question: If the continents used to all be connected millions of years ago, how did the deep underwater trenches between their separation happen?
comment: This is a bit complicated, so here's a brief Geology 101 lesson. There are areas called Mid-Ocean Ridges. MORs are areas where the ocean crust thins and spreads apart on the borders of convective cells in the mantle. This creates a lower pressure area in the mantle, and since the temperature remains the same while the pressure decreases you get melt inside the mantle that rises to the surface where it erupts creating new seafloor. Oceanic Crust and Continental Crust have vastly different chemical compositions. Oceanic Crust has a lot of heavier elements like Iron and Magnesium in it, while Continental Crust is lighter being made up of elements like Silicon Oxides. This means that Ocean Crust is heavier and is overridden by continental crust. This is why where ocean plates meet continental plates the ocean crust sinks creating volcanoes inland on the continental crust. Where continental crust meets continental crust though you get huge mountain ranges like the Himalayas. Ocean Crust is hottest closest to the MORs, and hot things are more buoyant than colder things, so when ocean crust meets ocean crust, the older ocean crust will sink below the newer, warmer ocean crust and in these places you get oceanic trenches like the Marianas Trench.
passages: A similar relationship between proximity to rivers, forearc width, and trench morphology can be observed from east to west along the Alaskan-Aleutian convergent margin. The convergent plate boundary offshore Alaska changes along its strike from a filled trench with broad forearc in the east (near the coastal rivers of Alaska) to a deep trench with narrow forearc in the west (offshore the Aleutian islands). Another example is the Makran convergent margin offshore Pakistan and Iran, which is a trench filled by sediments from the Tigris-Euphrates and Indus rivers. Thick accumulations of turbidites along a trench can be supplied by down-axis transport of sediments that enter the trench away, as is found for the Peru–Chile Trench south of Valparaíso and for the Aleutian Trench.
   Section::::History.:"Masterpiece Theatre" (2009–2011).
   Section::::Morphologic expression.:Filled trenches.
  
  The composition of the inner trench slope and a first-order control on trench morphology is determined by sediment supply. Active accretionary prisms are common in trenches near continents where rivers or glaciers supply great volumes of sediment to the trench. These filled trenches may lack the bathymetric expression of a trench. The Cascadia margin of the northwest USA is a filled trench, the result of sedimentation by the rivers of the western United States and Canada.
   Oceans between continents provide barriers to plant and animal migration. Areas that have become separated tend to develop their own fauna and flora. This is particularly the case for plants and land animals but is also true for shallow water marine species, such as trilobites and brachiopods, although their planktonic larvae mean that they were able to migrate over smaller deep water areas. As oceans narrow before a collision occurs, the faunas start to become mixed again, providing supporting evidence for the closure and its timing.
  
  Section::::Other evidence for past plate configurations.:Orogenic belts.
   The major geographical changes during this time period included the emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding (and return to salt water) by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and the European mainland; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water.
   Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper... (from thermal contraction and subsidence).
   The dense oceanic plate has a high tendency to sink. As it sinks, it breaks along the oceanic plate and the welded crust above and a gap is created. The extra space created leads to the decompression melting of mantle wedge materials. The melts flow upward and fill the gap and intrude the oceanic plate and welded crust as mafic dykes intrusion. Eventually, the oceanic plate completely breaks apart from the welded crust as it continues to sink. 
  
  Section::::Evolution of divergent double subduction system.:Final stage.
   The northern supercontinent of Laurasia began to break up, as Europe, Greenland and North America drifted apart. In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts. In Europe, the Tethys Sea finally vanished, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. India continued its journey away from Africa and began its collision with Asia, creating the Himalayan orogeny.
   At divergent boundaries, two plates move away from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.
   There are approximately of convergent plate margins, mostly around the Pacific Ocean—the reason for the reference “Pacific-type” margin—but they are also in the eastern Indian Ocean, with relatively short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea. Globally, there are over 50 major ocean trenches covering an area of 1.9 million km or about 0.5% of the oceans. Trenches that are partially infilled are known as "troughs" and sometimes they are completely buried and lack bathymetric expression, but the fundamental plate tectonics structures that these represent mean that the great name should also be applied here. This applies to the Cascadia, Makran, southern Lesser Antilles, and Calabrian trenches. Trenches along with volcanic arcs and zones of earthquakes that dip under the volcanic arc as deeply as are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. Trenches are related to but distinguished from continental collision zones (such as that between India and Asia forming the Himalaya), where continental crust enters a subduction zone. When buoyant continental crust enters a trench, subduction eventually stops and the area becomes a zone of continental collision. Features analogous to trenches are associated with collisions zones, including sediment-filled foredeeps, such as those the Ganges River and Tigris-Euphrates rivers flow along.
   The lithosphere, which rides atop the asthenosphere, is divided into a number of tectonic plates that are continuously being created and consumed at their opposite plate boundaries. Oceanic crust and tectonic plates are formed and move apart at mid-ocean ridges. Abyssal hills are formed by stretching of the oceanic lithosphere. Consumption or destruction of the oceanic lithosphere occurs at oceanic trenches (a type of convergent boundary, also known as a destructive plate boundary) by a process known as subduction. Oceanic trenches are found at places where the oceanic lithospheric slabs of two different plates meet, and the denser (older) slab begins to descend back into the mantle. At the consumption edge of the plate (the oceanic trench), the oceanic lithosphere has thermally contracted to become quite dense, and it sinks under its own weight in the process of subduction. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. The overall process of repeated cycles of creation and destruction of oceanic crust is known as the Supercontinent cycle, first proposed by Canadian geophysicist and geologist John Tuzo Wilson.
   The depth of the ocean basins is a function of the age of oceanic lithosphere (the tectonic plates beneath the floors of the world's oceans). As older plates age, they become denser and sink, allowing newer plates to rise and take their place. Therefore, a configuration with many small oceanic plates that rapidly recycle the oceanic lithosphere would produce shallower ocean basins and (all other things being equal) higher sea levels. A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, would result in deeper ocean basins and lower sea levels.
   Section::::Structural features.:Unconformity.
  
