Easter Plate
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Easter Plate is a tectonic
microplate A microplate, also known as a microtiter plate (''Microtiter'' is a registered trademark in the United States, therefore it should not be used generically without attribution), microwell plate or multiwell, is a flat plate with multiple "wells" ...
located to the west of
Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...
off the west coast of
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
in the middle of the
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, bordering the Nazca Plate to the east and the
Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...
to the west. It was discovered from looking at earthquake distributions that were offset from the previously perceived Nazca-Pacific
Divergent boundary In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent b ...
. This young plate is 5.25 million years old and is considered a microplate because it is small with an area of approximately .
Seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener an ...
along the Easter microplate's borders have some of the highest global rates, ranging from /yr.


Structure and tectonics (present)

From the 1970s to 1990s, multiple efforts were made to collect data on the area, including several
magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
and
gravitational anomaly In theoretical physics, a gravitational anomaly is an example of a gauge anomaly: it is an effect of quantum mechanics — usually a one-loop diagram—that invalidates the general covariance of a theory of general relativity combined with s ...
surveys. These surveys show that Easter plate is uniquely shallow, bordered by spreading centers and transform boundaries, with a triple junction located at the southern and northern tip. Along the eastern border, there are several spreading centers south of 27° S and 3 northward propagating rift to the north of 27° S. The axis further north is a
graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
reaching a depth of approximately 6000 m. Northward propagation of the eastern rifts is continuous at a speed of /yr. The spreading ridge between 26° S and 27° S has a spreading rate of /yr, but is asymmetrical on Nazca Plate side.
Bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
data shows the depth is near 26°30' S and progressively gets deeper to the north, reaching depths of in an axial valley. There is approximately a gap at the northern end of the east rift with no rift connecting the northern boundary to the eastern boundary. The northern border has wide ridges, greater than 1 km tall, linked side-by-side with the steeper slopes to the south. The southern trough area sits deeper than the areas to the north. The very eastern end of the northern border has pure
strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
motion, while the western end is marked by the Northern Pacific-Nazca-Easter triple junction. This triple junction is a stable rift-fracture-fracture zone with anomalous earthquakes occurring to the northeast portion, indicating a possible second spreading axis. The rest of the northern boundary to the east and west of the triple junction are colinear transform boundaries. A
trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...
, approximately deep, borders the north along this transform boundary to the east connecting to a deep hole, called the "Pito Deep" because of its close proximity to the Pito Seamount, at the northeastern limit. The western border is divided into two parts. The west section has 2 spreading segments running north to south with spreading rates that approximately range from /yr. These segments are connected by sinistrally slipping transform faults around 14°15' S. A
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...
basin runs north to south along the southernmost segment as a result of past counter-clockwise rotation. The southwest consists of one slower spreading center (/yr) that runs northwest to southeast until joining the southern transform boundary. Like the western end of the northerner border, the southern end also has an inferred rift-rift-fracture triple junction, but no data has been gathered yet to verify its existence. A single transform fault runs west to east and is home to the most rugged and shallow terrain with high seismic activity.


Evolution

In 1995, routine magnetic, gravity, and
echosounder Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; ...
data, supplemented with data from GLORIA (a long-range
side scan sonar Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side imaging sonar, side-imaging sonar and bottom classification sonar) is a category of sonar system that is used to efficiently create an image of large areas of the sea ...
), German Sea Beam, SeaMARC II, and data from the World Data Center in Boulder, CO were all utilized to construct a two-stage model for the evolution of the Easter microplate.


Stage 1: 5.25 to 2.25 million years ago

Approximately 5.25 million years ago, the boundary between the Pacific and Nazca plates was not connected and did not completely separate the two plates. The Easter microplate began to grow to the north-south throughout this period. The eastern rift, having not yet connected to the western rift, began to propagate northward by pseudofaults that appear to the west and east of the rift and continued until approximately 2.25 million years ago when the tip reached 23° S. While this was occurring, the west rift was propagating southward, north of the east rift, breaking into segments connected by transform faults that trend towards the southwest. The entire microplate continued a counter-clockwise rotation rate of 15° every million years throughout the entire history of the Easter microplate.


Stage 2: 2.25 million years ago to present

The Easter microplate grew at a slower rate in the east-west dimension during this period, as it stopped growing north-south due to the cessation of east rift propagation. The east rift did continue angular spreading while keeping the same growth rate, but did not propagate any further northward. The west rift continued adjusting with more segmenting until the southwest rift began to open and propagate to the east. The southwest rift continued propagation until the present day southern triple junction was created.


Future predictions

Though other evolution models have argued that the microplate was created approximately 4.5 million years ago, there is currently only one hypothesis for future evolution of the Easter microplate. It is believed that due to the slowing spreading rates at the southwest rift and the northern end of the east rift, the southwest and west rift will cease spreading activity and completely transfer the microplate from the Nazca to the Pacific Plate. This has been the case for other areas where extensive rift propagation studies have been conducted.


Dynamics


Driving forces

Divergence of the Nazca and Pacific plates generate a pulling force acting on the Easter microplate, causing its rotation. Two types of driving forces are believed to act on the Nazca-Pacific plate divergence:
shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...
and
tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches rocks in two opposite directions * Voltage or el ...
. Shear driving forces occur along the north and south boundaries, which explain failures due to compression in the northern end of the plate. Tension driving forces occur at the east and west rifts. Because of the fast spreading rates along these boundaries, the Easter microplate has a thin lithosphere. The normal tensional forces applied across the east and west rifts is enough to drive the microplate's rotation. Due to the slowing trend of these spread rates along these rifts to the north, it is believed the lithosphere gets thicker near the north and the shear forces are believed to contribute to the overall driving force.


Resisting forces

Mantle basal drag accounts for 20% of the forces applied to the Easter microplate. Mantle basal drag force is calculated using the equation: \vec_D = D\vec, where \vec_D is the mantle drag force per unit area, D is the proportionality constant, and \vec is absolute
velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
of microplate using a fixed hotspot as the reference frame. The value for \vec_D represents a quantification of the total resisting force that the ductile
asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...
applies to the brittle
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
floating on top. The other 80% of the resisting forces come from the rotation of the Easter microplate. As the microplate is rotating, normal resistances are applied to the microplate at the north and south ends where there are no rifts to help microplate adjustment. Both tension and compression contribute to the resistance, but compressional forces along the ends of the rifts have more of an impact. These compressional forces are what create the elevated regions that surround the "Pito Deep".


References

{{DEFAULTSORT:Easter Plate Tectonic plates Geography of Easter Island Geology of the Pacific Ocean