Chapter 20: The Earth Through Time. Introduction: Tracking Past Plate Motions (1) The evidence in support of plate motions comes from measurements made

Chapter 20: The Earth Through Time

Introduction: Tracking Past Plate Motions (1)

The evidence in support of plate motions comes from measurements made with the Global Positioning System (GPS).

However, evidence of past plate motions is not obtained through GPS.

Introduction: Tracking Past Plate Motions (2)

Evidence includes: Great arc-shaped belts of metamorphic rocks formed

by continental collisions. The eroded remains of island-arc volcanic rocks. Traces of Earth’s past magnetic field preserved in old

lava flows.

Plates have been moving and changing Earth’s surface for at least 2 billion years.

Former Ideas About Continents (1)

In the sixteenth century, it became apparent that the coasts were approximately parallel.

During the nineteenth century, the favored idea was that Earth was originally a molten mass that is cooling and contracting, with the crust being gradually compressed.

Former Ideas About Continents (2)

Scientists discovered at the beginning of the twentieth century that the Earth’s interior is kept hot by radioactive decay. The Earth might not be cooling but heating up (and

therefore expanding). Heating would cause the Earth that had been covered

largely by continental crust to expand and then the continental crust would crack into fragments.

As expansion continued, the cracks would grow into ocean basins and basaltic magma from the mantle would rise through the cracks to form the oceanic crust.

The New Idea: Plate Tectonics (1)

By the middle of the twentieth century, a totally new approach was developed: plate tectonics.

When the lithosphere (岩石圈 ), called plate, slides sideways across the asthenosphere(軟流圈 ): Some parts of the lithosphere can be in compression. Others can be in tension (that is, being pulled apart). When a plate split in two, the broken edges of

continental crust match perfectly.

The New Idea: Plate Tectonics (2)

The energy needed to move plates comes from the Earth’s internal heat energy, which causes convective flows in the mantle.

Plate tectonics is the only theory ever developed that explains all of the Earth’s major features.

from Cox and Hart, 1986

The greatest shortcoming of continental drift was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely.

Pangaea (1)

Alfred Wegener’s theory of continental drift originated when he attempted to explain the match of the shorelines on the two sides of the Atlantic, especially along Africa and South America.

He hypothesized an ancient land mass called Pangaea.

The northern half of Pangaea is called Laurasia, the southern half Gondwanaland.

Pangaea (2)

Laurasia includes: Eurasia. North America.

Gondwanaland includes: India. Africa. Antarctica. Australia. South America.

Figure 20.1 A

300 My BP

white area: covered by icesolid lines: contours of 2000-m below sea level, along which the continental crust meets oceanic crustblack area: overlapped region; red area: gaps

Present

white area: old ice sheetsarrows: direction of movement of the old icedashed lines: connecting the glaciated regions if the continents were in the present positions at 300 My BP.

Figure 20.1B

Pangaea (3)

During the late Carboniferous Period, about 300 million years ago, a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia. This is explained by continental drift: 300 million

years ago, the regions covered by ice lay in high, cold latitudes surrounding the south pole, while north America and Eurasia were close to the equator

It remained unconvinced that how the solid rock of a continent could possibly overcome friction and slide across the oceanic crust.

Breakup of Pangaea from200 My BP to the present

Reunite of Pangaeabackward in time

Apparent Polar Wandering (1)

From the mid-1950s to the mid-1960s, geophysicists discovered paleomagnetism (古地磁 ). Certain igneous and sedimentary rocks can preserve a

fossil record of the Earth’s magnetic field at the time and place the rocks formed.


Three essential bits of information are contained in that fossil magnetic record: The Earth’s polarity (the magnetic field was normal

or reversed at the time of rock’s formation). The location of the magnetic poles at the time the rock

formed. The magnetic inclination (磁傾角 , I) (indicating how

far from the point of rock formation the magnetic poles lay).

The earth’s magnetic field is close to a dipole field that originated inside the earth.

The earth’s magnetic field is nearly a dipole field, with the dipole axis tilted almost 12° away from the earth’s rotation axis.

Figure 20.2

blue lines: horizontal surface red diamonds: magnetic inclination


The paleomagnetic inclination (I) is a record of the place between the pole and the equator (that is, the magnetic latitude (m) where the rock was formed (use the dipole formula: tanI=2tan m).

In the 1950s, geophysicists determined that the strange plots of paleopole positions indicated apparent polar wandering.

