evolution of continents and oceans

8/13/2019 Evolution of Continents and Oceans

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Evolution of Continents and Oceans

The theory of plate tectonics is nowadays more or less universally accepted by geologists,

and I have mentioned the basic idea briefly at the beginning of this class. The basic thought

is, that instead of being permanent fixtures of the earth's surface, the continents and oceanbasins undergo continuous change. Both are parts of lithospheric plates that move against

each other, and in the process new crust is created at midoceanic ridges (spreading centers),

and old crust is consumed at convergent plate boundaries (subduction zones). Even before

the theory of plate tectonics, there were a variety of geologic observations that suggested that

the continents were on the move, but because nobody had a good idea what the underlying

driving mechanisms might be, the idea languished in obscurity for the first half of the 20th

century. For now we will take plate tectonics as a theory with a broad observational data

base in its support, and will assume that it essentially works as outlined inChapter 3.

PLATE MARGINS

Alfred Wegener, the pioneer of continental drift, thought that the continents as plates move

through the oceanic crust, implying thus that the shorelines of the continents are the margins

of the continental plates. However, even though that may be initially a reasonable assumption

(the shorelines being major geographic features), continental margins need not necessarily be

plate margins. Today scientists have a fairly good understanding of how the plates move and

how such movements relate to earthquake and volcanic activity. Most movement occurs

along narrow zones between plates where the results of plate-tectonic forces are most evident.

There are basically three different types of plate boundaries (divergent, convergent,

transform), and a fourth type (boundary zones) is sometimes designated when it is difficult todefine a clear boundary:

Divergent boundaries-- where new crust is generated as the plates pull away fromeach other.

Convergent boundaries -- where crust is destroyed as one plate dives under another. Transform boundaries-- where crust is neither produced nor destroyed as the plates

slide horizontally past each other.

Plate boundary zones -- broad belts in which boundaries are not well defined and theeffects of plate interaction are unclear.
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The three principal types of plate margins and various associated features are

illustrated in the picture above.

Divergent Boundaries

Divergent plate boundaries occur along spreading

centers where plates are moving apart (white

arrows) due to mantle convection and new crust is

created by magma pushing up from the mantle.

Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This

submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip

of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth.

The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year(cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but

because this process has been going on for millions of years, it has resulted in plate

movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million

years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents

of Europe, Africa, and the Americas into the vast ocean that exists today.

The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a

natural laboratory for studying on land the processes also occurring along he submerged parts

of a spreading ridge. Iceland is splitting along the spreading center between the North

American and Eurasian Plates, as North America moves westward relative to Eurasia. The

consequences of this type of plate movement are easy to see around Krafla Volcano, in the

northeastern part of Iceland, and the Thingvellir Fissure Zone.

Lava fountains (10 m high) spouting from eruptive fissures during

the October 1980 eruption of Krafla Volcano in Iceland. At Krafla,

existing ground cracks have widened and new ones appear every

few months. From 1975 to 1984, numerous episodes of rifting

(surface cracking) took place along the Krafla fissure zone. Some of

these rifting events were accompanied by volcanic activity; the

ground would gradually rise 1-2 m before abruptly dropping,

signaling an impending eruption. Between 1975 and 1984, the

displacements caused by rifting totaled about 7 m.


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Aerial view of the area around Thingvellir, Iceland, showing a fissure

zone (in shadow) that is the on-land exposure of the Mid-Atlantic

Ridge. Right of the fissure, the North American Plate is pulling

westward away from the Eurasian Plate (left of the fissure). Large

building (near top) marks the site of Lgberg, Iceland's first

parliament, founded in the year A.D. 930.

The evolution of a divergent plate boundary has

three recognizable stages. The birth of a divergent

boundary requires that an existing plate begins to

divide. This is happening today in east Africa, in an

area known as the East African Rift zone. The

African continent is slowly splitting in two. As the

continental crust divides, magma from the

asthenosphere fills in the gap. Several volcanoes are

present in the rift zone. Eventually the gap will form

a narrow ocean (youth) much like the Red Sea to the

north of the East African Rift Zone. The Red Sea

separates Saudi Arabia from Africa.

