) The continents look like they could fit together (like a puzzle).

2) Rock types are correlated from continent to continent (across the oceans).

3) The extinct plant fossil, Glossopteris, which grew in temperate climates, is found in South America, Africa, India, Antarctica, and Australia.

4) The extinct reptile, Mesosaurus, is found only in Brazil and South Africa.  The mesosaurus lived only in freshwater, and could not swim across oceans.

5) Ancient glacial deposits in South America, Africa, India, Antarctica, and Australia suggest these continents were all located near a polar region
.Wegener's Concept of Continental Drift: Polar Wandering

Alfred Wegener proposed that the continents were once assembled into a supercontinent called Pangaea.

Wikipedia Commons Image.

He also proposed that Pangaea split into two parts:

    1) Northern Pangaea (which includes present day North America and Eurasia) became Laurasia.

    2) Southern Pangaea (including South America, Africa, India, Antarctica, and Australia) became Gondwanaland.

After Pangaea fragmented, Laurasia drifted northward and Gondwanaland drifted southward.

Wegener knew that coral reefs form near the equator, deserts form about 30° north and south of the equator, and glaciation occurs near the poles.  Based upon this information, he determined the North and South Pole positions over geologic time.  He called the apparent movement polar wandering.

There were two possible explanations:

    1) The continents remained motionless and the poles actually moved (literal polar wandering).

    2) The poles stood approximately still and the continents moved (continental drift).  Wegener preferred this explanation.

Opposition to Continental Drift

-Some scientists argued that some fossils (especially fossil plants) could have been spread from one continent to another by wind or ocean currents.

-Land dwelling reptiles could have spread from continent to continent by land bridges that rose up from the seafloor (this idea was pure speculation, as no appropriate mechanism was known).

-Wegener lacked a plausible mechanism by which continents could actually drift.

-Wegener's Polar wandering might reflect true wandering of poles rather than drifting of continents.

 New Evidence for Continental Drift

  

1) Paleomagnetism- The study of Earth’s magnetic field through time; geologists look at the way magnetic minerals in rocks preserve the magnetic field.

When a magnetic mineral crystallizes and cools below its Curie point, its magnetic alignment is locked in.

The direction of the mineral alignment gives the direction of magnetic North.  The dip of the mineral alignment gives the latitudinal distance from magnetic North.

  

For example, Permian age rocks in North America point to an apparent magnetic North pole in Asia whereas Permian age rocks in Europe point to an apparent magnetic North Pole in Japan.

Note: Today, when geologists use the term polar wandering they are referring to an “apparent” wandering of the poles.  We know that the poles themselves did not wander (although, as we shall see, the poles flipped or reversed many times throughout geologic history).

2) The continental slope- If the continental slope is taken into account, the plates fit together extremely well.

3) GPS technology- Global positioning satellites allow us to actually watch and measure the drift of the continents (1-16 cm/year).

Seafloor Spreading

Wegener thought that the seafloor remained stationary whereas the continents moved.

In contrast, in the 1960's Harry Hess proposed that the seafloor was also moving.

According to Hess, oceanic crust is produced at mid-ocean ridges and subducted at trenches.  The driving force for seafloor spreading is convection (hot mantle rises near the mid-ocean ridges and cold mantle sinks near trenches). 

The best evidence for sea floor spreading was provided by magnetometer surveys perpendicular to mid-ocean ridges.  A zebra pattern of magnetic reversals reflects episodic flips in Earth's North and South poles.

Image from the USGS.

The zebra pattern of magnetic anomalies is symmetrical about the ridge crest.

The concept of seafloor spreading explains the age of the sea floor.  Near mid-ocean ridges: sea floor is young and lacks sediment.  Away from ridges: seafloor gets older and acquires a thick blanket of pelagic sediment.

 The Big Picture: Plate Tectonics

By the late 1960’s, the hypotheses of continental drift and seafloor spreading had been combined into a single, unified theory of plate tectonics.

Recall that a plate is a thick, mobile slab of the Earth’s surface made of lithosphere (crust + upper mantle).  Plates glide on the ductile asthenosphere.

There are 3 types of tectonic plate boundaries:

1) Divergent Plate Boundaries

Divergent plate boundaries can occur in the middle of oceans or in the middle of a continent.

