Chapter 4- Plate Tectonics (Week 2)

Question 1

What observation led Alfred Wegener to propose the idea of continental drift?

Answer 1

In 1911, Wegener discovered a scientific publication describing matching Permian-aged terrestrial fossils in South America, Africa, India, Antarctica, and Australia. He concluded that the continents must have been joined in the past, allowing the organisms to move from one continent to another

Wegener proposed that the continents were once joined in a supercontinent called Pangea, allowing the organisms to move freely across land. He termed the process of continents moving and reconfiguring themselves as “continental drift.”

Question 2

What does the term “Pangea” mean, and why did Wegener use it?

Answer 2

The term “Pangea” means “all land.” Wegener used this term to describe his vision of a supercontinent comprising all present-day continents.

Question 3

How did Wegener support his idea of continental drift?

Answer 3

He relied on matching geological patterns across oceans, such as sedimentary strata, coalfields, mountain structures, and rock types.

evidence of the Karoo Glaciation from South America, Africa, India, Antarctica, and Australia, suggesting that these continents were once connected as a single supercontinent.

Question 4

How did Wegener suggest the continents moved, and what criticism did he face?

Answer 4

Wegener proposed that continents were like icebergs floating on the heavier ocean crust, moved by Earth’s rotation and tidal forces. However, the main criticism was that he couldn’t explain how continents could move, given the prevailing view of Earth’s crust as continuous.

Wegener first published his ideas in 1912 and revised them up to 1929. The main criticism was the lack of a plausible mechanism for continental movement within the continuous crust model.

His ideas were tentatively accepted by a small minority and firmly rejected by most. However, within a few decades, plate tectonics emerged, validating many of Wegener’s ideas

Question 5

What was the prevailing view on the origin of mountain chains at the beginning of the 20th century?

Answer 5

At the start of the 20th century, one prevailing view on the origin of mountain chains was contractionism, suggesting that Earth, slowly cooling, was also shrinking, leading to the formation of mountains as the Earth’s crust wrinkled.

Question 6

What issues did the contractionism theory face?

Answer 6

The contractionism theory faced challenges such as Earth not cooling fast enough for the required amount of shrinking and the principle of isostasy, which prevented blocks of continental crust from sinking as needed for oceans to form.

The alternative view was permanentism, suggesting that continents and oceans have always been generally the same. The geosyncline theory proposed that geosynclines, thick deposits of sediments, could develop into fold-belt mountains through compression.

The idea that geosynclines develop into fold-belt mountains was first proposed by James Hall and later tested by Philip Kuenen in 1937 using layers of paraffin wax, causing layers within a geosyncline to fold up.

*image 7

Question 7

What was the main problem with the geosynclinal hypothesis for mountain building?

Answer 7

The geosynclinal hypothesis lacked an adequate explanation for the lateral forces required to cause compression. While an experiment by Kuenen used pistons to compress layers, the main force in nature was suggested to be gravity pulling the geosyncline downward, drawing the sides together as it folded.

Question 8

What challenge did proponents of the geosyncline theory face in explaining intercontinental terrestrial fossil matchups?

Answer 8

Proponents of the geosyncline theory struggled to explain intercontinental terrestrial fossil matchups. The proposed explanation was the existence of land bridges that once linked continents, allowing animals and plants to migrate back and forth.

Question 9

What is paleomagnetism?

Answer 9

Paleomagnetism is the study of the record of Earth’s magnetic field in rocks through time. Rocks, especially those with magnetic minerals like magnetite, can retain a remnant magnetism aligned with the Earth’s magnetic field when they form.

Question 10

How does paleomagnetism work in rocks?

Answer 10

Magnetic minerals in rocks, such as magnetite, become aligned with the Earth’s magnetic field during the rock’s formation. This alignment is locked in place as the rock cools, creating a remnant magnetism. By studying the horizontal and vertical components of this remnant magnetism, one can determine the direction to magnetic north and the latitude where the rock formed.

The vertical component of remnant magnetism points more sharply downward the closer it is to the magnetic north pole. By analyzing this component, researchers can determine the latitude where the rock formed relative to magnetic north.

Question 11

What are apparent polar wandering paths (APWP)?