Figure 20.3

If the continents were not moved in the past, the paleopole of the earth’s magnetic fields would wander along the track denoted by solid lines.

Seafloor Spreading (1)

In 1962, Harry Hess of Princeton University hypothesized that the topography of the seafloor could be explained if the seafloor moves sideways, away from the oceanic ridges.

His hypothesis came to be called the theory of “seafloor spreading,” and was soon proven to be correct.


Hess postulated that magma rose from Earth’s interior and formed new oceanic crust along the midocean ridges.

Three geophysicists (Frederick Vine, Drummond Matthews, and Lawrence Morley) proposed that lava extruded at any midocean ridge becomes magnetized and acquires the magnetic polarity that exists at the time the lava cools.


The oceanic crust contains a continuous record of the Earth’s changing magnetic polarity.

Successive strips of oceanic crust are magnetized with normal and reversed polarity.

The magnetic striping allows the age of any place on the seafloor to be determined.

Magnetic striping also provides a means of estimating the speed with which the seafloor had moved.

Figure 20.4

Relative velocity of plate motion v can be determined by distance to the ridge d divided by age of the seafloor t, i.e.,v = d / t.

Magnetic Record And Plate Velocities (1)

The most recent magnetic reversal occurred 730,000 years ago.

The oldest reversals so far found date back to the middle Jurassic, about 175 million years ago.

From the symmetrical spacing of magnetic time lines it appears that both plates move away from a spreading center at equal rates.

Figure 20.5


All that can be deduced from magnetic time lines is the relative velocity of two plates.

Absolute velocities requires information from GPS measurements (for present-day plate motions), or hot spot tracks, or past plate motions; Plates with only oceanic lithosphere tend to have high

relative velocities (Pacific and Nazca plates).


Plates with a great deal of thick continental lithosphere, such as the African, North American, and Eurasian plates, have low relative velocities.

Plate velocities vary with the geometry of motion of a sphere.

Plates of lithosphere are pieces of a shell on a spherical Earth.

The motion of a rigid plate on the surface of a sphere can be described by a rotation pole (Euler pole) and a constant angular velocity rotating around the pole.


One consequence of different plates velocities is that the width of new oceanic crust bordering a spreading center increases with the distance from the rotation pole.

Each transform fault lies on a line analogous to a line of latitude around the rotation pole.

Figure20.6

Figure 20.7 Figure 20.8

Relict Plate Boundaries In The Geologic Record (1)

Seafloor magnetic strips help us reconstruct plate motion only as far back in time as the Jurassic, some 175 million years ago.

All large expanses of older oceanic crust have been subducted back into the mantle at convergent plate boundaries.

Relict Plate Boundaries In The Geologic Record (2)

The paleomagnetism of continental rock can be used to follow plate motion further back in time, but without the breadth and continuity of seafloor data.

Today’s continents were assembled from many distinct plates or plate fragments.

Small fragments of continental crust that have drifted as a single unit in the Earth history are called terranes.

Ophiolites (1) (蛇綠岩系 )

The igneous rock formed at a spreading center, known as midocean ridge basalt (MORB), has a distinctive chemistry;

When MORB is found on land, it usually lies within a body of rock that appears to be a fragment of oceanic crust caught up in a continental collision.

Ophiolites (2)

The minerals that characterize basalt, if buried deep within the collision zone, transform into a assemblage dominated by a distinctive green fibrous mineral called serpentine (蛇紋石 ).

These serpentine-dominated fragments of oceanic crust found on continents are called ophiolites, from the Greek word for serpent, ophis.

Ophiolites (3)

Their structure matches well the crustal structure expected at a midocean ridge: At the top is a thin layer of sediment that was

deposited on the ocean floor. Beneath the sediment is a layer of pillowed basalt. Still deeper are sills of gabbro ( 輝長岩 ), the plutonic

equivalent of basalt. Beneath the gabbro sills there is often a layer of

peridotite. The contact between gabbro and peridotite ( 橄欖岩 ) is

interpreted to be the former Moho at the base of what was formerly oceanic crust.

Ophiolites (4)

Subduction Mélange (混雜 ( 成 ) 岩 ) And Blueschists (1)

Along convergent margins, a distinctive feature is the development of a mélange: a chaotic mixture of broken, jumbled, and thrust-faulted rock.

A sinking plate drags the sedimentary rock formed from accumulated sediment downward beneath the overriding plate.

