Lecture 4: Plate Tectonics: Mechanisms and Margins
Reading: Chapter 2, pages 42-61
Topics:
· Divergent Plate Boundaries
· Convergent Plate Boundaries
· Transform Plate Boundaries
· Hot Spots and Hot Spot Tracks
· Plate Tectonics and the Building of Continents
· Continental Rifting and the Development of Ocean Basins
· Tectonic Settings
· Significance of Plate Tectonics
Next lecture: Mineral Composition and Properties
4.1 Plate Boundaries
The interior of tectonic plates are usually geologically stable. The main geologic processes operating in the middle of plates are erosion and sedimentation. However, along plate boundaries dynamic interactions between plates are responsible for volcanic activity, faulting and earthquakes; the building of mountain ranges, the creation of new crust, and the recycling of old crust.
There are three types of plate boundaries:
1. Divergent plate boundaries occur where plates are being pulled apart.
2. Convergent plate boundaries occur where plates are moving together or colliding.
3. Transform plate boundaries occur where two plates are sliding past each other in different directions.
4.2. Divergent Plate Boundaries
New oceanic crust is created at divergent plate boundaries as two plates pull away from each other. Divergent plate boundaries are associated with extensional stress, shallow earthquakes, and basaltic volcanism. Examples include the mid-ocean ridges such as the Mid-Atlantic Ridge.
At divergent plate boundaries, extensional stresses pull two plates apart, allowing magma to rise from the asthenosphere towards the surface and create new oceanic crust. The extension is accommodated by normal faults in the crust, creating a rift valley.
The Mid-Atlantic ridge rises above sea level in Iceland, allowing easy observation of its features.
The central rift valley of the Mid-Atlantic Ridge in southwest Iceland. The photographer is standing at the edge of the North American Plate, looking across the rift valley towards the Eurasian Plate. The broad mountains in the distance are shield volcanoes, the characteristic style of volcanism in a divergent tectonic setting.
Where a continent is being pulled apart a new divergent boundary called a continental rift valley is created. The East African Rift Valley is an example of a continental rift. Continental rift valleys represent the earliest stage in the development of a new ocean basin as a single continental plate splits into two fragments as new oceanic crust grows between them.
Development of a rift in continental crust by mantle upwelling
Various stages of rifting are present in East Africa. The East African Rift Valley represents a very young rift not yet flooded by seawater. The Red Sea is a more advanced rift that is continuing to grow into a new ocean basin by seafloor spreading.
4.3 Convergent Plate Boundaries
Convergent plate boundaries occur when two plates collide with each other. There are three types of convergent boundaries:
1. Ocean-Ocean: collision between two plates composed of oceanic crust; older denser oceanic crust is subducted and destroyed.
2. Ocean-Continent: collision between oceanic and continental crust; oceanic crust is subducted by the continent and destroyed.
3. Continent-Continent: collision between two plates composed of continental crust; both are too light to be subducted.
4.3.1. Ocean-Ocean Convergence
In a collision between two plates of oceanic crust, the older and denser crust gets subducted beneath the younger and lighter crust. A deep-sea trench marks the subduction zone at the surface. As subducted slab descends into the mantle, it partially melts. The rising magma produces a chain of volcanoes on the overriding, younger oceanic plate called a volcanic island arc.
A convergent plate boundary between two oceanic plates. The older, denser plate is subducted by the younger, more buoyant plate. A trench marks the subduction zone at the surface. As the subducted plate descends into the mantle and melts, rising magma creates a volcanic island arc.
The subduction of the Pacific Plate by the Philippine plate is an example of an ocean-ocean convergent boundary. The subduction zone is marked by the Mariana Trench. Challenger Deep, its deepest point, is 10,911 meters (35,798 ft) deep and is the deepest place in the ocean.
4.3.2. Ocean-Continent Convergence
When a plate composed of oceanic crust collides with one made of continental crust, the denser oceanic crust is subducted beneath the lighter continental crust. As in the case of ocean-ocean subduction, there is a trench and volcanic arc, with the volcanic arc on the continent. Compression crumples the edge of the continent, producing mountains. Ocean sediments and anything else too light to subduct such as a volcanic island arc or fragment of continental crust is scraped off and added to edge of continent as accreted terranes. Both types of subduction zones (ocean-ocean and ocean-continent) are associated with compressional stress, deep earthquakes, and andesitic volcanism.