Answer 11

Apparent polar wandering paths are records of the apparent movement of the magnetic poles over time based on the remnant magnetism of rocks. In the early 1950s, geologists observed different magnetic pole positions for rocks of different ages in the same area, assuming Earth’s magnetic pole had shifted significantly. However, it was later realized that the paths were not true records of pole movement but rather reflections of continental drift.

Apparent polar wandering paths provided the first new evidence supporting continental drift in the 1950s. While not immediately convincing for all geologists, these paths indicated that continents had different magnetic pole positions in the past, supporting the idea that continents were not fixed but had moved over time.

Question 12

What advancements occurred in understanding ocean basin geology and geography during the 20th century?

Answer 12

Before 1900, knowledge about ocean basin bathymetry and geology was limited. By the end of the 1960s, significant progress was made, leading to detailed maps of ocean floor topography, insights into ocean floor sediment and solid rock geology, and a comprehensive understanding of the geophysical characteristics of ocean rocks, rivaling the knowledge of continental rocks.

Question 13

What are some important physical features of the ocean floor identified through bathymetric data?

Answer 13

Key ocean floor features include extensive linear ridges (at 2,000 to 3,000 m depths), fracture zones perpendicular to ridges, deep-ocean plains (4,000 to 5,000 m depths), relatively flat continental shelves (depths under 500 m), deep trenches (up to 11,000 m deep, mostly near continents), and seamounts and chains of seamounts.

Question 14

What is seismic reflection sounding, and how does it contribute to understanding the ocean floor?

Answer 14

Seismic reflection sounding involves transmitting high-energy sound bursts and measuring the echoes with geophones towed behind a ship. This technique, more advanced than acoustic sounding, allows mapping of the bedrock topography, crustal thickness, and sediment thickness. Seismic studies revealed that ocean sediments are thin or absent along ocean ridges and provided insights into the oceanic crust’s composition, which is mainly basalt.

Question 15

What did Bullard and his colleagues find regarding heat-flow rates along the ocean floor?

Answer 15

Edward Bullard developed a heat flow probe in the early 1950s.

They found higher than average heat-flow rates along the ridges and lower than average rates in trenches.

The data were interpreted as evidence of mantle convection, where areas of high heat flow correspond to upward convection of hot mantle material, and areas of low heat flow correspond to downward convection.

The data suggest that the observed higher heat-flow rates along ridges and lower rates in trenches provide evidence for mantle convection, with upwelling and downwelling of hot mantle material, respectively.

Question 16

What became possible with the advancements in seismographic networks?

Answer 16

It became possible to plot the locations and depths of both major and minor earthquakes with great accuracy.

There was a remarkable correspondence observed between earthquake locations and both the mid-ocean ridges and deep ocean trenches.

Question 17

What did Gutenberg and Richter show in 1954 regarding ocean-ridge earthquakes?

Answer 17

They showed that ocean-ridge earthquakes were all relatively shallow.

Question 18

What did Benioff show in the 1930s regarding earthquakes near ocean trenches?

Answer 18

Benioff, in the 1930s, demonstrated that earthquakes in the vicinity of ocean trenches were both shallow and deep. The deeper ones were situated progressively farther inland from the trenches.

Question 19

What did Harold Hess propose in 1960, and what elements of plate tectonics did it include?

Answer 19

In 1960, Harold Hess proposed a hypothesis with elements now accepted as plate tectonics. He suggested that new sea floor is generated at ocean ridges and that old sea floor is dragged down at ocean trenches, driven by mantle convection currents. He also proposed that continental crust does not descend into trenches but colliding landmasses are thrust up to form mountains.

*image 9

Question 20

Who provided the first understanding of magnetic stripes, and what was their interpretation?

Answer 20

Fred Vine suggested that magnetic stripes were related to magnetic reversals, with symmetrical patterns on either side of ridges.

Question 21

What did researchers studying magnetic reversals discover?

Answer 21

Earth’s magnetic field periodically weakens, becomes non-existent, and re-establishes with reverse polarity during reversals.

Question 22

What did the Vine-Matthews-Morley (VMM) hypothesis propose?

Answer 22

It suggested a link between magnetic patterns at ridges and magnetic reversals, with positive anomalies during normal events and negative anomalies during reversed events.