East Africa may be the site of the Earth's next majorocean. Plate interactions in the region provide

scientists an opportunity to study first hand how the

Atlantic may have begun to form about 200 million

years ago. Geologists believe that, if spreading

continues, the three plates that meet at the edge of the

present-day African continent will separate

completely, allowing the Indian Ocean to flood the

area and making the easternmost corner of Africa

(the Horn of Africa) a large island.

A similar narrow sea, the Gulf of California (see image at right),

lies between much of Mexico and Baja California. The view to

the south along the Gulf of California, between Baja peninsula

(right) and the mainland of Mexico (left). The Gulf is spreading,

pushing Baja further away from the Mexican mainland.

It takes millions of years to form a mature ocean, as rates of

plate motions are slow (10-100 mm/yr). At such rates it would

take millions years to form even a narrow ocean.


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Convergent Boundaries

The size of the Earth has not changed significantly during the past 600 million years, and

very likely not since shortly after its formation 4.6 billion years ago. The Earth's unchanging

size implies that the crust must be destroyed at about the same rate as it is being created. Such

destruction (recycling) of crust takes place along convergent boundaries where plates aremoving toward each other, and one plate sinks (is subducted) under another. The location

where sinking of a plate occurs is called a subduction zone. The type of convergence (some

call it a very slow "collision") that takes place between plates depends on the kind of

lithosphere involved. Convergence can occur between an oceanic and a largely continental

plate, or between two largely oceanic plates, or between two largely continental plates.

Convergence between continental and oceanic crust

Off the coast of South America, along the Peru-Chile

trench, the oceanic Nazca Plate is pushing into and is

being subducted under the continental part of the South

American Plate. In turn, the overriding South American

Plate is being lifted up, creating the towering Andes

mountains, the backbone of the continent. Partial

melting of the subducted oceanic crust gives rise to

andesitic volcanism parallel to the subduction zone.

Because continental crust is less dense than oceanic

crust, oceanic crust will always be subducted under

continental crust. Strong, destructive earthquakes and

the rapid uplift of mountain ranges are common in

these region. Earthquakes are often accompanied by

uplift of the land by as much as a few meters.

The convergence of the Nazca and South American

Plates has deformed and pushed up limestone strata to

form the towering peaks of the Andes, as seen

here in the Pachapaqui mining area in Peru.

Convergence between oceanic and oceanic crust

As with oceanic-continental convergence, when two

oceanic plates converge, one is usually subducted

under the other (the older one is subducted because

of its larger density), and in the process a trench is

formed. The Marianas Trench (paralleling the

Mariana Islands), for example, marks where the fast-

moving Pacific Plate converges against the slower

moving Philippine Plate. The Challenger Deep, at the


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southern end of the Marianas Trench, plunges deeper

into the Earth's interior (nearly 11,000 m) than Mount

Everest, the world's tallest mountain, rises above sea

level (about 8,854 m).

Subduction processes in oceanic-oceanic plate convergence also result in the formation of

volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean

floor until a submarine volcano rises above sea level to form an island volcano. Such

volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic

island arcs, which closely parallel the trenches, are generally curved. The trenches are the key

to understanding how island arcs such as the Marianas and the Aleutian Islands have formed

and why they experience numerous strong earthquakes. Magmas that form island arcs are

produced by the partial melting of the descending plate and/or the overlying oceanic

lithosphere. The descending plate also provides a source of stress as the two plates interact,

leading to frequent moderate to strong earthquakes.Continental-continental convergence

The Himalayan mountain range dramatically

demonstrates one of the most visible and spectacular

consequences of plate tectonics. When two

continents meet head-on, neither is subducted

because the continental rocks are relatively light and,

like two colliding icebergs, resist downward motion.

Instead, the crust tends to buckle and be pushed

upward or sideways.