At a convergent boundary between an oceanic plate and a continental plate, the denser oceanic plate is subducted by the lighter continental plate. A volcanic arc is built on the overriding continental plate as the subducted oceanic plate melts.
4.3.3. Continent-Continent Convergence
Continental crust is too light to be subducted. So when two continents collide, thickening, thrust faulting, and uplift produces mountain ranges and high plateaus. A suture zone marks the boundary where two formerly separate continents are joined.
When two continents collide, neither is subducted. Instead, the compressional stress produces thickening and thrust faulting, which uplifts mountain ranges and high plateaus along and behind the collision zone.
The collision of India and Asia is an example of an ongoing continental collision and is raising the Himalaya and Tibetan Plateau. The collision zone between India and Asia is marked by the abrupt Himalayan front and the world’s highest mountains. Behind the collision zone, an area of thickened continental crust supports the extensive Tibetan plateau, the largest high plateau in the world.
The collision of India with Asia began about 20 million years ago when a subduction zone completely destroyed the ocean basin that formerly separated the two continents. This same process is closing the Mediterranean Sea and bringing Africa into a collision with Europe. The ancient collision between North America and Africa during the formation of Pangaea is marked today by the heavily eroded Appalachian Mountains.
4.4. Transform Plate Boundaries
A transform plate boundary occurs where two plates are sliding past each other. The boundary is marked by a system of faults. Motion between the plates is not always smooth due to friction; instead the plates periodically lurch past each other, resulting in earthquakes. Shear stresses crumple the edges of the plates and rocks on either side of the faults.
A transform plate boundary occurs where two plates slip past each other in different directions.
California’s San Andreas Fault is an excellent example of a transform plate boundary between the Pacific and North American plates. The fault marks the plate boundary and runs through the center of the photo from top to bottom. Note how the edges of the two plates are crumpled due to friction along the fault.
4.4.1 Evolution of the San Andreas Fault
As we saw in the example of India and Asia, plate boundaries do not remain static, instead they evolve over time. The San Andreas Fault is another example. The San Andreas Fault originated when the subduction zone off the west coast of North America caught up to and swallowed the seafloor spreading center that separated two oceanic plates: the Pacific Plate and the Farallon Plate. The Farallon Plate and spreading center was split into two smaller pieces connected by a transform fault that we call the San Andreas Fault.
The San Andreas Fault originated when the subduction zone off central and southern California caught up with and swallowed the offshore spreading center. This severed the oceanic Farallon Plate into two smaller oceanic plates; connected by a transform fault we call the San Andreas Fault. Seafloor spreading and subduction still occurs further north off the Pacific Northwest and further south off Central America.
4.5. Hot Spots
Volcanism is usually associated with plate boundaries. However, some volcanic centers are located in the middle of a plate. Such centers are called hot spots and have a magma source in the deep mantle, perhaps extending all the way to the core boundary. Because the magma source is below the lithosphere, it does not move with the plate. Instead a chain of volcanoes called a hot spot track is produced as the plate moves over the stationary hot spot. The Hawaiian Islands are an excellent example of a hot spot track. The big island of Hawaii is currently located over the hot spot.
The Hawaiian Islands were produced as the Pacific Plate moved northwestwards over a fixed hot spot in the deep mantle, producing a chain of volcanic islands that get younger to the southeast. All of the Hawaiian Islands are volcanic, but the volcanoes become extinct as the plate carries the island off the hot spot. The big island of Hawaii is currently over the hot spot and is the only Hawaiian island with active volcanoes.
4.5.1. Hot Spot Tracks
The hot spot responsible for the Hawaiian Islands has existed for more than 70 million years. A chain of extinct volcanic islands and underwater seamounts extends west and north from the currently active island of Hawaii all the way to the Aleutian trench. Southeast of the island of Hawaii a new volcanic island is growing beneath the sea as the big island moves away from the hot spot. Hot spot tracks are good indicators of plate motion. Note the bend in the Hawaiian hot spot track, where the Hawaiian Ridge meets the Emperor Seamounts. This bend indicates a change in the direction of the Pacific plate from northerly to northwesterly about 40 million years ago.
