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Content: Quick Review
In this lesson, we are going to hit the key parts of modern plate tectonics that we need for the rest of the semester. If you want a deeper dive into plate tectonics, consider taking geology courses.
Before we begin, let's quickly review what we have already covered:
- Earth's interior is hot- we are still losing heat from the formation of the planet, but new heat is produced through radioactive decay of elements.
- Earth's structure is layered. While the planet was molten, gravity formed a dense metallic core as iron sank to the center of the planet. Less dense material on the outside of the planet cooled forming the solid layers of the Earth's crust and mantle.
- The outer layer of Earth is broken into pieces that are moving.
Content: Earth's Layers
Let's look more closely at Earth's layers and develop a common vocabulary we can use. Two terms important to plate tectonics we have not used yet are lithosphere and asthenosphere.
The lithosphere is the outer rigid (strong solid) layer of the Earth that is broken into pieces called plates. The lithosphere contains all of the crust and the upper part of the mantle (note the bracket on the figure below).
The asthenosphere is a weak solid layer directly below the lithosphere. The asthenosphere can flow very, very slowly in convection cells. The asthenosphere is in the mantle but does not encompass the entire mantle.
Below the mantle if the liquid part of the Earth's core. The planets magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in this layer. Our magnetic field protects us from solar winds. It also plays an important role in unraveling our geologic history as we learned in the last lesson.
Content: The Crust
The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways.
Continental crust forms the land (the continents, as the name suggests) that we see today. Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick). Continental crust contains some of the oldest rocks on Earth. Continental crust is mostly composed of granite.
Oceanic crust is below the oceans. Oceanic crust is composed of basalt, a very dense type of rock. Compared to continental crust, oceanic crust is thin (6-11 km). The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.
Both types of crust float on the denser upper mantle (Density of crust = 2.7-2.9 g/cm3; Density of the upper mantle = 3.3 g/cm3). Because so much of the lithospheric plates is composed of crust, the plates float on the asthenosphere. The plates float at a depth where they displace an equal mass of asthenosphere. This buoyant equilibrium of flotation is called isostacy. If the mass of the plate changes with time, the plate will float higher or lower until equilibrium is restored. This principle is important to understand when considering relative sea level.
Content: Tectonic Plates
There are 52 separate lithospheric plates; 14 are major. The map below shows many of them. The boundaries between the plates are marked with the maroon colored lines. The plates are in motion and as they move they interact with one another. For that reason, plate boundaries are geologically active (remember the earthquake maps).
Content: Plate Motion
This text is edited from: http://www.columbia.edu/~vjd1/driving_forces_basic.htm
After the discovery of radioactivity by Marie Curie and Henri Becquerel in 1896 it became clear that the Earth's interior would be heated by radioactive decay in the mantle and crust. Rock is a poor conductor of heat, so if there were not some other mechanism to cool Earth's interior, it would be largely molten. However, growing amounts of data showed that the crust and mantle were solid. During the late 1920's a scientist named Holmes proposed that the interior of the planet was cooled by convection of the solid, ductile mantle.
You can see how the model Holmes proposed would work below. Heat from the core would warm the lower mantle. The heated material would expand, becoming less dense, and begin to rise toward the surface of the Earth. It would rise until its density matched that of the surrounding rock where it would start to spread out laterally. As it moves, it would cool and become more dense, sinking back toward the center of the Earth. The heat intensity in areas where the hot material rises melts the crust and magma erupts at the spreading center (mid-ocean ridge).
The concept of a convecting solid mantle wasn't very palatable to the scientific mainstream of the time. However, now we know convection almost certainly does occur in the mantle. Seismic tomography of the mantle shows areas of lower density (hotter) mantle rock which presumably are rising and other areas of higher density (not as hot) rock which should be sinking. But some tectonic plates are very wide, thousands of kilometers, and it doesn't seem plausible to have a convection cell that wide. In fact, other data indicates that there are multiple convection cells beneath the Pacific plate. The combined effect would not be one that would drive the motion of the plate - some cells would help, some would hinder.
So, if convection is not the main driver of plate tectonics, then what is? There are two, more modern competing theories of major tectonic forces: ridge push and slab pull. Recent research indicates that slab pull is the major driving force, but like convection, ridge push is probably playing a small role, so we will briefly explore both.
newly solidified material being attached to the end of each plate (remember the magnetic striping we discussed in the last lesson).
Mid-ocean ridges are also called spreading centers. Two are especially well studied: The East Pacific Rise and the Mid-Atlantic Ridge. Let's take a closer look at each.
