Describe different geological processes in relation to plate tectonics.

This section demonstrations the implications of plate tectonics. You will be introduced to various formations and features that are attributed to plate tectonics.

Supercontinents

Early Continents

How did the first continents form?
We think something like modern plate tectonics occurred on the early earth.  Perhaps there were differences in overall planetary heat flow, and consequently differences in crustal thickness and plate velocities, but we do think that the early earth had upwelling asthenosphere and associated mid-ocean spreading centers.  We do think that the first crust was composed of basaltic rock, like the current ocean crust.

When this basaltic crust underwent partial melting (of its lower portion), over 4 billion years ago, the result was the generation of more silica rich continental crust!

Craton

Figure 1. Ice age glaciers scraped the Canadian Shield down to the 4.28 billion year old greenstone in Northwestern Quebec.

The earliest felsic continental crust is now found in the ancient cores of continents, called the cratons. Rapid plate motions meant that cratons experienced many continental collisions. Little is known about the paleogeography, or the ancient geography, of the early planet, although smaller continents could have come together and broken up.

Geologists can learn many things about the Pre-Archean by studying the rocks of the cratons.

• Cratons also contain felsic igneous rocks, which are remnants of the first continents.
• Cratonic rocks contain rounded sedimentary grains. Of what importance is this fact? Rounded grains indicate that the minerals eroded from an earlier rock type and that rivers or seas also existed.
• One common rock type in the cratons is greenstone, a metamorphosed volcanic rock (Figure 1). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones.

Shield

Places the craton crops out at the surface is known as a shield. Cratons date from the Precambrian and are called Precambrian shields. Many Precambrian shields are about 570 million years old (Figure 2).

Figure 2. The Canadian Shield is the ancient flat part of Canada that lies around Hudson Bay, the northern parts of Minnesota, Wisconsin and Michigan and much of Greenland.

Platform

In most places the cratons were covered by younger rocks, which together are called a platform. Sometimes the younger rocks eroded away to expose the Precambrian craton (Figure 3).

Figure 3. The Precambrian craton is exposed in the Grand Canyon where the Colorado River has cut through the younger sedimentary rocks.

Early Convection

During the Pre-Archean and Archean, Earth’s interior was warmer than today. Mantle convection was faster and plate tectonics processes were more vigorous. Since subduction zones were more common, the early crustal plates were relatively small.

Since the time that it was completely molten, Earth has been cooling. Still, about half the internal heat that was generated when Earth formed remains in the planet and is the source of the heat in the core and mantle today.

The presence of water on ancient Earth is revealed in a zircon crystal:

https://youtube.com/watch?v=V21hFmZP5zM%3Fenablejsapi%3D1

Supercontinent Cycle

Look at Figure 4; is this Earth?

Figure 4. Rodinia

The existence of Wegener’s supercontinent Pangaea is completely accepted by geologists today. The movements of continents explain so much about the geological activity we see. But did it all begin with Pangaea? Or were there other supercontinents that came before? What does the future of the continents hold?

Pangaea

Figure 5. Pangea broke up to become our modern continents.

Wegener had lots of evidence for his continental drift hypothesis. One line of evidence was the similarity of the mountains on the west and east sides of the Atlantic. Those mountains rose at convergent plate boundaries. The continents on both sides of the ocean (where the Atlantic is now) smashed together to create Pangaea. The proto-Atlantic ocean shrank as the Pacific Ocean grew.

The Appalachian mountains of eastern North America formed at this convergent plate boundary (Figure 6a). About 200 million years ago, they were probably as high as the Himalayas.

Figure 6. (a) The Appalachian Mountains in New Hampshire. (b) The Appalachians along the eastern U.S. These mountains began when North America and Eurasia collided as Pangaea came together.

Summary

• The ancient core of a continent, at and beneath the surface, is its craton.
• The cratonic rock that is seen at the surface is called the shield. Where the shield is covered by younger sediments is the platform.
• Convection on early Earth was faster and so plate tectonics was faster. Since then, Earth has been cooling.
• Pangaea came together as a set of continent-continent convergent plate boundaries.
• Pangaea is still breaking up as the continents move apart. The Atlantic Ocean is getting bigger, and the Pacific Ocean is getting smaller.
• The continents come together and break apart about every 500 million years. This is called the supercontinent cycle.

Volcanic Arcs

All subduction zones have, at some distance in from the edge of the upper plate, arcs or chains of composite cone volcanoes. The subducting plate, as it goes down deep into the mantle, releases water. This changes the chemistry of the already hot rocks in the mantle and causes them to melt, forming magma. The magma is less dense than the solid rocks around it, so it rises upward, culminating in volcanic eruptions at the earth’s surface.

The volcanic arc at an ocean-continent subduction zone is not only a chain of volcanoes. The stress of plate convergence compresses the crust there, causing it to thicken through a combination of folds and thrust faults. Igneous intrusions and volcanic eruptions also thicken the crust there. Deep within the crust, the igneous intrusions solidify into batholiths of rocks such as granite, and the pre-existing rocks that are intruded by the batholiths are regionally metamorphosed into new rocks. The result is a high mountain range with granitic and metamorphic rock at its core, folded and faulted sedimentary and volcanic around its margins, and a chain of composite cone volcanoes distributed along the crest of the range.