The Hawaiian hot spot track extends from the big island of Hawaii, which is currently over the hot spot, all the way to the western Aleutian Trench. The bend in the track of extinct volcanic islands and submerged seamounts indicates a change in the direction of the Pacific Plate about 40 million years ago.
4.5.2. Continental Hot Spot Tracks
The Yellowstone hotspot is the world’s only hot spot beneath a continent and is responsible for Yellowstone National park’s famous geysers and hot springs as well as three enormous volcanic eruptions in the last two million years. The hot spot track is marked by a chain of progressively older volcanic centers that extends southwest across Idaho’s Snake River Plain to north-central Nevada. The origin of the hot spot is unknown, but is probably related to the extensive Columbia Flood Basalts of eastern Washington and Oregon that were erupted about 18 million years ago.
The Columbia Plateau flood basalts were erupted about 18 million years ago. The flood basalts were followed by progression of volcanic calderas from the oldest in north central Nevada to the youngest in Yellowstone National Park. The flood basalts are believed to mark the first appearance of a mantle hot spot beneath the North American plate, which then produced a chain of volcanic calderas as the North American plate moves southwestwards over the hot spot. The hot spot is currently located beneath Yellowstone National Park. Note that the hot spot track does not line up with its presumed origin in the Columbia flood basalts; instead there is a pronounced offset to the south. This has not yet been satisfactorily explained, but may result from interactions in the mantle between the hot spot plume and the oceanic plate being subducted beneath North America.
It is well established that the hot spot currently beneath Yellowstone has left a track of progressively older calderas across southern Idaho to an earliest location beneath north central Nevada (16 million years ago). This is a good match for the age of the Columbia plateau flood basalts (18-16 million years old), however the caldera track does not quite line up geographically with the eruption points of the Columbia basalts. Geologists continue to debate the exact relationship between the Yellowstone hot spot and the Columbia flood basalts.
The very large volume of the Columbia basalts (several thousand cubic kilometers), their geologically short eruption time (less than 1.5 million years), and trace element compositions of the lavas themselves all suggest a deep mantle (hot spot) source. The Columbia basalts were erupted from three locations: the Chief Joseph dike swarm near Hells Canyon in Northeast Oregon; the Monument dike swarm near John Day, Oregon, and the Steens Mountain dikes south of Burns, Oregon. There is an age progression from north (older) to south (younger) within the Columbia basalts, so that also points to the hot spot (which next manifested itself as a caldera in north central Nevada).
The geography of the hot spot eruptions is likely also complicated by interactions between the rising mantle plume and the subducted oceanic plate. Remember, the subducting plate is not instantaneously destroyed at the trench, but instead slowly sinks into the mantle, gradually re-assimilating into the mantle by melting. So the oceanic plate being subducted by the Cascadia Subduction Zone extends into the mantle beneath western North America. This means that a rising mantle plume will have to get past the subducted slab. The hot spot plume also appears to be inclined, rather than exactly vertical, and may even wiggle around a bit, instead of being fixed as portrayed in textbooks. So the connection between the Yellowstone hot spot and the Columbia flood basalts is quite complicated.
What happens when a hot spot forms ?
Imagine a container filled with hot wax and oil. As the wax (deep mantle) is heated from below (Earth’s core), it becomes buoyant and begins to rise through the oil (mantle). The viscosity of the oil provides resistance, causing the rising wax to form a large glob, or head, followed by a narrow tail, or stem.
A mantle hot spot consists of leading glob, or head, followed by a narrower stem, or tail, because of the high viscosity of the mantle.
The hot spot forms a large glob (head) followed by a narrow tail (stem) because of the resistance of the mantle to getting out of its way (viscosity). When the head reaches the lithosphere, it flattens out and erupts a huge volume of magma onto the surface. This type of eruption is known as a flood basalt. As the plate moves away from the hot spot, it is dragged over the narrower stem of the hot spot. This creates a chain of smaller, but still very large, eruptions called a hot spot track.
A mantle hot spot plume has a wide head followed by a narrow stem. As the head reaches the surface, it erupts massive flood basalts. As the plate passes over the narrower stem, a hot spot track is produced leading away from the flood basalt.
The flowing sequence of figures illustrates the events marking the passage of the North American plate over the Yellowstone hot spot.
The head created the massive flood basalts that cover eastern Washington and Oregon.
