SLO 1: Plate Tectonics

This objective will be about interactions between the Earth, its tectonic plates, and the oceans around them, starting with their origins and general information. Billions of years ago, the big bang scattered space dust all around the cosmos, and parts of it stuck together. Through gravity this dust gathered together into pebbles, then rocks, then boulders, and further still into mountains. Eventually, enough came together to form a planet-sized mass, but it was not ready to support life yet. Water had to arrive through meteors falling to earth, and the formation of an atmosphere was millions of years away. After many years of melting and a planetary collision with its twin Theia, the Earth's crust cooled, water vapor in the sky following suit. However, the previous heat of the planet was far from gone, having retreated underneath the surface. This heat bubbles under and cracks the cool crust, increasing in intensity the deeper you go. The first below the crust is Earth's mantle, composed of partly liquid silicate rocks that slowly move under the surface. There is an outer and inner mantle, the difference between them being heat, much like what lies below both. The outer and inner cores are found deeper than the mantle and are composed of pure molten metals such as nickel or iron. This highly magnetic core is responsible for the creation of our magnetic field, and if it were to cool we would lose this crucial force (In Depth | Earth). All of this mass and heat is held below the Earth's crust, and it does take a toll on the highest layer of our planet. The temperature slowly cracked all the way through the weaker parts of the crust, eventually snapping it and forming the tectonic plates (Morton). These plates float on top of the mantle, and they move alongside the mantle they can crash into each other. The nature of the interaction depends on multiple factors, such as direction and if the crust of each is continental or oceanic. One such interaction is between convergent plate boundaries, and these can have some of the most noticeable results. As the name suggests, these interactions happen when two plates converge and push against each other in opposite directions. The result of this push depends on the crust of each plate, and the denser oceanic crust will almost always sink under less dense continental crust. An applicable weight difference will cause one plate to subduct under the other, creating a subduction zone and potential volcanoes as the sinking crust meet the mantle. If there is no weight difference, the converging plates will instead push each other up, creating high mountains both above and below water. Some real-life examples of converging plates include the Aleutian Islands and the New World Seamount. The next tectonic interaction is between transform boundaries, of which further real-life examples include the San Andreas Fault and the Dead Sea Rift (Imaging Israel's Dead Sea Fault to Understand How Continents Stretch and Rift). These interactions happen when two plates grind alongside each other, transforming the landscapes around the points of impact. Trenches and faults are the most common occurrences along transform boundaries, however, rifts and other crustal deformations have also been observed (Transform Plate Boundaries). One more interaction between plates is the divergent plate boundary, and as the name states this is when two plates move away from each other. The biggest geological change at divergent boundaries is crust growth as the plates move away, however common rifts and plains also form between these two plates. A few real-life examples of these include the East Africa Rift and the entirety of the Red Sea. There is also another minor boundary called the hotspot, although most aren't on a boundary at all. Hotspots are made where certain parts of the earth's core are hotter, and landmarks such as volcano arcs and islands pop up from them right in the middle of tectonic plates (Types of Plate Boundaries). The best example of a hotspot is the Hawaiian islands, with their chain confusing scientists for years until hotspots were identified. Speaking of islands, there's one thing to be said about the water surrounding them here before we move on to the ocean's dedicated segment. Even with all of the shifting plates and masses on our planet, our water is much more connected than one might think. First of all, 75% of our Earth is covered in water and 96.5% of that water is found within our oceans, and as you know oceans are everywhere (Water Science School). However, despite the distances between some bodies of water, all of it is connected. The ocean basins or "seven seas" are just sections of our Earth's one ocean, so all of these tectonic changes affect all of it.

