Learning Material

 

SCIENCE 10

 

LEARNING MATERIAL: TECTONIC PLATES

 

 

Prepared by:

BAÑAS, Margie M.

CARMELINA, Isaac Gainever M.

GUATNO, Aldrin R.

LLANDER, Divine Ruth M.

OMEREZ, John Gener H.

 

 

 

 

2024

PLATE TECTONICS

            Plate tectonics is a theory explaining how major landforms are created. In the 1960's, the theory was solidified, which transformed into explaining many phenomena such as mountain building events, volcanoes, and earthquakes. It also describes the interaction between the different layers of the Earth. Lithosphere, which is the outermost layer- is made up of the crust and mantle- is broken into large plates. Meanwhile, the plates lying on the top of a partially molten layer is the asthenosphere. The lithosphere and asthenosphere move relative to each other due to convection. This interaction is responsible for different geological formations such as the San Andreas Fault in California and the Himalaya mountain range in Asia.

            In 1912, German scientist Alfred Wegenes changed the scientific debate that continents moved over time. He published the concept about the continental drift. In this article, he suggested that 200 million years ago, Pangaea, which is a supercontinent, broke into pieces, and its parts were moving away from each other. The fragments of these supercontinents are the continents we have today. In order to support his claim, Wegener pointed to the matching rock formation and similar fossils found in Brazil and West Africa.

            In the 1950s and 1960s, the theory of continental drift was supported by new data. Harry Hess, an American geologist proposed that molten rock is rising from the asthenosphere resulting in the ridges to form. As it came to the surface, the rock cooled, making a new crust, and spreading the seafloor away from the ridge in a conveyer-belt motion. After a million years, the crust would disappear into ocean trenches at a place called the subduction zone. But there was one question with the plate tectonics theory:

Most volcanoes are found above subduction zones, but some form far away from these plate boundaries. How could this be explained?

            In 1963, a Canadian geologist, John Tuzo Wilson, answered this question by proposing the idea of volcanic island chains. It is like a Hawaiian island, as they are created by fixed “hot spots” in the mantle. At the mantle, the magma forces its way upward through the moving plate of seafloor as it moves to another hotspot, another volcanic island can be formed.

1.1 Distribution

1.1.1 Volcanoes

Volcanoes (relationship among the locations of volcanoes, earthquake, epicenters, and mountain ranges)

                        It is a feature in Earth’s crust where molten rock is squeezed out onto the Earth’s surface. Magma is the molten rock beneath the surface and it is called lava when it erupts, or flows out, from a volcano. Along with lava are the gasses, ash, and solid rock that are ejected on the Earth’s surface. The term volcano means that magma and other substances erupt on the surface, creating a landform of solidified lava and volcanic debris near the vent.

                        Volcanoes can be found throughout the world. It comes in many different sizes and shapes but cone-shaped hills or mountains are the most common. Moreover, there are about 1,900 active volcanoes on Earth, meaning they are showing some level of activity and are more likely to erupt again. They are also called dormant volcanoes; they are the ones that show no current signs of exploding but at some point, are more likely to become active again in the future. While others are considered extinct.

                        Volcanoes are an agent of change through volcanic eruption. They can create new landforms, but can also destroy everything. Volcanic eruption is a spectacular display of the Earth’s power. Yet, it can cause disastrous loss of life and property.

Active Volcanoes

-        Active volcanoes are mountains that have craters on their sides or summits that spew lava, rocks, ash, and other materials are known as active volcanoes. The movement of tectonic plates beneath the surface of the Earth is the cause of these volcanic activities.

Types of Volcanoes

Cinder cones – are steep, conical hills that have a prominent crater at the top. It is known as scoria cones. Scoria means having irregular-shaped, highly vesicular fragments of lava erupted in the air and are solid when they land. This type of volcano is surrounded by dark lava flows that erupt from their base. It frequently has an asymmetric shape in which it forms over a linear fissure vent and is elongated, and ones that are formed in areas with strong prevailing winds may be much taller on the downwind side. 

Composite Volcanoes (Stratovolcanoes) – it is the most picturesque among the types of volcanoes. It has a conical with a concave shape which is steeper near the top. They are active volcanoes for a long period, erupting periodically and generally, composed of lava flows, pyroclastic flows, mudflow deposits, and lava domes. They have multiple vents, but most of them have a main vent at the summit. Active composite volcanoes have a shallow magma chamber at depths of 3-6 miles (5-10 km). Thus, just like mountains, composite volcanoes are subjected to the forces of erosion. They may experience mass wasting events, including landslides, rock avalanches, lahars, and debris flows. In terms of its magma composition, it usually erupts a basalt to rhyolite, but intermediate (andesitic) and dacitic magmas are the most common.