Subduction Mélange And Blueschists (2)

Caught between the overriding plate and the sinking plate, the sediment becomes shattered, crushed, sheared, and thrust-faulted, forming a mélange.

As the mélange thickens, it undergoes metamorphism, common in many mélange zones, to form low-temperature metamorphism blueschists (藍片岩 )and eclogites (榴輝岩 ).

Figure 20.10

Back-arc Basins And Plate Extension (1)

When the sinking of a subducting plate is faster than the forward motion of the overriding plate, the margin of the overriding plate can be subjected to tensional (pulling) stress.

If the overriding plate is oceanic or if the extension of a continental margin has progressed to an extreme state, an arc-shaped basin forms behind and parallel to the magmatic arc of the subduction zone.

Figure 20.11

Figure 20.12

Back-arc Basins And Plate Extension (2)

Basaltic magma may rise into such a back-arc basin at a newly formed spreading center, and new oceanic crust may form.

Supercontinents And Vanished Oceans (1)

The average composition of continental lithosphere is quartz-rich, compared to olivine and pyroxene-rich oceanic lithosphere.

Continental lithosphere is always less dense than oceanic lithosphere and is not subducted.

Seismic evidence suggests that a root of mantle rock is attached to the base of old, cool continental crust.


Continental buoyancy can be enhanced by the detachment and loss of this dense mantle root during the final stages of a continental collision.

When plate movement bunches continental fragments together, the heat of the mantle beneath still must escape Earth’s interior. A supercontinent impedes heat flow from the deep

mantle to the surface, effectively forming a layer of insulation.


Convective motion within Earth’s mantle: Will accumulate heat and cause thermal buoyancy at

the base of the continental lithosphere. Warms and softens the lithosphere, which begins to

rift. The opening of the Atlantic ocean was heralded by the

eruption of large basalt flows; In some cases, these eruptions persisted and became hot

spots (example: the Parana flood basalt in Brazil).


Two supercontinents existed in the past: Pangaea, formed roughly 350 million years ago, Rodinia, an earlier Proterozoic* (原古界 ) continent

formed roughly 1100 million years ago and split apart roughly 750 million years ago.

*see geological time scale in Figure 11.10 of page 286.

Figure 20.13

Red strip marks the location of an orogen that once formed a common suture of several cratons.


The sutures between continental fragments can be found in the many mountain ranges and belts of ophiolites that formed in the late Paleozoic period, roughly between 450 and 350 million years ago.

Examples include: The Appalachian mountains in eastern North America. The Atlas mountains in Morocco. The Ural mountains in Russia. The Hercynian mountains in Europe.

Each of these mountain ranges is heavily eroded now.


As Pangaea formed, sediment sequences on continental shelves around the world record evidence that global sea level fell, draining shallow continental seas and exposing large areas of continental shelves.


Geologists interpret the sea level drop as evidence for: A general slowdown in plate movement. A slowdown in the rate of formation of young,

buoyant oceanic lithosphere at midocean ridges.

The oceanic lithosphere at the time of Pangaea was, on average, older, colder, and denser, leading to a deeper world ocean.


Pangaea lasted as a supercontinent for at least 150 million years.

New oceans formed when Pangaea separated: The Atlantic. Southern Oceans.

Ice Ages in Earth History (1)

Large continental ice sheets have existed several times during Earth’s history: In the Carboniferous period, 286 to 360 My ago. In the Ordovician and Silurian periods, roughly 450 to

425 My ago. Both of the glacial periods occurred at times

when large continental landmasses lays over Earth’s south pole, providing a platform for the accumulation of vast quantities of snow and ice at high latitudes.

Figure 20.14

Ice Ages in Earth History (2)

Factors leading to an ice sheet: The amount of carbon dioxide and other greenhouse

gases in the atmosphere. The tilt of the Earth’s axis. Changes in the Earth’s orbit around the sun. The height of continents and their positions in relation

to the north and south poles.

Phanerozoic (顯生宙 ( 元 )) Glaciations (1)

Three major periods of high-latitude glaciation have been recognized in the Phanerozoic: Pleistocene (0 to 5 million years ago). Carboniferous (roughly 300 million years ago). Ordovician (roughly 455 million years ago).

Rapid carbon burial and reduced volcanic activity because of slow plate motion and low spreading-ridge volcanism lower concentrations of CO2 in the atmosphere, reducing the greenhouse effect and encouraging glaciation.