Our earth being so enveloped in water at such high depths has meant that studying all of its surfaces was once an impossible task. The ocean floor was previously one of these impossibilities, but through the development of bathymetry, its secrets have been unveiled to us. This need to measure and understand the ocean floor first started with depth sounding, the process of leading a weighted line down to the ocean floor and measuring the distance. This distance was measured in fathoms, a unit of measurement equal to a sailor's arm length of 6 feet. This method and unit of measurement dated all the way back to the 1600s, however, it wasn't very accurate even by past standards. This all changed after WWII with the invention of S.O.N.A.R. in the 1930s, a process similar to echolocation that uses sound to measure distance. Because sound has a set speed in water, it could be used to accurately measure distances from the surface to the bottom. A pioneer in the field who made use of this technology was none other than Marie Tharp. Born in 1920, she gained an interest in the earth's oceans after graduating college, and after two master's degrees and years of work at the Lamont Geological Observatory, she met Bruce Heezan, who she would work with in plotting the oceans. Staying on the shoreline measuring readings that the rest of the crew took out on the water, she found different readings in different areas, proving that the ocean floor wasn't flat. Later studies of hers revealed the Mid-Atlantic Ridge to the world, spurring forth the theory of continental drift in addition to mapping a great portion of the ocean floor (Marie Tharp: Pioneering Mapmaker of the Ocean Floor). Marie Tharp's work was crucial for the evolution of bathymetry, and further findings on the ocean floor are still being found today because of it. The biggest of these findings is the topography of the earth's ocean basins, which change as depth increases. At the surface, we have the continental shelf, which is found in the Neritic zone and is the start of the dive into the depths. The zone is covered by a relatively small amount of water and is formed by tiny grains of inorganic and organic material over thousands of years. The next location on the way is the continental slope, which leads directly into the deeper parts of the ocean. The slope can be found in the Epipelagic zone 0-200 meters down all the way to the Bathypelagic 1000-4000 meters down and is formed by deposits of sediments that flow down from the shelf. Most of the ocean's sediments are deposited right here, and through forces called turbidity currents turbidites can form. Finally, we reach the abyssal plain in some of the deepest parts of the ocean at 4000-6000 meters down in the abyssopelagic zone. Not much sediment can reach here, so these "plains" are more made up of very uneven earth crust that can lead to landscapes as large as mountains being found here. Much less common than the others is the Hadal Zone, a depth that can only be reached through deep underwater trenches or rifts at 6000-11,000 meters down (Ocean floor features). At these depths the nature of the earth's oceanic crust can be seen despite the pitch blackness, however, it might've not always been this way. The less-dense continental crust can actually become oceanic crust through a special process we already partially talked about. The process of sedimentation brings particles from the surface floating in the water down to the ocean floor to rest as you know. However, given enough time this sediment can become part of the ocean floor itself, meaning that a lot of oceanic crust probably started on land (McGuire, 182–184). While the plates are sections of the earth's crust that have split apart from each other, individual sections of plates have their own subsections too. These continental margins have two distinct variations, and each is found on different areas of a tectonic plate. Passive margins are found in areas that have little geological and tectonic activity, and as such are much more settled topography-wise. Erosion has time to take its course, leading to much smoother plains, beaches, and systems. This erosion leads to a lot of sediment gathering up near the coast, causing flat and long continental shelves and slopes. In contrast, active margins are always found in areas of high geological activity, the constant crashing of an oceanic plate leading to a very jagged landscape. Volcanoes and mountains are built here, with igneous rock bubbling to the surface often, meaning a continental shelf is almost nonexistent. Both of these margins are found by coastlines, with passive margins reaching far out while active ones stay tightly packed (What's the difference between an active and passive continental margin?). However, they aren't the only interesting feature present on the sea-boarders of our world.

The shores, beaches, and coasts of the landmasses that serve as the gateway from the land to the sea have unique qualities and origins from a long time ago. Over hundreds or even thousands of years, fine particulates from the sea and eroded from the land are left behind to rest by the waves, much like the process of sedimentation. These grains of matter can be sand, calcium, and even organic material from a long-gone creature (Wynne). Some beaches remain stagnant as time passes while others can change every day, and these qualities have led to two coastal classifications. These types of beaches are determined based on multiple variables such as the makeup of their surroundings and the intensity of the waves. Erosional coasts are the first of these and possess high levels of tidal activity and hard land surfaces to clash against. As the description would suggest this activity doesn't leave much space for sediments to rest, leading to rocky and jagged beaches walled by tough eroded rock. The second classification for coasts is depositional, which is on the opposite side of the scale from erosional coasts. These coasts have relatively lower wave activity, leading to much more deposition of sediments along the coastline. The buildup of sediments leads to very smooth and sandy