Shield Volcanoes – are broad volcanoes with gentle slopes and are shaped somewhat like a warrior’s shield lying flat. Shield volcanoes have a convex shape as they are flatter near the summit. Also, they are constructed almost entirely of basaltic and/or andesitic lava flows which were very fluid when erupted and built by repeated eruptions over vast periods. The low viscosities of the erupted lavas create the great width of shield volcanoes relative to their height. It also produces thin widespread lava flows, eruptions from both the summit and fissure vents on the volcano’s flanks, and widening and subsidence along the summit and rift zones. Moreover, large shield volcanoes may have calderas that contain long-lasting lava lakes. They also have pit craters or smaller collapse structures, often with vertical sides. 

1.1.2 Earthquake Epicenters

How does an earthquake happen?  

            An earthquake is a shock wave from the underground that radiates to the Earth’s surface. It is caused by the sudden release of friction and pressure between tectonic plates. Earthquakes are a natural phenomenon that occurs frequently in certain areas of the world. Earthquakes cause a range of effects from unnoticeable, mild tremors to violent, prolonged shaking. The epicenter receives the most powerful shock waves and it is directly above the hypocenter. On the other hand, the hypocenter is the place in the underground where the earthquake begins.        

Earthquake Epicenters

        Earthquake is defined as the violent shaking of the ground which is produced by the sudden movement of rock materials below the Earth’s surface.

        When two blocks of the earth suddenly slip past one another, it results in an earthquake. The surface where they slip is called the fault or the fault plane. The hypocenter is the location below the earth’s surface where the earthquake starts. On the other hand, the epicenter is the location directly above the surface of the earth.

                        There are different shocks of an earthquake. The foreshocks, mainshocks, aftershocks. A foreshock is a smaller earthquake that happens in the same place as the larger earthquake that follows. Meanwhile, the mainshock is the largest, main earthquake that can destroy infrastructures and properties. On the other hand, the aftershock is what follows after the mainshock. Aftershocks can continue for weeks, months, and even years after the mainshock. There are two ways in which we can measure the strength of an earthquake, it is the use of magnitude and intensity. Magnitude is the proportional energy released by an earthquake at the location. A seismograph is the instrument used in measuring the magnitude of an earthquake. Intensity, on the other hand, is the strength perceived and felt by people in a certain locality. It is a numerical rating represented by Roman Numerals that are based on the relative effects on people, the environment, and even the structure of the surroundings. The intensity of an earthquake is generally higher when it is near the epicenter.

Two Types of Earthquakes

          Tectonic Earthquake – this kind of earthquake occurs at plate boundaries. It happens when the large, thin plates of the Earth’s crust and upper mantle become stuck as they move past one another. As they lock together, pressure builds up. When they release, earthquakes occur. When two plates push into each other, they form a convergent plate boundary.

       Volcanic Earthquake - it is much smaller than a tectonic earthquake and results from tectonic forces that occur in conjunction with volcanic activity.

1.1.3 Mountain Ranges

Major Mountain Belts (Margie)

Mountain belts differ from one another, but they also have a number of similarities that enable Earth scientists to group them into certain distinct categories.

Himalayas: A stunning mountain range spanning multiple nations in South Asia, including China, Nepal, Bhutan, and India. It is the result of the Indian and Eurasian plates colliding. The highest point on Earth is Mount Everest..

Andes: The South American Plate was subducted beneath the Nazca Plate, resulting in their creation. It is renowned for its deep valleys, active volcanoes, and stunning scenery. It extends through countries including Peru, Bolivia, Chile, and Colombia along South America's western coast.

Rocky Mountains: The North American Plate's uplift is what formed it. It passes through most of the United States and Canada as it travels across North America. Their breathtaking national parks, alpine lakes, and varied travel options have made them well-known.

Alps: The African Plate colliding with the Eurasian Plate shaped them. The alpine regions of the world provide stunning scenery, snow-covered peaks, and top-notch skiing areas. They are a well-known mountain range in Europe that is spread throughout France, Switzerland, Italy, Austria, and Germany, among other nations.

Andean Volcanic Belt: There are many active volcanoes there, and it runs parallel to the Andes. This stretches from southern Chile through Peru and Ecuador and is caused by the subduction of the Nazca Plate beneath the South American Plate.

