Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label Sea floor. Show all posts
Showing posts with label Sea floor. Show all posts

The Messinian Salinity Crisis


You will have heard of The Messinian Salinity Crisis no doubt. From learned articles, geology textbooks, probably lectures at your college or University. Or possibly not. This was not always the hot topic it is now. In fact, the very idea of this happening, was for a while, challenged, even ridiculed. It seemed too incredible that this could happen as it did and Dessication/Flood theories took time to gain traction. But, if you had heard about it, you would remember that The Messinian Salinity Crisis, was a time when the Mediterranean Sea, very much as we know it today, evaporated – dried out, almost completely.



You will have heard of the rates of desiccation, influx and yet more desiccation, repeated in endless cycles over tens, even hundreds of thousands of years. On a human temporal scale, this would have been a long drawn out affair, covering a time hundreds of generations deep, more than the span of Homo sapiens existence. In Geologic terms however, it was a string of sudden events. Of incredibly hot and arid periods followed by rapid ingress of waters, either via spillways through what is now modern day Morocco and the southern Iberian peninsular, or headlong through a breach in the sill between the Pillars of Heracles, the modern day Straights of Gibraltar.


There were prolonged periods of dessication, of desolate landscapes beyond anything seen today in Death Valley or The Afar Triangle. These landscapes were repeatedly transgressed by brackish waters from storm seasons far into the African and Eurasian interiors, or the Atlantic, and these in turn dried out. Again and again this happened. It had to be so because the vast deposits of rock salt, gypsum and anhydrites could not have been emplaced in a single evaporite event. The salt deposits in and around the Mediteranean today represent fifty times the current capacity of this great inland sea. You may have heard too of the variety of salts production, as agglomerating crystals fell from the descending surface to the sea floor, or as vast interconnected hypersaline lakes left crystalline residues at their diminishing margins, as forsaken remnant sabkhas, cut off from the larger basins, left behind acrid dry muds of potassium carbonates – the final arid mineral residue of the vanished waters.


Just under six million years ago, Geologic processes isolated what was left of the ancient Tethys ocean, the sea we know as the Mediterranean, home to historic human conflicts and marine crusades of Carthage, Rome, Athens and Alexandria, a Sea fringed by modern day Benidorm, Cyprus, Malta and Monaco. At a time 5.96 million years ago – evaporation outpaced replenishment. Indeed, just as it does today, but without the connecting seaway to replenish losses. Inexorable tectonic activity first diverted channels, then – sealed them. Cut off from the Atlantic in the West, water levels fell, rose briefly and fell again, and again. The mighty Nile - a very different geophysical feature of a greater capacity than today, and the rivers of Europe cut down great canyons hundreds and thousands of metres below present Eustatic sea and land surface levels, as seismic cross sections show in staggering detail. The cores taken at depth in the Mediterranean, show Aeolian sands above layers of salt, fossiliferous strata beneath those same salts, all indicating changing environments. The periods of blackened unshifting desert varnished floors and bleached playas, decades and centuries long, were punctuated often by catastrophic episodes, with eroded non conformable surfaces of winnowed desert pavement, toppled ventifracts, scours and rip up clasts. Species of fossilised terrestrial plant life, scraping an arid existence have been found, thousands of meters down, in the strata of the Mediterranean sea floor.
 


There is much evidence too, in the uplifted margins of Spain, France, and Sicily, of those hostile millennia when the sea disappeared. Incontrovertible evidence, painstakingly gathered, analysed and peer reviewed, demonstrates via the resources of statistical analysis, calculus and geophysical data that the Messinian Salinity Crisis was a period during the Miocene wherein the geology records a uniquely arid period of repeated partial and very nearly complete desiccation of the Mediterranean Sea over a period of approximately 630,000 years. But for the Geologist, the story doesn’t end there. The Geologists panoptic, all seeing third eye, sees incredible vistas and vast panoramas. Of a descent from the Alpine Foreland to the modern day enclave of Monaco, gazing out southwards from a barren, uninhabited and abandoned raised coast to deep dry abyssal plains, punctuated by exposed chasms, seamounts and ridges, swirling and shifting so slowly in a distant heat haze. A heat haze produced by temperatures far above any recorded by modern man and his preoccupation with Global Warming. An unimaginable heat sink would produce temperatures of 70 to 80 degrees Celsius at 4000M depth within the basins. 