Content: East Pacific Rise
The East Pacific Rise is a divergent boundary that extends southward from North America parallel to South America's west coast. The East Pacific Rise is classified as fast spreading (6 to 16 cm per year). Due to the fast spreading rates, there is no rift valley in the Pacific, just a smooth volcanic summit with a crack along the crest.
Images of the rise made from SONAR data show the gentle slopes on both sides of the spreading center.
Content: Mid Atlantic Ridge
The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean Basin, slowly spreading at a rate of 2 to 5 centimeters per year and forming a rift valley that is about the depth and width of the Grand Canyon. The slow spreading rates help to maintain steep slopes along the flanks of the boundary.
In Iceland, the ridge is exposed above sea level providing a glimpse into the rift.
Content: Activity 1
It is time to use that piece of paper and pencil. You will be making another drawing on the paper so use the space wisely.
Draw a cross section through a divergent plate boundary. Label it with the name of the boundary. Clearly label the crust, mantle, lithosphere and mantle. Use arrows to show the direction of plate movement.
Here is an example of a more complicated cross section that will give you an idea of what is expected. Yours should be simpler and restricted to the labels listed above.
When you are finished, you can move on to convergent boundaries, but keep track of your paper.
Content: Convergent Plate Boundaries
Convergent plate boundaries occur where two plates move toward each other. The impact of the colliding plates can cause crustal deformation where the edges of one or both plates buckle forming mountain ranges or one of the plates bends down into a deep seafloor trench. Chains of volcanoes often form parallel to convergent plate boundaries, and powerful earthquakes are common.
At convergent plate boundaries are defined by the types of crust that are interacting. Ocean-continent and ocean-ocean convergent boundaries always result in oceanic crust subducting down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into granite, the rock
that makes up the continents. Thus, at convergent boundaries, continental crust is created and oceanic crust is destroyed. At continent-continent boundaries, enormous mountain ranges are uplifted.
Next, we'll briefly look at each type more closely.
Content: Oceanic-Continental Convergence
This text is modified from: https://geology.com/nsta/convergent-plate-boundaries.shtml
When continental and oceanic plates collide, the thinner and more dense oceanic plate is overridden by the thicker and less dense continental plate. The oceanic plate is forced down into the mantle in a process known as "subduction." As the oceanic plate descends, it is forced into higher temperature environments. At a depth of about 100 miles (160 km), materials in the subducting plate begin to approach their melting temperatures and a process of partial melting begins.
This partial melting produces magma chambers above the subducting oceanic plate. These magma chambers are less dense than the surrounding mantle materials and are buoyant. The buoyant magma chambers begin a slow ascent through the overlying materials, melting and fracturing their way upwards. The size and depth of these magma chambers can be determined by mapping the earthquake activity around them. If a magma chamber rises to the surface without solidifying, the magma will break through in the form of a volcanic eruption.
The Washington-Oregon coastline of the United States is an example of this type of convergent plate boundary. Here the Juan de Fuca oceanic plate is subducting beneath the westward-moving North American continental plate. The Cascade Mountain Range is a line of volcanoes above the melting oceanic plate.
Content: Oceanic-Oceanic Convergence
This text is modified from: https://geology.com/nsta/convergent-plate-boundaries.shtml
When a convergent boundary occurs between two oceanic plates, one of those plates will subduct beneath the other. Normally the older plate will subduct because of its higher density. The subducting plate is heated as it is forced deeper into the mantle, and at a depth of about 100 miles (150 km) the plate begins to melt. Magma chambers are produced as a result of this melting, and the magma is lower in density than the surrounding rock material. It begins ascending by melting and fracturing its way through the overlying rock material. Magma chambers that reach the surface break through to form a volcanic eruption cone. In the early stages of this type of boundary, the cones will be deep beneath the ocean surface but later grow to be higher than sea level. This produces an island chain. With continued development the islands grow larger, merge, and an elongate landmass is created.
The Aleutian Islands in Alaska are examples of islands formed through this type of plate boundary. The frequent volcanic activity often causes disruption to commercial flight patterns.
Content: Continental-Continental Convergence
This text modified from: https://geology.com/nsta/convergent-plate-boundaries.shtml
Measured plate velocities are on the order of centimeters to 10s of centimeters per year. Plate velocities are often displayed on maps as vectors so the direction and speed of motion can be easily visualized. Three examples of maps are below. You can find many online using Google.
Content: Where are you headed?
Thought you must be finished with this lesson by now? You nearly are. Let's depart with a look at a few future predictions based on the plate motion we observe today. This figure is what NASA scientists predict the world will look in 50 million years:
- Atlantic will enlarge, Pacific will shrink
- New sea from East Africa rift valleys
- Further Himalaya uplift
- Separation of North and South America
- Part of California in Alaska