The tail, or stem, has produced a chain of progressively younger volcanic calderas from north-central Nevada to the present location of the hot spot beneath northwest Wyoming.
As the calderas move away from the hot spot stem, they are buried by younger basalt lava flows fed by remnant pockets of magma in the lithosphere. This is typical of the style of volcanism on Idaho’s Snake River Plain.
The location of hotspots worldwide. The red dots mark the present location of a plate over a hot spot. The blue dots mark their earliest known location. The Hawaiian hot spot has no point of origin because its track disappears into the Aleutian subduction zone, meaning all earlier traces of the hot spot have been destroyed by subduction. Note that some hotspots first appeared at locations that have been divergent plate boundaries or mid ocean ridges, so the earliest evidence of these hotspots is now found on the coastlines of continents now separated by oceans, for example in the North Atlantic (Iceland) and South Atlantic.
4.6. Plate Tectonics and the Building of Continents
Over hundreds of millions of years the modern continents have been assembled by plate tectonics from smaller fragments of material too light to be subducted. The pieces that make up a continent include:
1. Cratons: very ancient rock that make up the cores of continents. Exactly how cratons formed is still debated.
2. Orogens: belts of deformed rocks representing mountain building episodes during collisions between continental cratons. Only the youngest orogens still stand as mountain ranges. Most orogens are eroded roots of ancient mountain ranges.
The collision of two cratons, for example India and Asia
a. Subduction of oceanic crust crumples the edge of the overriding plate and creates a volcanic arc
b. As collision begins, sediments along subducting plate are deformed and welded onto overriding plate
c. As collision ends, subducted oceanic crust breaks off and continues its descent into mantle
The two continents are now joined along a high mountain range (the orogen)
4.6.1. Continental Shields
Continental shields are assemblages of ancient cratons and their associated orogens sutured together by tectonic collisions. The North American continental shield is also called the Canadian Shield because glacial erosion has exposed the ancient rocks of the shield at the surface throughout much of central and eastern Canada, as well as the upper Midwest of the United States. Further south the shield is covered by younger sedimentary rocks. The shield is surrounded by five younger orogenic belts: the Grenville, Innuitian, Caledonian, Appalachian, and Cordilleran Orogens marking younger episodes of tectonic collision and uplift.
The North American continental shield consists of several very ancient cratons joined by belts of deformed rocks called orogens that mark collisions between cratons. Note that the different parts of the shield are all older than 1.6 billion years. The shield is surrounded by younger orogens, the Grenville, Appalachian, and Cordilleran orogenies.
4.6.2. Accreted Terranes
Accreted terranes are small pieces of crust too buoyant to be subducted that are carried along by oceanic plates and added to the edge of a continent. Accreted terranes may include old volcanic island arcs, small pieces of continental crust called micro continents, or margins of continents sliced off by transform faults. Accreted terranes can be rafted great distances by plate motions and get added to the edge of continent as the oceanic crust is subducted. The entire west coast of North America is complex jumble of more than 40 accreted terranes added to North America by 200 million years of oceanic subduction in this location. This has gradually shifted the plate boundary further west.
Accreted terranes added to the western margin of the North American plate by more than 200 million years of subduction along the west coast.
4.7. Continental Rifting and the Development of New Ocean Basins
In addition to building continents, plate motions also tear continents apart. This process is called continental rifting.
1. The process begins when crust is heated by rising magma from the mantle, causing uplift of a broad area
2. As rising magma continues to push the crust aside, the uplift collapses to form a rift valley
3. New oceanic crust is created as the rift spreads. The area is flooded to form a narrow sea such as the Red Sea
4. The narrow sea may grow to become a new ocean basin with a mid-ocean ridge and seafloor spreading center.
Rifting can end at any point, not all rifts develop into a new ocean basin.
The four stages of continental rifting and development of a new ocean basin, with modern day examples of each stage
Two active spreading centers, the Red Sea and Gulf of Aden, have rifted the Arabian Plate from the African Plate. The East Africa Rift is less active and appears to be a failing rift that does not develop into an ocean basin. The more active part of the rift extends through the Red Sea.
At the north end of the Red Sea, the direction of rifting is changing again. Instead of continuing into the Mediterranean, the active rift turns northeast and is separating Sinai and Israel from Jordan. Only the southern portion is flooded by the sea, the remainder stands as much as 1000 feet below sea level and is partly filled by the Dead Sea, a salt lake.