Looking down upon this Venusian landscape, the sun might glint on remaining lakes and salt flats so very far away and so very much farther below. Hills and valleys, once submerged, would be observed high and dry – from above, as would the interconnecting rivers of bitter waters hot enough to slowly broil any organism larger than extremophile foraminifer. All this, constantly shimmering in the relentless heat. Only the imagination of the geologist could see the vast, hellish, yet breathtaking landscape conjured up by the data and the rock record. And finally, the Geologist would visualise a phenomenon far greater in scope and magnitude than any Biblical flood – The Zanclean Event.
Also known as The Zanclean Deluge, when the drought lasting over half a million years was finally ended as the Atlantic Ocean breached the sill/land bridge between Gibraltar and North West Africa. Slowly perhaps at first until a flow a thousand times greater than the volumetric output of the Amazon cascaded down the slopes to the parched basins. Proximal to the breach, there would be a deafening thunderous roar and the ground would tremor constantly, initially triggering great avalanches above and below the Eustatic sea level as the far reaching and continuous concussion roared and rumbled on, and on, and on. For centuries great cataracts and torrents of marine waters fell thousands of metres below and flowed thousands of kilometers across to the East. Across to the abyssal plains off the Balearics, to the deeps of the Tyrrhenian and Ionian seas, into the trenches south of the Greek Islands and finally up to the rising shores of The Lebanon. The newly proximal waters to the final coastal reaches and mountains that became islands, must have had a climatological effect around the margins of the rejuvenated Mediterranean. Flora and Fauna both marine and terrestrial will have recolonised quickly. Species may have developed differently, post Zanclean, on the Islands. And in such a short period, there must surely have been earthquakes and complex regional depression and emergence. Isostacy compensated for the trillions of cubic meters of transgression waters that now occupied the great basins between the African and Eurasian plates, moving the land, reactivating ancient faults and within and marginal to the great inland sea, a region long active with convergent movements of a very different mechanism.

Hollywood and Pinewood have yet to match the imagination of the Earth Scientist, of the many chapters of Earths dynamic history held as fully tangible concepts to the men and women who study the rocks and the stories they tell. The movies played out in the mind of the geologist are epic indeed and – as we rightly consider the spectre of Global Warming, consider too the fate of future populations (of whatever evolved species) at the margins of the Mediterranean and the domino regions beyond, when inexorable geologic processes again isolate that benign, sunny holiday sea. Fortunately, not in our lifetime, but that of our far off descendants who will look and hopefully behave very differently from Homo Sapiens.


Note: This blog is written and contributed by Paul Goodrich. You can also contribute your blog or article on our website. See guidelines here.

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.
However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary: Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions. 

Banded-iron formation (BIF)
is 
consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.



                     Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John



In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that
the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in ocean environments today, where sediments and silica-shelled microorganisms accumulate gradually on the seafloor and eventually turn into rock. But iron is less common in younger oceanic sedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.


Banded iron-formation (10 cm), Northern Cape, South Africa.
Specimen and photograph: A. Fraser
Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas, it becomes insoluble (it cannot be dissolved in water) and forms a solidparticle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).
Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:
4Fe3 + 2O2 → 2Fe2O3
6Fe2 + 4O2 → 2Fe3O4
The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygen gas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.
Banded Iron Formation
Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extreme glaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with the atmosphere, limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate.

References:

1. Misra, K. (1999). Understanding Mineral Deposits Springer.
2. 
Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope. Science, 160(3829), 729–736.
3. 
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.

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.

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.