  When the two overriding plates collide and the ocean basin is closed, sedimentation ceases. Sinking of the oceanic plate drag down the welded crust to form a basin that allows continued sedimentation. After the oceanic plate completely detaches from the crust above, isostatic rebound occurs, leaving a significant unconformity in the sedimentary sections. 
  
  Section::::Factors controlling the evolution of divergent double subduction system.
   In contrast to the persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle.
   There are three types of lithospheric plate boundaries: 1.) divergent (where lithosphere and oceanic crust is created at mid-ocean ridges), 2.) convergent (where one lithospheric plate sinks beneath another and returns to the mantle), and 3.) transform (where two lithospheric plates slide past each other).
   A well documented example of geodispersal in between continental ecosystems was the Great American Biotic Interchange (GABI) between the terrestrial faunas and floras of North America and South America, that followed the formation of the Isthmus of Panama about 3 million years ago. Between 69 and 47 million years ago, the Thulean Land Bridge facilitated gene flow by allowing bees from the Old World to travel to the New World, an example of geodispersal from the Old World to the New World. Another example was the formation of the modern Amazon River Basin about 10 million years ago, which involved the merging of previously isolated Neotropical fish faunas to form what is now the most species-rich continental aquatic ecosystem on Earth (Oberdorff et al., 2011).
   Throughout the Cenozoic (66 million to the dawn of the Neogene, 23 Mya), a combination of the northern migration of Africa and global sea levels fall, lead eventually to the connections between the Atlantic and Indian Oceand across the Tethys to closed off in what is now the Middle East during the Miocene. This decupling occured in two steps, first around 20 Mya and another around 14 Mya.
   In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen) and the other two continue spreading to form an ocean. The opening of the south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea, from where it continued to the west. The NE-trending Benue Trough is the failed arm of this junction.
   During break-up of the supercontinent, rifting environments dominate. This is followed by passive margin environments, while seafloor spreading continues and the oceans grow. This in turn is followed by the development of collisional environments that become increasingly important with time. First collisions are between continents and island arcs, but lead ultimately to continent-continent collisions. This is the situation that was observed during the Paleozoic supercontinent cycle and is being observed for the Mesozoic–Cenozoic supercontinent cycle, still in progress.
  
  Section::::Relation to climate.
   At about 10,000 BP, the exit continued to rise and the lake/sea broke through Denmark creating the first Great Belt channels. The total opening was less than 1 km wide and included two channels at the northern end. The Great Belt channels was blocked again by rising land from the post-glacial rebound that created Ancylus Lake.
   BULLET::::- Tongue of the Ocean – A deep oceanic trench in the Bahamas between Andros and New Providence islands
  
  BULLET::::- Oceanic trenches of the Indian Ocean
  
  BULLET::::- Diamantina Deep – Part of the Diamantina Trench southwest of Perth, Western Australia
  
  BULLET::::- Diamantina Fracture Zone – An escarpment, separating two oceanic plateaus in the southeast Indian Ocean
  
  BULLET::::- Sumatra Trench – Subduction trench in the Sumatra-Andaman subduction zone in the eastern Indian Ocean
  
  BULLET::::- Sunda Trench – An oceanic trench in the Indian Ocean near Sumatra where the Australian-Capricorn plates subduct under a part of the Eurasian Plate.
   BULLET::::2. "Divergent boundaries (Constructive)" occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise), and continent-to-continent rifting (such as Africa's East African Rift and Valley and the Red Sea), are examples of divergent boundaries.
   Earthquakes are common along convergent boundaries. A region of high earthquake activity, the Benioff zone, generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670  km along the Benioff. Both compressional and extensional forces act along convergent boundaries. On the inner walls of trenches, compressional faulting or reverse faulting occurs due to the relative motion of the two plates. Reverse faulting scrapes off ocean sediment and leads to the formation of an accretionary wedge. Reverse faulting can lead to massive earthquakes, such as the magnitude 9.1 Sumatra earthquake of 2004.  Tensional or normal faulting occurs on the outer wall of the trench, likely due to bending of the down going slab.
   In western Laurentia (North America), a tectonic episode that preceded this rifting produced failed rifts that harboured large depositional basins in Western Laurentia. The global ocean of Mirovia, an ocean that surrounded Rodinia, started to shrink as the Pan-African ocean and Panthalassa expanded.
  
  Between 650 million and 550 million years ago, another supercontinent started to form: Pannotia, which was shaped like a "V". Inside the "V" was Panthalassa, outside of the "V" were the Pan-African Ocean and remnants of the Mirovia Ocean.
  
  Section::::Reconstruction of ocean basin.
   The opposite of transgression is regression, in which the sea level falls relative to the land and exposes former sea bottom. During the Pleistocene Ice Age, so much water was removed from the oceans and stored on land as year-round glaciers that the ocean regressed 120 m, exposing the Bering land bridge between Alaska and Asia.
  
  Section::::Characteristic facies.
  