OC103 Lesson #5: The Shape of the Ocean Floor Look at the surface of Earth with the water drained away (as in the map below), and you will notice that about 70% of Earth’s surface is at conspicuously low elevations, and is covered by seawater to make the ocean basins, while about 30% of the surface is at much higher elevations, and makes up the continents. You will also notice that the seafloor is not flat in many areas; there are mountain ranges and other features. This lesson covers why there are low areas on Earth that can be filled by water to create the ocean basins, and describes some of the bathymetric features that are found on the ocean floor and how they formed.The Surface of Earth One of the most obvious features on that ocean floor map is the sharp contrast between the elevated continents and the depressed ocean basins. With the exception of some narrow shallow areas (shown in orange along the margins of the continents) that are underwater now, but have been exposed in the past when sea level was lower, Earth’s surface is distinctly divided into two areas: continents that are elevated, and project well above sea level; and ocean basins that are mostly far below sea level and filled in with seawater. We essentially only have one or the other – continents or deep oceans, and not much middle ground. What causes this stark difference in elevation between continental and oceanic areas? Remember last time we showed how the outer layer of Earth is a rocky crust made up of the lighter components of the primordial Earth, and that the crust has a range of thicknesses, from as thin as 3 km in some areas to as thick as 50 km in others? Those variations in thickness also explain the variations in height of the Earth’s surface. Continental crust is much thicker (30–50 km thick) than oceanic crust (3–10 km thick), and therefore projects to higher elevations. Geophysicists can actually determine the thicknesses of the crust at different places by measuring how high-energy sound waves travel through and bounce back from different layers in the crust (see figure below; usually the “sound” source for one of these geophysical experiments is a controlled explosion; pretty extreme, but effective at penetrating rock layers).The Crust: Continents vs Oceans These geophysical experiments showed that the oceanic crust is much thinner than the continental crust, and is also slightly denser (see figure below, showing a side view of the differences between continental and oceanic crust). Next week we will cover how these two different types of crust form, and it will become obvious why they are so different, but for the moment just picture continental crust as a crumpled heap of rocks and sediments that accumulated over many millions of years, whereas oceanic crust is a thin skin of volcanic rock that formed relatively quickly. Notice in the figure below that the continental crust not only projects up to higher elevations, but also has a deep “root” below it. This deep root was obvious as soon as geophysicists figured out how to measure the thickness of the crust, and fundamentally changed our understanding of the interior of Earth.Isostasy and the Continental “Root” If Earth’s mantle was solid, immobile rock, then the thick continental crust should rest on top of it like a block of wood on a table, and the base of the continental crust should be even with the base of the oceanic crust. Picture a large and a small block of wood set on a table: the bases of the two wooden blocks are at the same level because they are both resting on the table, but the top of the larger block sticks up higher because it is larger. That is not what we see with Earth’s crust though. The bases of the two different crusts are not at the same level as if they were resting on solid rock below. Instead, Earth’s crust acts more like blocks of wood that are floating in water (see figure below). The top of the large block of wood still projects above the smaller block, but the larger block also has a much deeper “root” below it, exactly as we observe in continental crust (compare blocks of wood and icebergs in figure below to crust in figure on previous slide). This fundamental observation led scientists to conclude that Earth’s mantle must not be an immobile solid, but is actually able to flow and move slowly, so that the crust was, in effect, "floating" on top of the mantle. The mass of topography is buoyantly supported by the mass of a “root” below, whether the floating object is a block of wood, an iceberg, or the Earth’s crust. The exact balance between the mass projecting above and the mass buoyantly holding it up from below is called isostasy, and can be carefully calculated using the formula shown in the figure below (although for this class we are far more interested in the principle than the formula). So the difference in the thicknesses of oceanic versus continental crust explains why we have low-lying areas on Earth that can be filled in by the oceans, and why we also have high-standing areas where we can live more-or-less free from crashing waves and wandering jellyfish. We will come back to the geologic explanation for how these two different types of crust form in a later lesson. For now let's continue with a description of some of the geologic features we observe on the seafloor.Seafloor Features Another thing you will notice on the global map is that the ocean floor is fairly flat in some places, but in many other places it is not: there are mountain ranges, trenches, and miscellaneous bumps. (Remember that on this map, blue and purple areas are deepest, green and yellow areas are shallower, and orange areas are shallowest.) We will describe some of those features now, and over the next few lessons provide explanations for how they formed. Although there are a variety of seafloor features, right now we are only concerned with the major ones because they are important pieces of the puzzle for how oceanic crust forms, and are also important for determining what habitats are available for many of the marine organisms we will cover later in the term. I will point out where these features are visible on the global map below, and idealized versions of them are included in the drawings in the figures on the following slides.Continental Margin Continental Margins are the areas surrounding most parts of the continents on the map. These shallow areas often