Phanerozoic Glaciations (2)

Carboniferous coal deposits occur within repetitive layered sedimentary sequences of marine and nonmarine sediments called cyclothems.

Cyclothems indicate repeated transgressions and regressions by the sea. Timing connected to the longer Milankovitch orbital

cycles of 100,000 years. glacial and interglacial climate may have alternated in

a regular pattern.


New continental glaciers: Would have stolen water from the oceans, causing sea

level to drop. Drained Coastal coal swamps as the sea level fell. Decreased carbon consumption because of reducing

photosynthesis. Atmospheric CO2 levels could return to normal

levels. An increase in greenhouse warming would melt the

glaciers and raise the sea level, rejuvenating the coal swamps and repeating the glacial cycle.


There is evidence for continental glaciation near the end of the Proterozoic Eon, between 600 and 800 million years ago. The extensive low-latitude glaciation occurred during

a climatic extreme in which the whole Earth must have experienced glaciation.

Extensive low-latitude glaciation did not occur at any time during the Phanerozoic.

Snowball Earth Hypothesis (I)

Many lines of evidence support a theory that the entire Earth was ice-covered for long periods 600-800 My ago. The extensive continental glaciation existed at low latitudes, possibly even at the equator near the end of Proterozoic Eon. Each glacial period lasted for millions of years and ended violently under extreme greenhouse conditions. These climate shocks triggered the evolution of multicellular animal life, and challenge long-held assumptions regarding the limits of global change.

Snowball Earth Hypothesis (II)

A potential explanation for the rare occurrence of "snowball" events in Earth history is an unusual continental configuration. Paleomagnetic evidence suggests that there were few if any continents at high latitudes 600-700 million years ago. When most continents are close to the Equator, the Earth is deprived of a mechanism that keeps the amount of carbon dioxide in the atmosphere above a critical level. If carbon dioxide in the atmosphere were to slowly drop over millions of years due to a slow reduction in volcanic activity, global temperatures would drop and glaciers would cover the high-latitude continents, just as ice sheets cover Antarctica and Greenland today. The ice sheets prevent chemical weathering, the process that converts carbon dioxide to carbonate, from proceedin. This stabilizes the levels of carbon dioxide in the atmosphere. But if all the continents were in the tropics, such a "safety switch" would not work, as the continents would remain ice-free even as the Earth grew colder, approaching the critical threshold for a snowball.

Regional Structures Of Continents (1)

Because the most ancient oceanic crust known to exist in the ocean dates only from the mid-Jurassic Period, the only direct evidence concerning geologic events more ancient than the mid-Jurassic comes from the continental crust.


Two kinds of structural units can be distinguished within the continental crust: The craton is a core of very ancient rock that has

attained tectonic and isostatic stability. Orogens are elongate regions of crust that have been

intensely folded and faulted during continental collision.

Only the youngest orogens are mountainous today. Ancient orogens reveal their history through the kinds of

rock they contain and the kind of deformation present.


An assemblage of cratons and ancient orogens that has reached isostatic equilibrium is called a continental shield.

Because it is a stable platform, a continental shield is covered by a thin layer of little-deformed sediments.

Figure 20.15

(Green) Four young orogens:NE: the CaledonideE: AppalachianW: CordilleranN: InnuitianEach is younger than 600 My.

Canadian Shield consists of ancient orogens (brown) and cratons older than 2 billions years (orange).


The fragmentation, drift, and welding together of pieces of continental are responsible for the five types of continental margins we know of today: Passive. Convergent. Collision. Transform fault. Accreted terrane.

Passive Continental Margins (1)

A passive continental margin occurs in the stable interior of a plate. The eastern coast of North America, for example, is in

the stable interior of the North American Plate.

Passive continental margins develop when a new ocean basin forms by the rifting of continental crust (for example, the Red Sea).

Figure 20.16

The rifting of continental crust formed a ocean basin bounded by passive continental margin.

Figure 20.17


Passive continental margins are places where a great thickness of sediment accumulates.

This accumulation apparently occurs in the following manner: Basaltic magma, associated with formation of the new

spreading edge, splits the continent. Heating expands the lithosphere so that a plateau

forms as the lithosphere expands, with an elevation of as much as 2.5 km above sea level.


Tensional forces cause normal faults and form a rift with pronounced topographic relief between the plateau and rift.

Before the rift floor sinks low enough for sea water to enter, clastic nonmarine sediments shed from the steep valley walls accumulate in the rift.

