Dynamic Planet/Tectonics

This article is about the topic of Dynamic Planet for the 2017 and 2018 seasons. For information about the event in general, see Dynamic Planet Topics.

Template:EventLinksBox Tectonics is the topic of Dynamic Planet for the 2017 and 2018 seasons.

History of Plate Tectonics Theory

In 1912, Alfred Wegener composed the theory of Plate Tectonics. The idea of a supercontinent was derived from inspecting fossil patterns and crust thickness. Wegener theorized that around 250-300 million years ago, the supercontinent named Pangea, was slowly split apart to their current positions (continental drift). He suggested that the continental coastlines on the opposite sides of the Atlantic Ocean could be “fit together” like jigsaw puzzle pieces, and found similar plant and animal fossils across oceans.

While the theory certainly made sense from a practical point of view, Wegener didn't have a geological way of proving this was true. After looking at various pieces of evidence such as the aforementioned fossil patterns, mountain formation statistics, and glaciation in the Appalachian Mountains during the Pennsylvania Period of the Carboniferous Era, he came up with a geological theory that the Earth were made up of plates that moved around thanks to the rotation of the Earth, causing Pangaea to split up. However, that theory was shot down very quickly, as was the idea that America's movement came from the gravitational forces of the sun and the moon. Thus, Wegener's theory was soon discredited for lack of an explanation.

At around the same time, in 1929, Arthur Holmes came up with a way to explain plate tectonics through mantle convection, which would cause the plates to move around the Earth on a bed of mantle convection belts. This idea was not paid attention to at the time until the 1960s, when scientists discovered geomagnetic anomalies along the ocean floor near trenches and the existence of island arcs near continental margins, suggesting the possibility of convection. These pieces of evidence led Harry Hess in 1962 and Robert Dietz in 1961 to publish independently a theory known as sea-floor spreading, which thus became the main way to explain why plate tectonics exists.

1863 - Frederick J. Vine, Drummond H. Matthews, and Laurence w. Morely suggested that new crust would have a magnetization aligned with Earth’s geomagnetic field. Over a geologic time, this would appear as bands of crust that exhibit alternating patterns of magnetic polarity. This provided more evidence that earth’s plates separate at mid-ocean ridges.

1968 - Vessel Glomar Charllenger- (in mid-ocean ridge between South America and Africa collected core sample obtained from drilling holes through the overlying sediments and into the oceanic crust revealed that rocks close to deep ocean ridges are younger than rocks that are farther away from the ridges. It was part of a research effort called Deep Sea Drilling Project.

Mid 1970s - seismic tomography- enables scientists to investigate the dynamic processes in the deep interior of Earth. Scientists created three-dimensional images of Earth’s interior by combining information from many earthquakes using an approach similar to computed tomography (CT) scanning.

Elements of Plate Tectonics

Layers

Mechanical/Physical Divisions of the Earth's Layers

Lithosphere: the upper 100 km of the earth, comprised of the crust and the uppermost mantle.

Asthenosphere: the layer beneath the lithosphere comprising part of the mantle. The Asthenosphere is known as the Low Velocity Zone, as seismic waves travel more slowly through it than they do through the Lithosphere. The boundary between the Lithosphere and the Asthenosphere is generally considered the 1300 C isotherm. Above this boundary, the mantle behaves rigidly and below it behaves in a more ductile manner.

Mesospheric Mantle: the mantle beneath the Asthenosphere and above the outer core (about 660 km deep to 2700 km deep) that is distinguished by a sharp increase in seismic wave velocity and density.

Outer Core: a liquid layer composed of iron, nickel, and other elements in trace quantities. Convection in the outer core is thought to be the cause of the Earth's magnetic field. About 2,300 km thick.

Inner Core: The central layer composed primarily of iron with some nickel. The inner core is too hot to hold a permanent magnetic field and is slowly becoming thicker as more of the outer core solidifies slowly over time due to the gradual cooling of the Earth. Radius of about 1,220 kilometers.

Chemical Divisions of the Earth's Layers

The Earth's layers are also classified chemically as the Crust, Mantle, Outer Core, and Inner Core.

Crust: the uppermost layer ranging from about 5 km - 70 km in depth. Continental crust is considerably thicker than oceanic crust and is primarily composed of granite and other felsic sodium potassium aluminium silicate rocks, while oceanic crust is thinner and composed mostly of basalt and other mafic iron magnesium silicate igneous rocks.