Question 23

In 1963, who proposed the idea of a mantle plume or hot spot, and what evidence did they cite?

Answer 23

J. Tuzo Wilson proposed the idea based on the distribution of Hawai’ian and Emperor Seamount island chains, where volcanic rock becomes progressively younger toward the southeast.

*image 10

Question 24

What is a mantle plume?

Answer 24

A mantle plume is a stationary, semi-permanent upwelling of hot mantle material beneath Earth’s surface.

Question 25

How does the Hawaiian volcanic chain relate to mantle plumes?

Answer 25

J. Tuzo Wilson proposed that the Hawaiian volcanism results from a mantle plume. The Pacific Plate moves northwest over the stationary plume, forming the Hawaiian Islands. The change in direction of the volcanic chain near Midway Islands is attributed to the Pacific Plate's movement over the plume. Mantle plumes, like the one at Yellowstone and the Anahim Volcanic Belt, are long-lived phenomena, lasting for tens of millions to possibly hundreds of millions of years.

Question 26

Where are most mantle plumes found, and can they be under continents?

Answer 26

Most mantle plumes are within ocean basins (e.g., Hawaii, Iceland, Galapagos Islands), but some are found under continents (e.g., Yellowstone hot spot in the U.S., Anahim Volcanic Belt in central British Columbia).

Question 27

How long do mantle plumes typically last, and can they endure for hundreds of millions of years?

Answer 27

Mantle plumes are very long-lived, lasting for at least tens of millions of years, and possibly for hundreds of millions of years in some cases. *image 11

Question 28

What are transform faults associated with oceanic spreading ridges?

Answer 28

Transform faults are faults perpendicular to oceanic spreading ridges, composed of a series of straight-line segments. They offset the ridge at intervals.

Question 29

Who introduced the concept of transform faults and the idea that Earth's crust is divided into rigid plates?

Answer 29

Tuzo Wilson introduced the term "transform faults" in 1965, describing faults associated with oceanic spreading ridges. He also proposed the concept of Earth's crust being divided into rigid plates, coining the term "plate tectonics." *Image 12

Question 30

When did the ideas of continental drift and sea-floor spreading become widely accepted?

Answer 30

By 1965, the ideas of continental drift and sea-floor spreading were widely accepted, leading more geologists to think in these terms.

Question 31

By the end of 1967, how had Earth's surface been mapped?

Answer 31

Earth's surface had been mapped into a series of plates, including major plates like Eurasian, Pacific, Indian, Australian, North American, South American, African, and Antarctic plates, as well as numerous smaller plates and sub-plates.

Question 32

How can plate motions be tracked, and what are the rates of motion for major plates?

Answer 32

Plate motions can be tracked using Global Positioning System (GPS) data. Rates of motion for major plates range from less than 1 cm/year to more than 10 cm/year. The Pacific Plate is the fastest, followed by the Australian and Nazca Plates.

Question 33

What explains the variation in motion rates within a single plate, such as the North American Plate?

Answer 33

Plates move as rigid bodies, but they also rotate. For example, the North American Plate rotates counter-clockwise, leading to different rates of motion in different places.

Question 34

What are the three types of plate boundaries, and what materials make up the plates?

Answer 34

Plate boundaries are divergent (moving apart), convergent (moving together), and transform (moving side by side). The plates are made up of crust and lithospheric mantle.

Question 35

How do plates move along their boundaries, and why is there never a significant amount of space between them?

Answer 35

Plates move along the lithosphere-asthenosphere boundary because the asthenosphere is relatively weak and deforms as the plates move, preventing significant space between them. *image 13

Question 36

Why is the lithospheric mantle relatively thin at spreading centers?

Answer 36

At spreading centers, the upward convective motion of hot mantle material generates temperatures too high for the existence of a significant thickness of rigid lithosphere, causing plates to fall away from each other.

Question 37

Why can a single plate include both oceanic and continental crust?

Answer 37

Plates can include both crustal material and lithospheric mantle material, allowing a single plate to encompass both oceanic and continental crust.

Question 38

Can you provide examples of plates including both oceanic and continental crust?