beaches with shallow wetlands and barrier islands (Bralower). In fact, these depositional coasts also have their own unique landmark called a coastal cell. These cells are closed-off systems that constantly filter sediment in and out of depositional beaches with paths, sinks, and sources for sediment (Mangor). If these descriptions sound familiar, it's because these coastal classifications are pretty much the same as the continental margins, with the types of beaches being found in the same areas as their respective margins. Much like the active erosional coast's jagged features, passive depositional coasts also have their own rounded features formed from sediments and waves. Starting from the surface we have the backshore-zone berms, sediment found here is flat and has been completely out of the water for some time. Next, there is the beach face, which marks the edge of the water's reach and is found in the swash zone, however, a particularly strong wave can get past the beach face, and continuous force on the face can actually move it up the coast. Finally, we have the longshore bars and troughs, the former being found in the breaker zone, where water first "breaks" against the land to make waves. The force of water hitting sediment is what rides up these bars, and the waves moving back and forth across them make them look like hills parallel to the coast. The trough is found behind the longshore bar facing the ocean and is what composes the sand leading up to the beach in the surf zone (17.3 Landforms of Coastal Deposition). The topography of these zones is important to their function, however, the creatures that live in them are just as interesting. One such creature even forms its own ecosystem for thousands of other creatures to live in, and this system is known as the coral reef. These landmark ecosystems form over many years through the growth of creatures called polyps that form coral around them. When polyp larvae attach to steady rocks, their feeding creates calcium growths that the polyp sheds and enough of them combined will form a coral structure. Enough coral structures will form a complete coral reef, and different types of these reefs are defined based on their age. The first stage of development is the fringing reef, which grows slightly separated from the shoreline in lagoons or other shallow bodies of water. The Coral that grows here is new and so this type is the most common, even being found near volcanoes. The next part of a reef's development is the barrier reef, which is much larger than a fringe reef and is found farther away from the coastline. Often the product of multiple fringe reefs becoming one, these reefs will form a large line in front of coasts and a ring around islands, hence the name. The final stage of development is the atoll reef, which is only formed after an island surrounded by a barrier reef has almost or fully sunken into the water. These reefs are most often found in the middle of the ocean, now fully surrounding a lagoon it most likely developed in. These atolls become islands themselves of diverse marine life in the middle of the open ocean, with near millions of creatures being able to thrive in just a small space of reef (Reef Types and How Coral Reefs are Formed). The wonderous coral reefs of our ocean develop greatly on the coastlines of the landmasses, however, the coastlines themselves also have even further classifications and developments. Estuaries are prime possessors of some of these qualities, and what they're called is dependent on their location. The first estuary is the bar-built estuary, which is formed when longshore bars partially block in a portion of water. The water flowing into these estuaries is often low relative to the sediment buildup which allows for this to happen. If the difference is great enough in the sediment's favor, it can even fully block off the water from the ocean and form an inland lagoon. In the United States, these are most often found in lower parts of the country such as Texas and Florida. Another type of estuary is the tectonic estuary, which is found near areas of high tectonic activity such as the west coast of the United States. These bodies of water are formed quickly and drastically when a coastal part of land sinks below sea level due to tectonic activity. What fills the newly formed basins is a mix of seawater and freshwater that flows into the estuary from displaced streams and rivers. The next classification is the fjord, which is exclusively found in northern regions of our world. The reason why they are found only in these areas is that a fjord must be carved into the land by a moving glacier from long ago. These fjords have very little sea water within them due to being much closer to land, and inversely have a very high level of freshwater flowing from the land and from melting ice, this water possessing a low oxygen count because of it. Notable locations for these estuaries include Greenland, Norway, and Canada. The final estuary type is called the drowned river valley and is much older than the other types of estuaries talked about so far. During the ice age when glaciers started to melt on a global scale, the sea level rose drastically in a short amount of time. This led to the existing low-lying river valleys near glaciers drowning in seawater and creating a new environment in its place, which is how it got its name. Places where drowned river valleys are found include many rivers such as the Seine in France, the Ki-Sang in China, and the Thames in England. Different estuaries are found all over the world, however, this isn't the only way they are categorized (Classifying Estuaries: By Geology). An estuary's salinity and pattern of mixing also determine sub-categorization, of which there are four. Well-mixed or vertical estuaries possess completely mixed salt and fresh water from the surface all the way to the bottom. The only exception to this balance is where the sources of fresh and seawater enter the estuary, and this type of mixing is almost always found in shallower estuaries. The next profile is the slightly stratified mix, and it is closely similar to the