The result of a divergent plate boundary is to create a new ocean basin.

When a supercontinent like Pangaea breaks up, the divergent boundary is found in the middle of a continent, marked by a continental rift.

During the rifting event, the continental crust is stretched and thinned, producing a normal fault.  Topographically this results in a rift valley with a central graben.

The fault provides a pathway for basaltic magma, which rises up from the mantle to form basalt flows and cinder cones.

As divergence continues, sea water will eventually fill the split.

True oceanic crust is eventually produced at a mid ocean rift between the two diverging continents.

The trailing edge of the continent on each side of the rift becomes a passive margin.

2) Transform Boundaries

Transform boundaries involve strike-slip motion of plates (a conservative boundary).

Transform boundaries are characterized by shallow earthquakes.

The San Andreas fault is a transform boundary.  A previous plate was subducted, and after subduction was complete a subsequent plate arrived with a strike-slip orientation to the mainland.

Transform boundaries also occur along the fracture zones of mid-ocean ridges.

3) Convergent Plate Boundaries

At convergent plate boundaries, two plates move towards each other.

The character of the convergent boundary depends on the types of plates that are converging:

Ocean-ocean convergence-  One of the oceanic plates will subduct.  The subduction results in an island chain of volcanoes called an island arc (for example, the Phillipines).

Ocean-continent convergence- The dense oceanic crust is subducted below the continental crust.  A chain of volcanoes forms on the continental crust as a magmatic arc.

Regional metamorphism will occur due to the rising hot magmas and also due to the convergent forces.  On the landward side of the arc, a fold and thrust belt will form.

Continent-continent convergence-  In the case of continental-continental convergence, neither plate will subduct.  First, ocean floor in-between them the continents is subducted.  Eventually, when all the oceanic crust is subducted, the continents will collide.  The two continents will weld together along a suture zone.  Fold and thrust belts will form from the convergence and regional metamorphism will occur (Himalayan Mountains).

A closer look at subduction (for ocean-ocean or ocean-continent subduction).

A Benioff zone defined by shallow, intermediate, and deep earthquakes will define the top of the down-going plate.

At a depth of ~100 km, magmas will be generated in the asthenosphere overlying the down-going plate.  The magmas will rise upward, creating a chain of volcanoes on the overlying plate that parallel the subduction zone.

What Causes Plate Motions?

Rock deep within the Earth’s interior is heated and rises whereas shallow, colder denser rock sinks.  This sets up a convection current.

Huge convection cells may extend from the heat source at the core-mantle boundary all the way to the base of the lithosphere.

Several mechanisms assist in the movement of plates:

1) Ridge-push- Plates move apart at the midocean ridge due to down slopes. 

2) Slab-pull- Subducting slabs pull the surface part of the plate away from the ridge. 

3) Trench-suction- The subducting plate falls into the mantle.  As a result, the overlying trench and plate are pulled horizontally, seaward, toward the subducting plate.

Mantle Plumes and Hot Spots

A modification to the convection model suggests that the mantle transfers heat in the form of narrow columns of hot rock called mantle plumes.

The mantle plumes may rise from the core-mantle boundary and stay stationary.

The mantle plumes have a wide, mushroom-shaped head and a long, narrow tail.

Mantle plumes are proposed to produce “hot-spot” volcanic activity on the earth’s surface, sometimes far away from any plate margins.

Hawaii volcanism is “hot-spot” volcanism.  Hawaii is located in the middle of the Pacific plate, on oceanic crust.

Yellowstone volcanism is another example of “hot-spot” volcanism.  Yellowstone is located on the North American plate in the middle of continental crust.

Hot spot volcanism fed by a mantle plume is proposed to be the reason for the breakup of Pangaea.

The Sea Floor

The Sea Floor

Introduction

The Earth is covered by 71% ocean.

Most of what we know about the ocean floor was discovered after 1950, when advances in technology permitted its exploration.