Answer 38

Yes, examples include the North American Plate, which includes most of North America and half of the northern Atlantic Ocean; the South American Plate, extending across the western part of the southern Atlantic Ocean; and the European and African plates, each including part of the eastern Atlantic Ocean. The Pacific Plate is almost entirely oceanic but includes the part of California west of the San Andreas Fault.

Question 39

What characterizes divergent boundaries?

Answer 39

Divergent boundaries are spreading boundaries where new oceanic crust is created from magma derived from partial melting of the mantle.

Question 40

How does partial melting occur in the mantle at divergent boundaries?

Answer 40

Partial melting occurs when hot mantle rock is moved from deep within Earth, where pressures are too high for it to be liquid, to shallower depths where the pressure is much lower.

Question 41

What is the thickness of the triangular zone of partial melting near the ridge crest, and what is the proportion of magma?

Answer 41

The triangular zone is approximately 60 km thick, and the proportion of magma is about 10% of the rock volume, resulting in crust about 6 km thick.

Question 42

What is the character of the crust created at divergent boundaries, and where are most divergent boundaries located?

Answer 42

The crust created is always oceanic in character, composed of mafic igneous rock (basalt or gabbro, rich in iron and magnesium). Most divergent boundaries are located in the oceans.

Question 43

What are some processes occurring at divergent boundaries?

Answer 43

Processes include melted rock (magma) from the mantle rising to fill voids left by plate divergence, the formation of pillow lavas where melted rock emerges on the ocean floor and is cooled by seawater, intrusion of vertical sheeted dykes into cracks resulting from spreading, and the slower cooling of magma in the lower part of the new crust, forming bodies of gabbro. *image 14

Question 44

How is spreading thought to start at divergent boundaries?

Answer 44

Spreading is thought to start with lithosphere being warped upward into a dome by buoyant material from an underlying mantle plume or series of mantle plumes.

Question 45

What causes the dome to fracture in a radial pattern at divergent boundaries?

Answer 45

The buoyancy of the mantle plume causes the dome to fracture in a radial pattern, with three arms spaced at approximately 120°. *image 15

Question 46

How can a rift valley form, and what may it evolve into?

Answer 46

When a series of mantle plumes exists beneath a large continent, the resulting rifts may align, leading to the formation of a rift valley. This type of valley may eventually develop into a linear sea (e.g., the present-day Red Sea) and finally into an ocean (e.g., the Atlantic).

Question 47

What example is provided for the formation of a rift valley, linear sea, and ocean?

Answer 47

As many as 20 mantle plumes, some still existing, were likely responsible for initiating the rifting of Pangea along what is now the mid-Atlantic ridge, leading to the formation of the Atlantic Ocean.

Question 48

What characterizes convergent boundaries, and how many types are there?

Answer 48

Convergent boundaries involve two plates moving toward each other. There are three types based on the type of crust on either side: ocean-ocean, ocean-continent, and continent-continent.

Question 49

What happens at an ocean-ocean convergent boundary?

Answer 49

At an ocean-ocean convergent boundary, a plate margin consisting of oceanic crust and lithospheric mantle is subducted beneath the margin of the colliding plate. *image 16

Question 50

Which plate is often the one that subducts, and why?

Answer 50

Often, it is the older and colder plate that is denser and subducts beneath the younger and hotter plate.

Question 51

What commonly forms along ocean-ocean convergent boundaries, and why?

Answer 51

Ocean trenches commonly form along these boundaries.

Question 52

How does water play a role in the subduction process at ocean-ocean convergent boundaries?

Answer 52

As the subducting crust is heated and pressure increases, water is released from within the subducting material, primarily from alteration of minerals like pyroxene and olivine to serpentine. This water mixes with the overlying mantle, lowering the melting point of mantle rocks, leading to flux melting or fluid-induced melting.

Question 53

What does the newly produced magma create, and what is the resulting geological feature?

Answer 53

The newly produced magma rises through the mantle and sometimes through the overlying oceanic crust to the ocean floor, creating a chain of volcanic islands known as an island arc.

Question 54

How does a mature island arc develop, and can you provide examples of ocean-ocean convergent zones?