The collision of India into Asia50 million years ago

caused the Eurasian Plate to crumple up and override the

Indian Plate. After the collision, the slow continuous

convergence of the two plates over millions of years pushed

up the Himalayas and the Tibetan Plateau to their present

heights. Most of this growth occurred during the past 10

million years. The Himalayas, towering as high as 8,854 m

above sea level, form the highest continental mountains in

the world. Moreover, the neighboring Tibetan Plateau, at an

average elevation of about 4,600 m, is higher than all the

peaks in the Alps except for Mont Blanc and Monte Rosa,

and is well above the summits of most mountains in the

United States.


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The collision between the Indian and Eurasian plates has

pushed up the Himalayas and the Tibetan Plateau. The cross

sections show the evolution of the Himalayas and the

displacement of slivers of continental crust during this

collision. The reference points (small squares) show theamount of uplift of an imaginary point in the Earth's crust

during this mountain-building process.

Transform Boundaries

The zone between two plates sliding horizontally past

one another is called a transform-fault boundary, or

simply a transform boundary. The concept of transform

faults originated with Canadian geophysicist J. Tuzo

Wilson, who proposed that these large faults or fracture

zones connect two spreading centers (divergent plate

boundaries) or, less commonly, trenches (convergent

plate boundaries). Most transform faults are found on the

ocean floor. They commonly offset the active spreading

ridges, producing zig-zag plate margins, and are

generally defined by shallow earthquakes.

However, a few occur on land, for example the San

Andreas fault zone in California. This transform fault

connects the East Pacific Rise, a divergent boundary to the

south, with the South Gorda -- Juan de Fuca -- Explorer

Ridge, another divergent boundary to the north.

The Blanco, Mendocino, Murray, and Molokai fracture

zones are some of the many fracture zones (transform

faults) that scar the ocean floor and offset ridges.

The offset that is marked by the San Andreas Fault also

implies that there is mantle upwelling beneath

Southwestern North America. The resulting extension is

seen at the surface in form of a series of NE-SW trending

mountain ranges and valleys, the so called Basin and Range

Province, a result of Horts and Graben tectonics.
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The San Andreas fault zone, which is about 1,300 km long and in

places tens of kilometers wide, slices through two thirds of the

length of California. Along it, the Pacific Plate has been grinding

horizontally past the North American Plate for 10 million years, at

an average rate of about 5 cm/yr. Land on the west side of the faultzone (on the Pacific Plate) is moving in a northwesterly direction

relative to the land on the east side of the fault zone (on the North

American Plate).

The picture at left shows and aerial view of the San Andreas

faultslicing through the Carrizo Plain in the Temblor Range east of

the city of San Luis Obispo.

Other Pictures from the San Andreas Fault:

Orange grove offset

Highway offset

Plate-Boundary Zones

Not all plate boundaries are as simple as the main types discussed above. In some regions, the

boundaries are not well defined because the plate-movement and deformation occurs over a

broad belt (called a plate-boundary zone). One of these zones marks the Mediterranean-

Alpine region between the Eurasian and African Plates, within which several smaller

fragments of plates (microplates) have been recognized. Because plate-boundary zones

involve at least two large plates and one or more microplates caught up between them, they

tend to have complicated geological structures and earthquake patterns.

Regardless of these complications, however, it is now a well established fact that the Earth's

crust is broken into a dozen or so rigid slabs (called tectonic plates by geologists) that

are moving relative to one another.