aforementioned vertical mix. The main differences are slight increases in salinity as the depth of the estuary increases, and as such, this profile is mostly found in estuaries slightly deeper than the vertical profile. Another mix profile is the salt wedge, a profile that actually doesn't mix salt and freshwater much at all. A key quality of a salt wedge is a strong current of freshwater near the surface and a flow of salt water closer to the bottom. These forces don't mix together due to the high strength of the freshwater current, and each tends to stay at its respective depth, however, brackish water has been observed. The final pattern of mixing we know of is the highly stratified estuary which is found in estuaries with a very large mass. Mixing only occurs near the surface and the level of salinity increases as you near the source of salt water. However, salinity doesn't change with depth, unlike partially stratified estuaries, leading to a unique sub-environment (Webb). All of this information about coasts can be combined together to describe the topography of real-life coastlines. Some that are relevant to this essay include the three coasts of the United States. The United States' west coast is definitely an active margin zone with rocky beaches and topography due to being directly on top of a transform plate boundary. In contrast, the east coast of the U.S. is very much in a passive margin with long beaches and smooth terrain. This passive margin is shared by the United States "third coast" located by the great lakes. While much colder due to its location the lake beaches also possess smooth beaches and aren't located near large geological activity . Despite their size and very sturdy qualities, these beaches are still at risk of damage. The culprit being activity that also threatens many other natural systems of our earth, that of humans. The most egregious form of this harmful activity is hard stabilization in the form of many manmade constructs. Seawalls, groins, breakwaters, and many more are designed to block the longshore transport of water and sediment from going past them. While these are designed to prevent erosion of beaches, they can instead cause it to increase in other areas and cause a myriad of other issues due to their intrusive and ephemeral nature (Webb).

The source of all of this information about coasts and estuaries and tectonic plates can all be tied back to one thing, the sediments themselves that form all of them. Much like what is made from them, these sediments have several ways they can be classified into different groups, and one of those methods is through size. This categorization starts with "clay" at between 0.001 and 0.004 millimeters and goes up through "silt", "sand", "pebbles", and finally to "boulders" which measure between 84 and 256 millimeters. Another way these sediments can be categorized is through their "source" of which there are four agreed-upon types. The first of these is lithogenous sediments, which are formed through the erosion process on land, can be of any grain size, and are part of the terrigenous bio-group. These sediments can be found littering the ocean floor after they are swept into the water by winds, and examples of it include red clay and quartz. The second of the source classifications is biogenous sediments, which are formed from the remains of animals that have not fully dissolved, can be of various sizes, and are part of the calcareous bio-group. These sediments are most often found in biological hotspots such as reefs, and the presence of coral in these areas also adds to its number. Examples of biogenous sediments include teeth, shells, corals, and bones. The third type of source sediment is hydrogenous sediment, which is formed out of natural chemicals and minerals from the water, is mostly found in smaller sizes, and is part of the evaporites bio-group. These sediments are most often found on the ocean floor in manganese modules and near hydrothermal vents, examples include the aforementioned manganese modules and even materials such as salt. The fourth category is cosmogenous sediment, which is carried to the earth from space in the form of meteors, is usually on the larger side, and has two bio-groups, those being tektites and spherules. Both groups are exceedingly rare on the ocean floor, with larger spherules being composed of various common metals such as iron and nickel and smaller tektites being made of glass (3.1: Sources and Types of Marine Sediment). Those were the processes of how individual grains of sediment are identified, but there are also two categories for whole buildups of sediment. These types are defined by their location and also through what kinds of the previous four sediments are present within them. Pelagic sediments are the first of these and tend to be far away from coastlines, depositing slowly and covering around 75% of the ocean floor. Due to their location and finer grains, pelagic sediments are mostly composed of biogenous sediments. Neritic sediments are the second of the categories, are usually found close to coastlines, and take up the rest of the seafloor not taken by pelagic sediments. Due to being right next to eroding coastlines, neritic sediments are mostly comprised of lithogenous sediments and other unrefined grains (6.5: Neritic and Pelagic Sediments). There is a lot to be learned from every type of sediment dotting the ocean floor, but nothing can be learned if the right people have none to study with. Over the years scientists have come up with improved ways to study these sediments and found out what they can tell us about the world. The biggest example of what sediments have told us is the existence of previous life on earth before humankind. Sediment traps anything buried in it when more of it is stacked on top and has preserved countless fossils for us to discover (Colson). Alongside these fossils, sediments also trap oil from long-dead organisms that is ripe to be harvested. Without these sediments, the industrial revolution might have never happened, and the dinosaurs would never be discovered.

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