Answer 54

A mature island arc develops into a chain of relatively large islands (e.g., Japan or Indonesia) as more volcanic material is extruded, and sedimentary rocks accumulate around the islands. Examples of ocean-ocean convergent zones include the subduction of the Pacific Plate south of Alaska (Aleutian Islands), west of the Philippines, subduction of the Indian Plate south of Indonesia, and subduction of the Atlantic Plate beneath the Caribbean Plate.

Question 55

What occurs at an ocean-continent convergent boundary?

Answer 55

At an ocean-continent convergent boundary, the oceanic plate is subducted beneath the continental plate, similar to what happens at an ocean-ocean boundary. *image 17

Question 56

What geological features are formed at an ocean-continent convergent boundary?

Answer 56

Rocks and sediment on the continental slope are thrust up into an accretionary wedge, and compression leads to faults forming within the continental plate.

Question 57

How does mafic magma contribute to the geological features at ocean-continent convergent boundaries?

Answer 57

Mafic magma produced adjacent to the subduction zone rises to the base of the continental crust, leading to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes.

Question 58

Can you provide examples of ocean-continent convergent boundaries and the resulting geological formations?

Answer 58

Examples include the subduction of the Nazca Plate under South America, creating the Andes Range, and subduction of the Juan de Fuca Plate under North America, forming the Cascade Range with mountains like Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta.

Question 59

What happens in a continent-continent collision at a convergent boundary?

Answer 59

A continent-continent collision occurs when a continent or large island, along with subducting oceanic crust, collides with another continent. The colliding continental material is not subducted due to its lower density. *image 18

Question 60

What are the consequences of a continent-continent collision in terms of geological features?

Answer 60

There is tremendous deformation of pre-existing continental rocks, resulting in the creation of mountains from that rock. Mountains also form from any sediments that had accumulated along the shores of both continental masses, as well as from some ocean crust and upper mantle material.

Question 61

Can you provide examples of continent-continent convergent boundaries and the resulting geological formations?

Answer 61

Examples include the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating a series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.

Question 62

What may happen when a subduction zone is jammed shut by a continent-continent collision?

Answer 62

When a subduction zone is jammed shut by a continent-continent collision, plate tectonic stresses that are still present can sometimes cause a new subduction zone to develop outboard of the colliding plate.

Question 63

What characterizes transform boundaries?

Answer 63

Transform boundaries exist where one plate slides past another without producing or destroying crust, except in cases with bends and jogs, where collisions and divergence may occur on a small scale.

Question 64

What is the special case where crust may be produced or destroyed at transform boundaries?

Answer 64

Crust may be produced or destroyed at transform boundaries in the special case where the boundary has bends and jogs.

Question 65

What typically connects segments of mid-ocean ridges at transform boundaries?

Answer 65

Most transform faults connect segments of mid-ocean ridges, making them primarily ocean-ocean plate boundaries.

Question 66

Can transform faults connect continental parts of plates, and can you provide an example?

Answer 66

Yes, some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the southern end of the Juan de Fuca Ridge with the northern end of the East Pacific Rise in the Gulf of California. *image 19

Question 67

Do transform faults only connect divergent boundaries?

Answer 67

No, transform faults do not only connect divergent boundaries. An example is the Queen Charlotte Fault, which connects the north end of the Juan de Fuca Ridge to the Aleutian subduction zone.

Question 68

What was the name of the supercontinent that existed before the present continents, and who named it?

Answer 68

The supercontinent that existed before the present continents was named Pangea by Alfred Wegener.

Question 69

When did Pangea begin to rift apart, and what significant events occurred during this process?

Answer 69

Pangea began to rift apart around 200 million years ago (Ma). During this process, the Atlantic Ocean started to open up between northern Africa and North America, and India broke away from Antarctica.

Question 70

What is a Wilson cycle, and who proposed this concept in 1966?

Answer 70

A Wilson cycle is an on-going cycle where continents break up, drift apart, collide again, and form a new continent. Tuzo Wilson proposed this concept in 1966.

Question 71

What are the names of some supercontinents that preceded Pangea?

Answer 71

Some supercontinents that preceded Pangea include Pannotia (600 to 540 Ma) and Rodinia (1,100 to 750 Ma). *image 20

Question 72

What is the current stage of the Wilson cycle, and what continents are mentioned as part of this stage?