The cause of plate movementis not accessible to direct observation. The various features of

plate movement, and the increased heatflow along midoceanic ridges are consistent with the

idea that plate movement is caused byconvection in the mantle.The driving force behind the

convection is heat generated by radioactive decay in the earth. The heat released by thisdecay (radiogenic heat) is transferred by convection (slow movement of hot, plastic rock) to
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the surface of the earth. Friction between the convecting mantle and the lithosphere (includes

the rigid crust and that part of the mantle that lies above the plastic/soft behaving

astheneosphere) causes the crustal plates (form the top of the lithosphere) to move according

to the movement of the convection currents. Heat production in the earth will cease as

radioactive decay diminishes, and then convection will cease and the final cooling phase ofthe Earth will begin. No more mountain ranges will be built, and the continents will become

very flat. Eventually the oceans may cover the continents again (shallow seas, buildup of

carbonate platforms, change of seawater composition because terrestrial input cut off,

possibly a new stage in evolution). Tectonic movements will still occur, but this time they

will mainly be a response to differential cooling of the earth (surface already cold, but interior

shrinks now as well, volume reduction, pressure ridges will form due to shrinking, may

resemble folded mountain belts).

THE OCEAN BASINS, THEIR EVOLUTION AND IMPORTANT FEATURES

Most of the earth's surface is covered by oceans, but for a long time the oceans have been an

essentially white spot on the map of the world. Early expeditions like that of the Beagle

(Charles Darwin) brought some preliminary knowledge, compilations of data by ship captains

brought some initial knowledge about ocean currents and migration of fish swarms (mention

Melville, Captain Ahab), but by and far we did not know much about the topography of the

ocean floor, much less about its geological features. Starting at around 1930, however, a vast

amount of knowledge has been gathered about the oceans, about their water chemistry, the

cycling of elements, biological aspects, bathymetry, bottom sediments and their stratigraphy.

Though much less spectacular and not as well publicized, the progress in knowledge aboutthe oceans is far more important for the future of mankind than to send a few men to the

moon. Ocean research has implications for food resources, the supply of raw materials for a

growing population, and possibilities of ocean population by man (giant raft cities in shallow

seas, platforms moving with food-rich ocean currents, etc.). Even populating the deep sea is

probably cheaper and more feasible than to have people live in colonies on the moon.

Work on the bathymetry of the ocean basins (mainly with echo-sounding devices) has

revealed many morphologic features that were previously unknown, such as oceanic ridges,

abyssal plains (and hills), seamounts, trenches, and continental margins, all of these features

are now easily explained by plate tectonics.

Map of the Atlantic and Eastern Pacific Basin. Mid-

Oceanic Ridges(marked with white arrows) are

extensive. These are the youngest portions of the

ocean basins where new ocean crust is generated

through mantle upwelling and plate divergence.

Taken together the oceanic ridge system of the earth

is about 65000 km long and extend all around the

globe.


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Map of the Pacific Basin and parts of the central

Atlantic. Continental Shelf= flooded edges of the

continents; Continental Margin = the edge/border region

of the continent; Deep Sea Trenches= deepest parts of

ocean basins (due to subduction of oceanic crust); AbyssalPlains= older parts of oceanic crust, smoothed due to

sediment deposition; Seamounts= submarine volcanic

cones; the can also form linear arrangements, so

called Seamount Chains.

Continental margins are in a geological sense not part of the

oceanic crust. They consist of continental crust and material

that was eroded from the continents and is now piled up

along the margins of the continents. The margins are

subdivided into CONTINENTAL SLOPE and SHELF with

the latter simply being a submerged part of shield or

platform.

Closeup of central Pacific Basin. Shows how the Hawaiian Islands(Hawaii marked with

white arrow) are the youngest portion of a long chain of seamounts. The linear arrangement

of many seamounts indicates that they formed because the plate moved over a stationary site

of magma upwelling, a so called mantle "Hot Spot". Seamounts are submarine volcanoes

that may finally build above the water level (e.g. Hawaii), in which case they are called

islands. If seamounts rise above sea level (rises for two reasons, buildup of material in a

cone, upwelling mantle pushes up plate), they are subject to wave erosion and colonization by

reefs, with both processes tending to create a flat top on the original volcanic cone. Later,when the oceanic plate cools down and the island finally drowns we get flat-topped

seamounts, so called GUYOTS.


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Closeup of the eastern Pacific Basin. Shows triple junctionof spreading ridges in center.