We study the ocean floor using:

1)            Sonar

2)           Core drilling

3)           Submersibles

4)           Gravity and magnetic surveys

The ocean floor consists of sediment lying on top of basaltic crust

Therefore oceanic crust (basaltic) is composition-ally and structurally simpler than continental crust (chiefly granitic).
 Structure of the Ocean Floor 

Continental shelf – An underwater platform of continental crust at the edge of a continent.  It is inclined very gently seaward at an angle of less than 1°.

On the Atlantic coast of the US, the shelf is 500 km wide.  On the Pacific coast, it is only a few kilometers wide. 

The shelf is covered with young, loose, sediment derived from the land via rivers.

Continental slope – A relatively steep (~4-5°) slope extending from a depth of 100-200 meters at the edge of the continental shelf down to the deep ocean floor.

This is where the continental crust transitions into oceanic crust.

Abyssal plain – The very flat region of the deep-ocean floor, consisting of oceanic (basaltic) crust and overlying sediments.

The plain starts at the base of the continental slope.  The water depth is about 5 km.

This is the flattest feature on the Earth (overlying sediment “fills in” the rugged volcanic oceanic crust). 

Submarine Canyons- V-shaped erosional canyons incised in the continental shelf and slope, and end at the abyssal plain.

Sediment transported within these canyons is deposited in fan-shaped features called abyssal fans (analogous to alluvial fans on land).

These underwater canyons may have initially been carved by rivers during the most recent period of glaciation when sea level was lower.

Currents related to tides move up and down the canyons aiding in the transport of sediment and the erosion of the canyon.

Also, turbidity currents (underwater landslides triggered by earthquakes or strong storms) contribute to the formation of these canyons.

Types of Margins

Passive Continental Margins (East Coast of US)

 

Passive continental margin- A margin that connects continental crust to oceanic crust without any tectonic boundaries.

This is a geologically "quite" boundary without volcanoes, earthquakes, or young mountain belts.  The main activity is sediment deposition.

Passive margins include a large continental shelf, a continental slope, an abyssal plain, and a “continental rise”.

Continental rise- A wedge of sediment that lies at the base of the continental slope on passive margins.  It connects the continental slope to the abyssal plain, and has a gentler slope than the continental slope (~0.5°).

Active Continental Margins (West Coast of US)

 

Active continental margins are characterized by tectonic boundaries, volcanoes, earthquakes, and young mountain belts.

They include a continental shelf and continental slope.

The continental rise is typically absent.

Instead, oceanic trenches are present.

Oceanic trenches are the deepest (8-10 km) parts of the ocean.  They parallel the edge of a continent and are related to a subduction zone.

Trenches are characterized by earthquakes associated with the subducting slab of oceanic crust (the Benioff zone).  Volcanoes are produced above the subduction zone on the continent.

The continental slope occurs on the landward side of the trench.  The continental slope angle changes from 4-5° on the upper part to 10-15° or more near the bottom of the trench.

The Mid-Oceanic Ridge

Mid-ocean ridges are giant undersea mountain ranges.  There are 49,700 miles of mid-ocean ridges on earth.  They are 930-1550 miles wide and 1.2-1.8 miles high.

The crests of the mid-ocean ridges are rift valleys: normal fault-bounded, down-dropped areas where the crust is undergoing extension.  They are about the size of the Grand Canyon.

Mid-ocean ridges are characterized by:

1. Basalt eruption (pillow basalt's)

2. Shallow earthquakes

2. High heat flow

4. Black smokers (sulfide minerals) and associated exotic organisms (that survive toxic chemicals, high temperatures, high hydro-static pressure, and total darkness).  These organisms may give some evidence for how life first evolved on earth.

Sediments of the Sea Floor

Sea floor sediment varies in thickness but can be up to thousands of meters thick in spots.

Terrigenous sediment – sediment derived from land that finds its way to the sea floor (via turbidity currents).

Pelagic sediment – sediment that settles slowly from ocean water.  It is made of:

a)            Fine grained clay- washed to deep sea.

b)           Volcanic ash- airborne fallout

c)            Skeletons of microscopic organisms (foraminifera and radiolarian).

The Age of the Seafloor

The age of oceanic crust and seafloor sediments do not exceed 200 million years in age (~ Jurassic).  In contrast, the oldest crustal rocks are 3.7 - 4.3 billion years old.

The young ages reflect recycling of dense oceanic crust at subduction zones.