Answer 72

At present, we are in the stages of a Wilson cycle where fragments are drifting and changing their configuration. North and South America, Europe, and Africa are moving with their respective portions of the Atlantic Ocean.

Question 73

Why is the Atlantic Ocean slowly getting bigger, and what may happen in the future?

Answer 73

The Atlantic Ocean is slowly getting bigger because the oceanic crust formed by spreading along the mid-Atlantic ridge is not currently being subducted (except in the Caribbean). In the future, as the Atlantic Ocean floor gets weighed down by continental sediments, it may break away from the continental lithosphere and begin to subduct. *image 21

Question 74

What will happen once a subduction zone develops and the oceanic plate begins to descend under the continent?

Answer 74

Once a subduction zone develops, and the oceanic plate begins to descend under the continent, the continents will no longer continue to move apart because spreading at the mid-Atlantic ridge will be taken up by subduction.

Question 75

What may occur if spreading along the mid-Atlantic ridge is slower than spreading within the Pacific Ocean?

Answer 75

If spreading along the mid-Atlantic ridge is slower than spreading within the Pacific Ocean, the Atlantic Ocean may start to close up, and eventually, North and South America will collide again with Europe and Africa, potentially leading to the formation of a new supercontinent.

Question 76

What evidence supports the idea that the Atlantic Ocean may have closed up before, leading to the formation of a supercontinent?

Answer 76

There is strong evidence around the margins of the Atlantic Ocean, such as roots of ancient mountain belts along the eastern margin of North America, the western margin of Europe, and the north-western margin of Africa, which suggest that these landmasses once collided with each other to form a mountain chain. The mountain chain might have been as big as the Himalayas.

Question 77

What is the approximate line of collision between the continents, and what regions does it run through?

Answer 77

The apparent line of collision runs between Norway and Sweden, between Scotland and England, through Ireland, through Newfoundland and the Maritimes, through the north-eastern and eastern states, and across the northern end of Florida.

Question 78

Why can some mountain chains formed during earlier collisions be traced from Europe to North America and from Europe to Africa?

Answer 78

When rifting of Pangea started at approximately 200 million years ago, the fissuring was along a different line from the line of the earlier collision. This is why some mountain chains formed during the earlier collision can be traced from Europe to North America and from Europe to Africa.

Question 79

Why might the Atlantic Ocean rift have occurred in approximately the same place during two separate events several hundred million years apart?

Answer 79

The series of hot spots identified in the Atlantic Ocean may have existed for several hundred million years, contributing to rifting in roughly the same place on at least two separate occasions. *image 22

Question 80

What is the role of mantle convection in plate tectonics, and what are the debated forces that make the plates move?

Answer 80

Mantle convection is considered critical to plate tectonics. The debated forces for plate motion include traction caused by mantle convection and the ridge-push and slab-pull mechanisms. *image 23

Question 81

What is the argument for the traction model in plate motion, and what does it propose?

Answer 81

The traction model suggests that plates are moved by the traction caused by mantle convection, and friction between the asthenosphere and lithosphere pulls the lithosphere along as the mantle convects.

Question 82

What are ridge-push and slab-pull, and why are they considered important in plate motion?

Answer 82

Ridge-push refers to gravity causing lithosphere to slide downhill away from elevated mid-ocean ridges. Slab-pull involves the weight of subducting slabs dragging the rest of the plate down into the mantle. These mechanisms are considered important in plate motion.

Question 83

What are some compelling arguments in favor of the ridge-push/slab-pull model, as listed by Kearey and Vine (1996)?

Answer 83

Plates attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move faster than plates that are not attached. The traction model would require the mantle to move about five times faster than the plates, which is not supported by geophysical models. Plate velocity is not related to plate area, contradicting the potential for higher convection traction in larger plates.

Question 84

Why is the coupling between the partially liquid asthenosphere and the plates not considered strong in the traction model?

Answer 84

In the traction model, the coupling between the partially liquid asthenosphere and the plates is not considered strong, requiring the mantle to move about five times faster than the plates, which is not supported by geophysical models.

Question 85

How does the velocity of plates relate to the presence of subducting slabs according to the ridge-push/slab-pull model?

Answer 85

Plates attached to subducting slabs move faster, while plates without subducting slabs move significantly slower, supporting the ridge-push/slab-pull model.