Also shown are the subduction zone/trench along the western edge of central America, and

the associated slope and shelf regions. Thetrenchesare the deepest parts of the oceans and

are the topographic expression of subduction zones. They are marked by intense volcanism

(island arcs, volcanic mountain ranges, e.g. Andes, Cascades), and high frequency of

earthquakes. hey are usually asymmetrical with a gentle slope towards the subducted plate,

and a steeper slope towards the subducting plate. Some trenches are as deep as 11 km, and

may extend for thousands of kilometers across the seafloor.

A map of the ocean basins where the locations of some major deep sea fans are marked.

Deep Sea Fans are large sediment accumulations that are deposited on the slope and the

adjacent seafloor. The sediments are supplied to the slope regions through submarine

canyons, deep incisions in the continental shelf that probably originated during prior

episodes of low sea level (ice ages). Along the continental margins sediment that is conveyed

to the deep sea via submarine canyons (sliding, mass movement, turbidity currents) forms

large cone-shaped or fan-shaped sediment accumulations at the toe of the continental slope,

so called SUBMARINE FANS or DEEP-SEA FANS (not unlike alluvial fans). Turbidity

currents move down these fans, spread out on the abyssal plain, decelerate, and deposit

graded sand and silt layers (so called turbidite sequences). Sediment spreading by turbidity

currents helps to smoothen the relief in abyssal plain regions.

The floor of the ocean basins (abyssal plains) is essentially basaltic crust that is covered by

sediment (settling from suspension, of organic material such as foram tests, radiolarian tests,

etc., and also clay swept in from the rivers, volcanic ash [large ashclouds may circle the globe


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several times], and material transported by winds from the continents [Atlantic west of

Sahara desert]). We call that material PELAGIC SEDIMENT.

COMPOSITION AND STRUCTURE OF OCEANIC CRUST

The oceanic crust is not simply a pile of basalt, but can be subdivided into several distinct

layers, that form in response to the processes operating at a midoceanic ridge.

The top layer (1.)consists of pelagic sediments that were

deposited above the basalts of the oceanic crust. The

second layer (2.)consists of lavas that were extruded onto

the ocean floor at the spreading center. These lavas are

called pillow basalts, because of the way they appear in

cross-section. The molten basalt is extruded onto the

ocean floor through fractures (extension), and as soon asthe molten material comes in contact with seawater it will

cool down and solidify. The next batch of lava will come

out to the side of the first one, and also will solidify, etc.

We will slowly pile up small batches of magma, that in

their geometric arrangement are not unlike a pile of

sausages, or squirts out of a toothpaste tube. In cross

section we will have mainly elliptical cross-sections

(pillow shape), thus the name pillow basalt. The surface

topography of this layer is irregular and rough. The thirdlayer (3.) consists essentially of complexly cross-cutting,

near vertical basaltic dikes, which are the feeder channels

for the pillow basalts. They form as fractures at the

spreading center (highest extensional stress), and finally

fill up with basalt and become part of the sheeted dike

complex as they move away from the spreading center.

The fourth layer(4.)consists of the magma chambers that

feed the dikes of layer three, and these leftover magma

chambers are filled by the plutonic equivalent of basalt,

gabbro. The magma itself originated by partial melting in

the mantle below the spreading center (higher heatflow,

rising of accumulating melt). Below that layer is the

mantle (asthenosphere), consisting of peridotite.

That the oceanic crust is layered has been known from seismic refraction data, but nobody

has ever drilled through the oceanic crust (too hot). Fortunately, once in a while bits and

pieces of oceanic crust are incorporated into the uplifted material of flooded mountain belts,

and is thus available for direct and detailed study. In Iceland, where the Mid-Atlantic Ridge

rises above the sea surface, is another opportunity to examine the structure of the oceanic

crust.