4.7.1. Passive Continental Margins
As a continental rift develops into a new ocean basin, the edges of the continent become a passive continental margin. A passive continental margin is a transition between oceanic and continental crust that does not coincide with an active plate boundary. Instead the plate boundary is at the mid-ocean ridge. Passive continental margins are characterized by cooling and subsidence of the crust as it moves away from rising magma at the mid-ocean ridge and the development of thick sequences of sediments eroded from the continent. The east coast of North America is an example of a passive continental margin
Passive continental margins are transitions between continental and oceanic crust without an active plate boundary. The major features of a passive continental margin are a broad continental shelf followed by the abrupt continental slope which drops to the ocean floor. A thick sequence of sediments buries the ancient faults that mark the episode of rifting. The active plate boundary is out at the active spreading center at the mid-ocean ridge.
4.7.2. Tectonics and Ocean Basins
The Atlantic Ocean is an example of a maturing ocean basin with passive continental margins. Its major topographic features such as the mid-ocean ridge, abyssal plain, and continental shelf are explained by plate tectonics. The mid-ocean ridge is the site of seafloor spreading and the active plate boundary. The abyssal plain is underlain by older oceanic crust that has moved away from the ridge, cooled, and subsided. The abyssal plains end abruptly at the continental slope where the crust transitions to lighter and more buoyant continental crust. The continental shelf is the low-lying portion of the continent flooded by shallow seas.
Tectonic features of a mature ocean basin, such as the Atlantic Ocean
4.7.3. The Wilson Cycle
Eventually, a passive continental margin develops into a new subduction zone as the adjacent oceanic crust becomes older, colder, and denser. The ocean basin, while still spreading at the mid-ocean ridge, starts to grow smaller as oceanic crust is subducted. As the intervening ocean basin disappears the continents are drawn into a new round of collision, albeit in a new place. This cycle of continent formation, breakup, and re-assembly is called the Wilson Cycle.
Stages in the Wilson Cycle with modern-day examples of each stage
The Wilson Cycle and Drifting Continents
Most of the continents were joined as the supercontinent Pangaea 200 million years ago (A). The continents became fragmented as first the Atlantic Ocean, between North America and Africa (B) and then the Indian Ocean, between Antarctica and India, (C) opened and grew (D). A new round of continental collision began as the Tethys Ocean separating India and Asia closed (E). The Atlantic and Indian continue to grow as the Pacific Ocean, ringed by subduction zones, continues to close (F). Eventually a new supercontinent may be formed.
4.7.4. Active vs. Passive Continental Margins
Active continental margins are found where subduction occurs between an oceanic and a continental plate. Active continental margins coincide with plate boundaries. Passive continental margins are found where there is a transition between oceanic and continental crust that is still moving as one plate. In this case the plate boundary is out at the mid-ocean ridge.
Active continental margins are plate boundaries marked by subduction zones with earthquakes, volcanoes, and mountain building. Passive continental margins are transitions between oceanic and continental crust that do not coincide with a plate boundary.
The western U.S. is an active continental margin with all four tectonic settings we have discussed:
1. Divergent: Gulf of California spreading center and extension in the Basin and Range and Rio Grande Rift (a failing continental rift)
2. Transform: the San Andreas and related fault systems
3. Convergent: the Cascadia Subduction Zone
4. Hot Spot: the Yellowstone hot spot
4.8. Tectonic Settings
Tectonic setting describes the geologic environment of an area relative to any nearby plate boundaries or hot spots. This figure summarizes the different tectonic settings we have talked about. Each tectonic setting is associated with a particular style of volcanism as we will see in a future lecture.
Tectonic settings include divergent, convergent, and transform plate boundaries as well as hot spots. Each tectonic setting produces a characteristic style of volcanism.
Summary of Tectonic Settings
4.8.1. Tectonic Setting of the Pacific Northwest
The tectonic setting of the Pacific Northwest is an active plate boundary where small oceanic plates, the Gorda (south) Juan de Fuca (central), and Explorer (north) plates are colliding with and being subducted by the continental North American Plate. The active plate boundary lies just offshore and is called the Cascadia Subduction Zone. As the oceanic plates are subducted, materials too light to be subducted are scraped off the oceanic plate and accreted to the western margin of North America to form the Costal Ranges of Vancouver Island, Washington, Oregon, and Northern California. Further inland, a volcanic arc, the Cascade Range, is formed on the overriding plate above the depth where the subducted oceanic plates begin to melt and produce magma, which rises to the surface. The coastal and Cascade ranges are separated by a fore-arc basin, a topographic depression produced by down-warping of the continental plate like a rug due to the tremendous stresses from the subduction zone. This depression is occupied by Puget Sound and the Williamette Valley.