Mantle: the layer below the crust that is composed of silicate rocks that are richer in iron and magnesium. The boundary between the Crust and the Mantle is the Moho (Mohorovicic discontinuity). Above the Moho are rocks containing plagioclase feldspar, below it are rocks containing no feldspars.

Outer Core: see above

Inner Core: see above

Plates

Tectonic plates are the pieces of the Earth's crust that "float" on the asthenosphere and make up the lithosphere. They are driven by convection currents in the mantle caused by the heat of the core due to pressure. There are two types of plates: oceanic and continental plates. Continental plates are thicker than oceanic plates, but oceanic plates are denser than continental plates. Oceanic plates are made of denser rocks due to cooling quickly and having fine-crystals.

Plate Boundaries

Plate boundaries are where the different pieces of the crust, known as tectonic plates, meet.

Convergent

When two plates collide with each other or come together, the boundary between the two plates is known as a convergent boundary.

Oceanic-Oceanic

When two oceanic plates converge, it causes subduction and creates a trench. The trench is the deepest part of the ocean because one plate is actually going underneath the other. Oceanic-oceanic convergent boundaries are also responsible for hotspots as magma from the mantle gets trapped between the two plates and is forced upwards.

Oceanic-Continental or Continental-Oceanic

When a continental and an oceanic plate converge, the oceanic plate always subducts underneath the continental. These boundaries create volcanoes and active margins and trenches, but the trenches are not as deep.

Continental-Continental

When two continental plates converge, the plate is pushed upwards creating mountains.

Divergent

Oceanic

Oceanic divergent plate boundaries are marked by mid ocean ridges. At these boundaries, seafloor spreading occurs as plates separate and new oceanic lithosphere is formed along the spreading center. The Mid Atlantic Ridge is a well-known example of an oceanic divergent plate boundary.

Continental

Divergent boundaries over continents can cause rifting, in which the continental lithosphere stretches and thins before being broken apart, after which seafloor spreading begins. The East African Rift is an example of an active continental divergent boundary and is currently splitting the African Plate into two plates: the Somali Plate and the Nubian Plate.

Transform

When two plates slide past each other, the boundary is known as a transform boundary. No old plate is subducted and no new plate is formed at transform boundaries, which are marked by large faults and often earthquake activity. The San Andreas Fault is one of the most famous faults that occurs along a transform boundary.

Basins

Rift Basin: They are found on all passive continental margins. They provide record of early stages of continental breakup, and are affected by displacement geometry on bounding normal fault systems. They are created by first having early rifting associated with some minor and isolated normal faults. Then, there is mature rifting going through boundary fault zones; widespread deposition; and footwall uplift and erosion.

Back arc basin: They are submarine basins that form behind an island arc. They are usually located on the western margin of the Pacific next to a convergent boundary.

Foreland basin: They are mountain belts associated with uplift of rock materials to several kilometers in height, and are bordered by regions of subsidence called foreland sedimentary basins.

Tectonic Plate Movement

Driving Forces

Tectonic plate movement is primarily driven by convection currents in the Earth's lower mantle.

Isostasy: Isostasy is the gravitational equilibrium between the lithosphere and the asthenosphere. The asthenosphere is weaker and more ductile than the lithosphere and flows laterally under the force of the lithosphere. The depth at which pieces of the lithosphere "float" in the asthenosphere is determined by their size and density. Denser and larger sections of crust will sink lower in the asthenosphere, which accounts for some differences in elevation of the Earth's crust. These different topographic heights can be accounted for by differing crustal thicknesses (Airy-Heiskanen model) and by differing densities (Pratt–Hayford model). Events that cause the crust to thicken, such as accumulation of sediment or collisions resulting in the development of mountains cause the crust to depress, while events that take weight off of the land, such as glacial retreat, cause the crust to "rebound".

Basal Drag: Basal drag is plate movement due to friction between the asthenosphere's convection currents and the lithosphere.

Slab Suction: Slab suction is plate movement due to local convection currents that pull plates down at subduction zones in ocean trenches.