Associated with these sediments are basaltic lavas, dikes, and sills that form by magma rising up the normal faults.

As the rift widens, a point is reached where seawater enters. The high rate of evaporation results in the deposition of a

strata of evaporate salts laid down on top of the clastic nonmarine sediments


The formation of three-armed rifts with one of the arms not developing into an ocean is a characteristic feature of passive continental margins.

The Gulf of Aden, the Red Sea, and the northern end of the African Rift Valley meet at angles of 1200. Such a meeting point formed by three spreading

edges is called a plate triple junction.

Figure 20.18Figure 20.17

Spreading is very slow

Triple junction

SW Indian Ridge

Pangaea

Continental Convergent Margins (1)

At a continental convergent margin, the edge of a continent coincides with a convergent plate margin in which oceanic lithosphere is being subducted beneath continental lithosphere.

Subduction produces intense deformation of a continental margin.


The Andean coast of South America is an example. The Nazca Plate (capped by oceanic crust) is being

subducted beneath the South American Plate. Wet partial melting in the mantle activated by water

released by the subducted Nazca Plate produced the andesitic magma that formed the Andes ( a continental volcanic arc).


Sediment subjected to deformation in such a setting forms a mélange (see slide 42 or Figure 20.10). Sediments subjected to high-pressure and low-

temperature metamorphism. Adjacent and parallel to the belt of mélange, the crust

beneath the continental volcanic arc is metamorphosed, but here the metamorphism is regional. One distinctive feature of a continental convergent margin, therefore, is a pair of parallel metamorphic belts.

Figure 20.19

An example of the continental volcanic arc is the Cascade Range. Each volcano has been active during the last 2 Myr. Magma to form the volcano comes from wet partial melting above the oceanic crust of the Juan de Fuca Plate subducting beneath the North American Plate. Where the volcanoes have been eroded, granitic batholiths, the remnants of the magma chambers that once fed the vocalnoes can be oberved

Continental Collision Margins (1)

The edges of two continents, each on a different plate, come into collision at a continental collision margin. This results in intense folding and thrust faulting.

The Himalayan Mountain chain.

Within a fold-and-thrust mountain system, strata are compressed, faulted, folded, and crumpled, commonly in an exceedingly complex manner.

Figure 20.20

Collision between the Australian-Indian Plate and the Eurasian Plate is an example of continental collision margins. The downgoing plate of lithosphere capped by oceanic crust breaks off and continues to sink, but the edge of the remaining segment of plate capped by buoyant continental crust is partly thrust under the edge of the overriding plate, causing further elevation of the collision zone.

Continental Collision Margins (2)

Metamorphism and igneous activity are always present. The Alps, the Himalayas, and the Carpathians are all

young fold-and thrust mountain systems formed during the Mesozoic and Cenozoic eras.

The Appalachians and the Urals are older, Paleozoic-aged, examples.

From www.geology.ucdavis.edu

http://www.geology.ucdavis.edu/~gel100/F05_cowgill/Lec/19Tibet/Lec19.html

Continental Collision Margins (3) -- Fold-And Thrust Mountain System

These systems develop from piles of sedimentary strata commonly 15,000 m or more in thickness. Sediments are predominantly marine, accumulated

along passive continental margins. The American geologist J.D. Dana coined the

term geosyncline to describe a great trough that received thick deposits of sediment during slow subsidence through long geologic periods.

One distinctive feature of mountain systems formed by collision is that an ocean disappears and a new mountain system lies in the interior of a major landmass.

Transform Fault Margins

A transform fault continental margin occurs when the margin of a continent coincides with a transform fault boundary of a plate. The San Andreas Fault.

The San Andreas Fault apparently arose when the westward-moving North American continent overrode part of the East Pacific Rise. The San Andreas is the transform fault that connects

the two remaining segments of the old spreading center.

Figure 20.21

San Andreas Fault (SAF) is an example of the transform fault continental margins. 20 My ago, the edge of N. America overrode the Farallon Plate creating two small plates, the Cocos and Juan de Fuca Plate. The SAF is the transform fault connects the two segments of East Pacific Rise.

Accreted Terrane Margins (1)

An accreted terrane continental margin is a former convergent or transform fault margin that has been further modified by the addition of rafted-in, exotic fragments of crust such as island arcs.

The northwestern margin of North America, from central California to Alaska, is an example of an accreted terrane margin.


Eventually, any fragment not consumed by subduction is added (accreted) to a larger continental mass.