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As new oceanic crust forms at mid-oceanic ridges, cold sea water invades the hot new crust

through the abundant fractures (crustal extension). As the sea water heats up its density

decreases and it rises upwards. When it leaves through fractures at the seafloor we

havesubmarine hot springs,better known as black smokers. These hot springs have created

quite a bit of excitement in the scientific community because they open up all sorts ofunexpected angles on the chemistry of the oceans, the transfer of chemical elementsbetween

the oceans and the oceanic crust (elemental cycles), and the origin of life. The latter was

prompted by the discovery ofunusual communities of microbes, worms, clams, and

crustaceans that live at hot spring sites and instead of sunlight depend on energy supplied by

the hot springs in the form of sulfides.

EVOLUTION OF CONTINENTS

The characteristic features of continents are shield areas, stable platforms, and folded

mountain belts (introduced earlier in this lecture). With the theory of plate tectonics we can

now relate these features to each other and describe them as different phases in the evolution

of continents.

When we examine the continental crust in some

detail, we see that in many areas (e.g. Texas) it

consists of a thin surface cover of horizontally

stratified sediments that is underlain by complexly

deformed metamorphic rocks that have been intruded

by granites. In places where vast areas of this lower

complex of rocks are exposed, we speak of

a "shield". In places where the shield material is

covered by sediments we speak of a"stable

platform". This kind of situation is typical for large

portions of continents, except along some of the

margins where we have subduction and compression.

In the latter case mountain ranges develop end we

have "folded mountain belts".

Pertinent features of the continental crust are:

It consists overall of material with granitic composition (granites and gneisses ofgranitic composition, other compositional rock types, such as basalts are present, but

volumetrically not important)

From the travelling velocity of seismic waves in the continental crust we know thatthe lower portions of the continental crust are denser than the upper portions, probably

because of a downwards increase of rocks of more basaltic composition

Continental crust is thickest beneath mountain ranges (root zones, 50-60 km),elsewhere the thickness is about 30 km.

The structure of the continental crust is considerably more complex than the simplelayer structure of the oceanic crust. It consists of intensely deformed metamorphic

rocks (derived from sediments and volcanic rocks) that are intruded by granites, andmay have been partially remolten to granites.
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The oldest continental crust has been determined to be about 3.8 b.y. old, and itappears that the continents grew throughout geologic history. They cover nowadays

about one third of the earth's surface, but initially the proportion of the oceans may

have been much larger.

The continental crust is the end product of planetary differentiation (accumulation oflight materials), and within the crust of each continent we can distinguish three basic

components: shields, stable platforms, folded mountain belts.

SHIELDScontain the bulk of the rock record of continental evolution and growth, and are

thus the key to the understanding of the origin of continents. As noted earlier, they are

essentially flat and consist of a complex arrangement of igneous and metamorphic rocks. The

mere fact that these rocks are exposed at the surface now, implies that many kilometers of

rock were eroded from the continent before these rocks finally came to the surface. If the

shield rocks of a continent are studied with respect to their metamorphic age, it often turns

out that those on the center are the oldest ones, and that there are several belts of

metamorphic rocks that get progressively younger outward. The oldest portions of the shields

consist of a mixture of volcanic rocks (basalts, andesites) and volcanic derived sediments

(erosion of volcanoes), and the rocks show similarity to the material accumulating in modern

day island arcs. Only when these basically mafic rocks were later on intruded by granites, did

the overall composition become granitic (75% granite). Later metamorphic belts were

accreted onto these old continental cores (will discuss a little later) and have overall a

considerably more granitic composition (because the sediment was derived from a crust that

was already 75% granite).

STABLE PLATFORMSAs time goes by, the shields are eroded down to within a few tens

of meters of sea level, and any rise of sea level will lead to flooding of vast areas of the shield(plate tectonics, increased spreading, rise of ridges, flooding). At present only 18% of the

continental crust is flooded, but there were times in the past where vast portions of the

continents were covered by a shallow sea (interior of North America).