Ocean Basins

This video provides an introduction to the Wilson cycle, a theory that describes the lifecycle of the ocean basin:

As you can see, it is very common for oceans to form and then disappear. Continental rifting plays a key role in the formation of an ocean. But how did the oceans form in the first place? Watch the following video for some current theories:

https://youtube.com/watch?v=hwVU0-2Qnso%3Fenablejsapi%3D1

The low lying areas mentioned in the video are the ocean floors. Oceanic crust is basalt and continental crust is composed granite. Basalt is heavier so when they both rest on the top of the mantle, oceanic crust sinks lower forming a basin for the water to drain into.

Hot Spots

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. They may be on, near to, or far from tectonic plate boundaries. Currently, there are two hypotheses that attempt to explain their origins. One suggests that they are due to hot mantle plumes that rise as thermal diapirs from the core-mantle boundary. An alternative hypothesis postulates that it is not high temperature that causes the volcanism, but lithospheric extension that permits the passive rising of melt from shallow depths. This hypothesis considers the term “hotspot” to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well known examples include Hawaii and Yellowstone.

Background

The origins of the concept of hotspots lie in the work of J. Tuzo Wilson, who postulated in 1963 that the Hawaiian Islands result from the slow movement of a tectonic plate across a hot region beneath the surface. It was later postulated that hotspots are fed by narrow streams of hot mantle rising from the Earth’score-mantle boundary in a structure called amantle plume. Whether or not such mantle plumes exist is currently the subject of a major controversy in Earth science. Estimates for the number of hotspots postulated to be fed by mantle plumes has ranged from about 20 to several thousands, over the years, with most geologists considering a few tens to exist. Hawaii, Réunion, Yellowstone, Galápagos, and Iceland are some of the currently most active volcanic regions to which the hypothesis is applied.

Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, in which water is trapped under the overriding plate. Where hotspots occur in continental regions, basaltic magma rises through the continental crust, which melts to form rhyolites. These rhyolites can form violent eruptions. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history. However, when the rhyolite is completely erupted, it may be followed by eruptions of basaltic magma rising through the same lithospheric fissures (cracks in the lithosphere). An example of this activity is the Ilgachuz Range in British Columbia, which was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.

The hotspot hypothesis is now closely linked to the mantle plume hypothesis.

Comparison with island arc volcanoes

Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes. The latter form over subduction zones, at converging plate boundaries. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench. This plate, as it is subducted, releases water into the base of the over-riding plate, and this water mixes with the rock, thus changing its composition causing some rock to melt and rise. It is this that fuels a chain of volcanoes, such as the Aleutian Islands, near Alaska.

Hotspot volcanic chains

The joint mantle plume/hotspot hypothesis envisages the feeder structures to be fixed relative to one another, with the continents and seafloor drifting overhead. The hypothesis thus predicts that time-progressive chains of volcanoes are developed on the surface. Examples are Yellowstone, which lies at the end of a chain of extinct calderas, which become progressively older to the west. Another example is the Hawaiian archipelago, where islands become progressively older and more deeply eroded to the northwest.

Geologists have tried to use hotspot volcanic chains to track the movement of the Earth’s tectonic plates. This effort has been vexed by the lack of very long chains, by the fact that many are not time-progressive (e.g. the Galápagos) and by the fact that hotspots do not appear to be fixed relative to one another (e.g., Hawaii and Iceland.)

Postulated Hotspot Volcano Chains

Figure 7. Kilauea is the most active shield volcano in the world. The volcano has erupted nonstop since 1983 and it is part of the Hawaiian-Emperor seamount chain.

• Hawaiian-Emperor seamount chain (Hawaii hotspot)
• Louisville seamount chain (Louisville hotspot)
• Walvis Ridge (Gough and Tristan hotspot)
• Kodiak–Bowie Seamount chain (Bowie hotspot)
• Cobb-Eickelberg Seamount chain (Cobb hotspot)
• New England Seamount chain (New England hotspot)
• Anahim Volcanic Belt (Anahim hotspot)
• Mackenzie dike swarm (Mackenzie hotspot)
• Great Meteor hotspot track (New England hotspot)
• St. Helena Seamount Chain – Cameroon Volcanic Line (Saint Helena hotspot)
• Southern Mascarene Plateau–Chagos-Maldives-Laccadive Ridge (Réunion hotspot)
• Ninety East Ridge (Kerguelen hotspot)
• Tuamotu–Line Island chain (Easter hotspot)
• Austral–Gilbert–Marshall chain (Macdonald hotspot)
• Juan Fernández Ridge (Juan Fernández hotspot)

Intraplate Activity

A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line (figure 8). Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot.

Here is an animation of the creation of a hotspot chain.

Figure 8. The Hawaiian Islands are a beautiful example of a hotspot chain. Kilauea volcano lies above the Hawaiian hotspot. Mauna Loa volcano is older than Kilauea and is still erupting, but at a lower rate. The islands get progressively older to the northwest because they are further from the hotspot. Loihi, the youngest volcano, is still below the sea surface.

Geologists use some hotspot chains to tell the direction and the speed a plate is moving (figure 9).

Figure 9. The Hawaiian chain continues into the Emperor Seamounts. The bend in the chain was caused by a change in the direction of the Pacific plate 43 million years ago. Using the age and distance of the bend, geologists can figure out the speed of the Pacific plate over the hotspot.

Hotspot magmas rarely penetrate through thick continental crust. One exception is the Yellowstone hotspot (figure 10).

Figure 10. Volcanic activity above the Yellowstone hotspot on the North American Plate can be traced from 15 million years ago to its present location.