Impacts of Plate Movement

Plate movement can cause many effects on Earth, such as earthquakes, tsunamis, volcano formation, and continent formation/destruction. All of these things are important to Earth's history, and many of these events caused mass extinctions, changing the flow of evolution on Earth.

Effects of Tectonics

Tectonic Hazards

For more detailed information on earthquakes, volcanoes, and tsunamis, please see the Earthquakes and Volcanoes page.

General Hazards: Plate tectonics can cause a large range of hazards, including Earthquakes, Volcanoes, Tsunamis, and Landslides.

Earthquakes:This one is pretty obvious. Earthquakes happen when two tectonic plates at a fault boundary suddenly slip. This creates a shaking sensation on the Earth's surface, which can be very bad for buildings (especially in 3rd world countries) and other structures.

Volcanoes Melting of subducting plates and upwelling of magma can produce volcanoes, which leads to eruptions. This is very bad for the surrounding area, as a lot of land and structures in the immediate area can be destroyed.

Tsunamis: Earthquakes in the ocean can cause a tidal wave (or tsunami), which can advance towards shore and grows in height. Tsunamis can devastate coastal cities upon landfall, killing up to hundreds of people and damaging many structures.

Landslides: Any geologic process in which gravity causes rock, soil, artificial fill or a combination of the three to move down a slope. Several things can trigger landslides, including the slow weathering of rocks as well as soil erosion, earthquakes and volcanic activity. Can destroy many things in its way.

Effects on Geology

Rock Deformation

Rocks can deform when under pressure. There are 2 main ways that rocks can break (or deform): brittle and ductile. Brittle deformation is when the rock fractures and breaks under pressure, and ductile deformation is when a rock bends and flows under pressure.

A great website to learn more about deformation.

Geological Composition

The effects of plate tectonics can create settings of various types of rocks and minerals within existing rock. At a divergent plate boundary, igneous rocks are commonly formed from magma coming to the surface and cooling. This is how we get the basalts and gabbros on the bottom of the ocean. Likewise, metamorphic rocks can form at transform boundaries due to the immense pressure of the plates rubbing together. Sedimentary rocks are usually not associated with plate boundaries.

Geologic History of North America

One specific topic included in the rules for 2017 and 2018 was the study of specific aspects of North American geologic history. This was focused on four different topics: the North American craton, the Rocky Mountains, the Appalachian Mountains, and the Yellowstone Hot Spot.

North American Craton

Laurentia is a large continental craton that forms the ancient geological core of the North American continent. A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates.

Rocky Mountains

In the south, an older mountain range was formed 300 million years ago, then eroded away. The rocks of that older range were reformed into the Rocky Mountains. The Rocky Mountains took shape during an intense period of plate tectonic activity that resulted in much of the rugged landscape of the western North America.

Appalachian Mountains

A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor. Strong evidence that these rocks were deformed during plate collision. The forces that are generated cause massive folding and faulting of rock layers into mountain ranges. The birth of the Appalachian ranges, some 480 million years ago, marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangea with the Appalachians near the center. By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present formed. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.

Yellowstone Hotspot

The Yellowstone hotspot, also referred to as the Snake River Plain-Yellowstone hotspot, is a volcanic hotspot responsible for large scale volcanism in Oregon, Nevada, Idaho, and Wyoming (in the United States) created as the North American tectonic plate moved across the Yellowstone hotspot. Last supereruption was 640,000 years ago

Common Tasks

Ordering Rock Layers

Ordering rock layers appears to be a commonly tested skill in the 2017 season. Because rock layers are deposited from the bottom up, it is fairly easy to order these rock layers. The oldest rocks are at the bottom, and the newest rocks are on top. This is known as the Law of Superposition.

The Law of Original Horizontality states that layers of sediment, rocks, or other geologic material are always deposited in horizontal layers. If rock layers are not horizontal, that indicates some sort of geologic activity.

The Law of Crosscutting Relationships states that geologic features, such as faults, and igneous intrusions are younger than the rocks they cut. Similarly, if rocks are folded, the folding is younger that the youngest rock affected.

The Law of Lateral Continuity states that all rock layers are continuous, but may be broken up or displaced by later events.

Links

Dynamic Planet/Earthquakes and Volcanoes - similar, but not equivalent, topic; however, overlap exists

GMOA Notes - old note page containing some relevant info

USGS Plate tectonics Publication