FOLDED MOUNTAIN BELTSare usually found along the margins of continents, and the

folding and thrusting indicates that as much as 30% of crustal shortening has taken place

during their formation. We know now that his shortening is a direct reflection of the

compressive stress regime and subduction of oceanic crust along convergent plate margins,

but before plate tectonics the missing crust was very troublesome thing to explain. The

location of these fold belts along continental margins implies that by convergence of plates

material is piled up along the continents, and finally becomes part of the continental crust.

Fold belts that are terminated abruptly at the continental margin, such as the Appalachians

and the Caledonides, suggest that he fold belts were once much longer, and have been

separated when continents broke up by continental rifting.

From Mountain Belt to Continent


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When a mountain belt is formed along a continental

margin by subduction, sedimentary and volcanic rocks

are buried deeply and undergo high-pressure and high-

temperature metamorphism in the root zone of the

mountain belt. Also, parts of the buried material aswell as of the subducted oceanic plate melt, and

granitic and andesitic magmas rise. A considerable

portion of the granites never rises to the upper portions

of the mountain range, and crystallizes within the

realm of the metamorphic rocks in the lower portions.

The newly formed mountain range (A) is of course in isostatic

equilibrium with the mantle (that's why we have a root zone), but

as erosion wears down the top portions of the fold belt, the root

zone has to rise in order that equilibrium is maintained (B&C). In

that way the volcanic and sedimentary unmetamorphosed portions

of the range are eroded away, and the metamorphosed and granite

intruded lower portions move upwards (B&C). This process

continues until the fold belt is eroded down to sealevel, then

erosion stops and isostatic uplift ceases (D). By that time the

outcropping rocks will be the high grade metamorphics and

granites of the root zone. We started with a folded mountain

belt, and through continued erosion we have produced a new

piece of shield material.

Formation of a fold belt and a metamorphosed root zone on convergent plate boundaries is

also known as orogeny (or creation of mountain ranges). Within the context of different

types of plate convergence (mentioned earlier) we can distinguish three different main types

of orogeny (ocean/ocean = island arc; ocean/continent = fold belt/volcanic arc;

continent/continent = fold belt/high plateaus).

EVOLUTION OF A CONTINENT

We can use these different types of orogenies and the underlying plate tectonic processes toexplain the evolution of continents and the continental crust.

Initially (A)we might for example have only oceanic crust,

convergence of oceanic plates and formation of island arc

complexes (andesitic material, too light to be again subducted).

Sediment is shed from the arc (B), is compressed and pushed

against the arc, the mountains rise, and the root zone grows, until

finally high-P/T metamorphism and granite plutonism occur (C).

We start accreting material (folded mountain belts) to the initial

arc, an embryonic continent is formed(C). The continent is erodedand quartz, feldspar, and clay-rich sediments accumulate around


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its margins. Renewed subduction pushes up new folded mountain

belts, accompanied by metamorphism and granite plutonism.

Finally the new fold belt is worn down and another segment has

been added to the growing continent (D). Continued accretion etc.

etc., the cycle repeats and the continent grows (D).

Crustal recycling and the differentiation of the continental crust is intimately related to the

composition of the oceans, the supply of nutrients for the global biomass, and thus is also

linked to those global feedback mechanisms that we consider essential for climate regulation

(carbon cycle etc.). In part, the biosphere has adapted opportunistically to whatever chemical

components were provided in the process, but it also has an active role through the

weathering of continents, the deposition of carbonate banks, the carbon cycle feedbacks with

climate, etc.

I hope that in the course of this lecture you have gained insights into three topical complexes:

the Earth system really is highly complex, and consists of many nested andinterlinked element cycles and feedback loops

we are a long way from understanding how the Earth system works in detail, butwe are making progress

the biosphere is an important component of the Earth system. Simply throughevolutionary selection pressures it may have evolved to participate in climate

regulation for most of Earth history.

Eventually, all things merge into one, and a river runs through it.

The river was cut by the world's great flood and

runs over rocks from the basement of time.

On some of the rocks are timeless raindrops.

Under the rocks are the words, and some of the words are theirs.

I am haunted by waters.

evolution of continents and oceans

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