SHAPE * MERGEFORMAT
The tectonic setting of the Pacific Northwest in map view (left) and in cross section (right), showing the major tectonic features of western and central Oregon.
Cross section of the Cascadia subduction zone with major geologic processes and topographic features
Although the Coastal Ranges and Cascades are both a product of subduction, the specific processes responsible for their formation are very different. Coastal Ranges like the Olympics of Washington and the Coast Range and Siskiyous of Oregon are a product of accretion and consist primarily of oceanic rocks added to the western margin of the continent. The Cascades, in contrast, are a volcanic range, and are a product of rising magma produced by heating and melting of the subducted plate.
In Oregon, tectonic extension is occurring in the back arc basin (east of the Cascades). Why? Two reasons: 1. The Pacific Northwest is being rotated clockwise, creating extension in southeastern Oregon and compression in northwestern Washington. 2. The gradual ending of subduction with the destruction of the last remnants of the Farallon Plate is relieving compression on the western margin of North America, allowing the edge of the plate to “rebound”
4.9. Significance of Plate Tectonics
Plate tectonics explains the global distribution of earthquakes, volcanoes, and mountain ranges. Plate tectonics is also responsible for creating and destroying ocean basins as well as building and rearranging the continents. Through the processes of subduction and volcanism, plate tectonics recycles earth materials. Our atmosphere is also maintained through volcanic degassing and climate regulated by controlling the atmospheric carbon dioxide content and the magnitude of the greenhouse effect.
Plate Boundaries Earthquakes Volcanoes
SHAPE * MERGEFORMAT
SHAPE * MERGEFORMAT
Compare the map of plate boundaries to the global distribution of earthquakes and volcanoes.
The locations of volcanoes do a pretty good job of outlining plate boundaries. Earthquakes do an even better job of outlining plate boundaries. Why? Not all types of plate boundaries are associated with volcanism (there are no volcanoes at transform plate boundaries), but all plate boundaries are associated with earthquakes!
Terms to Know
| Divergent plate boundary | Hot spot track | Continental rifting |
| Continental rift valley | Tectonic setting | Passive margin |
| Convergent plate boundary | Craton | Abyssal plain |
| Transform plate boundary | Orogen | Continental slope |
| Volcanic arc | Continental shield | Continental shelf |
| Hot spot | Accreted terrane | Wilson Cycle |
Questions for Review
1. What are the three types of plate boundaries? Sketch simple diagrams showing the geologic processes occurring at each boundary.
2. Provide a modern-day example of each type of plate boundary.
3. What are hot spots and hot spot tracks?
4. Why is plate tectonics important?
5. Describe the different parts of a continent and how they are pieced together.
6. Contrast the processes and resulting features occurring along active and passive continental margins.
7. Describe the stages and features associated with continental rifting and continental collision.
8. Describe the tectonic setting of the Pacific Northwest and the major geologic features of this setting.
rifts
Iceland
faults
central rift valley
magma
extension
asthenosphere
lithosphere
Oceanic crust
Rift Valley lakes
The Red Sea is a more advanced rift
African Rift Valley
Ocean-Ocean Ocean-Continent Continent-Continent
Overriding plate
Today
20 million years ago
Challenger Deep
Philippine Plate
Pacific Plate
40 million years ago
Divergent:
Plates move away from one another.
Lithosphere created.
Transform:
Plates slide past one another.
Lithosphere neither created nor destroyed.
Convergent:
Plates move toward one another.
Lithosphere destroyed.
Hotspot:
Plate rides over plume of hot mantle.
4.
3.
2.
1.
6. Continental collision
5. Accreted terranes
4. Subduction zone
3. Ocean basin with passive margins
2. Spreading center develops
1. Continental rifting
Cascade Range
Volatiles from heating of subducted slab
Coast Range
Sediment and Basalt Scraped off Subducting Plate