In the western Pacific Ocean, there are several examples: Taiwan. The Philippine Islands. The many islands of Indonesia.


Each fragment, called a terrane, is a geologic entity characterized by a distinctive stratigraphic sequence and structural history.

An accreted terrane is always fault-bounded and differs in its geologic features from adjacent terranes (also called suspect terranes).

Some terranes have moved 5000 km or more.

Figure 20.22

The western margin of N. America is a jumble of terranes accreted during the Mesozoic and Cenozoic eras.

Mountain Building

Today’s fold-and-thrust ranges are the orogens that formed during the last few hundred million years.

Examples include: The Appalachians. The Alps. The Canadian Rockies.

The Appalachians (1)

The Appalachians are a Paleozoic fold-and-thrust mountain system 2500 km long, that borders the eastern and southeastern coasts of North America.

They contain mud cracks, ripple marks, fossils of shallow-water organisms, and, in places, fresh water materials such as coal.

The sedimentary strata thicken from west to east.

Figure 20.23

Figure 20.24


Many of Pennsylvania’s oil pools were found in these gently folded strata.

In the region known as the Valley Ridge Province, the strata have been bent into broad anticlines and synclines.

In the west, the strata are nearly flat lying, but as one moves further east the strata dip more steeply to the east.

Figure 20.25


The surface along which movement occurred is known as a detachment surface, and the slice that moved is commonly referred to by its French name, decollement. The style of deformation above the detachment

surface is usually different from that below. The brittle sedimentary strata above the

detachement surface has been fractured, moved along thrust faults, and stacked like a series of thin cards, while the older rocks below tend to have resisted faulting and large-scale translation and to have been deformed by ductile deformation.

Figure 20.26

A sequence of subduction, collision, thrusting, and accretion produced the present-day structure. Iapetus ocean disappeared ~350 Myr ago when Africa collided with N. America.

The Alps (1)

The Alps and associated mountain ranges in southern Europe were formed during the Mesozoic and Cenozoic eras, as a consequence of a collision between the European and African plates.

The Jura Mountains, which mark the northwestern edge of the Alps, have the same folded form and origin as the Valley and Ridge Province in North America.

The Alps (2)

The Jura Mountains were formed from shallow-water sediments deposited on an ancient continental shelf.

In the high Alps, thrusting appears to have developed on a much grander scale than in the Appalachians.

The high Alps are composed of deeper-water marine strata.

Figure 20.27 Figure 20.28

The Canadian Rockies

In the Canadian Rockies, a central zone has been intensely metamorphosed.

The thrust sheets in the Canadian Rockies moved eastward away from the core zone.

Each sedimentary unit becomes thinner from west to east.

Revisiting Plate Tectonics And The Earth System (1)

For more than 2000 years, Arab traders sailed to India during the hot summer months, because summer winds blow from the west. They sailed back while winter winds blow from the east.

The Arabic name for this seasonal reversal of wind and weather is mausim, from which we derive our word monsoon.


The monsoon winds of India and Southeast Asia are a consequence of the collision between the Eurasian Plate and the Australian-Indian Plate.

The high mountains and plateau divert the normal flow of westerly winds.

Figure 20.31

Summer heating of high plateau causes warm air to rise and form a low pressure region draws moist air inland

In winter, the snow-covered plateau reflects solar radiation and cools down, creating a high pressure region from which dry cold air flows outward and downward.


Chinese geologists have discovered that these high lands are rather recent topographic features. They base their conclusion in part on evidence of

plant fossils of the Pliocene Epoch (5.3 to 1.6 million years ago), collected at altitudes of 4000 to 6000 m, that include many subtropical forms that today exist only at altitudes below 2000 m;


Sediment cores from the northern Indian Ocean show rates of sedimentation until about 10 million years ago. Two peaks in sediment supply (9 to 6 and 4 to 2 million years

ago) are evidence of major intervals of Himalayan uplift.

Some evidence suggests that monsoon winds started only during the second major phase of uplift. About 8 Myr ago, fine dust eroded by strong winds from desert

basins north of the Tibetan Plateau began accumulating in central China. An increase in the rate of dust accumulation occurred during 3.5 to 2.5 Myr ago. The onset of this vigorous wind erosion coincided with the beginning of the East Asian monsoon climate system.

Documents

Chapter 20: The Earth Through Time. Introduction: Tracking Past Plate Motions (1) The evidence in support of plate motions comes from measurements made