Japanese Archipelago: It arises from the Pacific Plate and Eurasian Plate convergent boundaries. It is made up of several volcanic islands in the Pacific Ocean, including the well-known Mount Fuji in Japan.

1.2 Plate Boundaries

            Plate Tectonic Boundaries

            Tectonic plates are also known as lithospheric plates that fit together on the Earth’s surface like a jigsaw puzzle. It is believed by the scientists that the plates float on the asthenosphere which is a hot, semi-solid region of the mantle. The movement is called plate tectonics. The movements are easily observed at the plate boundaries, where the plate converges, diverges, or slips sideways. It is also near or along lithospheric plate boundaries where earthquakes or volcanoes occur.

            Convergent Plate Boundaries - it is where two plates converge or collide into each other. It is sometimes called subduction zones, because the heavier, denser plate pushes beneath the lighter plate in a process called subduction. Subduction zones are associated with strong earthquakes and spectacular volcanic landscapes. A direct result of plate convergence and subduction is the Ring of Fire which is around the margins of the Pacific Ocean. Moreover, continental plates with similar density collide and neither is enough to create a subduction zone. This created the Himalayan Mountains where the brittle crust of the continental plate folds up and splinters as the plates collide.

            Divergent Plate Boundaries - it is where the lithospheric plates are moving away, or diverging from each other under the sea. Divergent boundaries create a new crust through a form of volcanism, in contrast to convergent boundaries that destroy old crust by subduction. When the plates move apart, magma wells up from beneath the surface to fill the spaces left by the diverging plates. This magma rises and cools in a continuous process forming mid-ocean ridges. Mid-ocean ridges are chains of volcanic mountains and rift valleys. The process of oceanic spreading occurs when the magma cools and forms a new crust, and it pushes the plates apart.

            Transform Plate Boundaries - it is sometimes called conservative boundary. It is because the crust is neither created nor destroyed at the boundary. This occurs in a region where plates are sliding horizontally past each other. It is typically found on the ocean floor but occasionally occurs on land. The San Andreas fault in California is the most visible manifestation of transform boundary movement. Earthquakes in this boundary are generally shallow. They are caused by the accumulation and sudden release of stress and tension as the plates slip past each other.

1.3 Internal structure of the Earth

Internal Structure of the Earth

Crust and Lithosphere

-        Earth’s outer surface is its crust; a cold, thin, brittle outer shell made of rock. The crust is very thin, relative to the radius of the planet. There are two very different types of crust, each with its own distinctive physical and chemical properties.

Oceanic crust is composed of magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro. Sediments, primarily muds and the shells of tiny sea creatures, coat the seafloor. Sediment is thickest near the shore where it comes off the continents in rivers and on wind currents.

Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic igneous rocks of the oceanic crust. Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planet’s oceans.The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick. The definition of the lithosphere is based on how earth materials behave, so it includes the crust and the uppermost mantle, which are both brittle. Since it is rigid and brittle, when stresses act on the lithosphere, it breaks. This is what we experience as an earthquake.

Mohorovičić Discontinuity or Moho - it is the substantial change in seismic velocity at the base of the crust. In 1909, it was discovered by Andrija Mohorovičić by studying the earthquake wave paths in Croatia. Underneath the ocean, the Moho is about 5 km down. On the other hand, it is about 30-40 km under continents, except near a sizable mountain-building event which is known as an orogeny, in which the thickness doubled.

Mantle

-        The two most important things about the mantle are: (1) it is made of solid rock, and (2) it is hot. Scientists know that the mantle is made of rock based on evidence from seismic waves, heat flow, and meteorites. The properties fit the ultramafic rock peridotite, which is made of the iron- and magnesium-rich silicate minerals. Peridotite is rarely found at Earth’s surface.Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties.

Core

-        At the planet’s center lies a dense metallic core. Scientists know that the core is metal for a few reasons. The density of Earth’s surface layers is much less than the overall density of the planet, as calculated from the planet’s rotation. If the surface layers are less dense than average, then the interior must be denser than average. Calculations indicate that the core is about 85 percent iron metal with nickel metal making up much of the remaining 15 percent. Also, metallic meteorites are thought to be representative of the core.If Earth’s core were not metal, the planet would not have a magnetic field. Metals such as iron are magnetic, but rock, which makes up the mantle and crust, is not. Scientists know that the outer core is liquid and the inner core is solid because S-waves stop at the inner core. The strong magnetic field is caused by convection in the liquid outer core. Convection currents in the outer core are due to heat from the even hotter inner core. The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core.

1.4 Mechanism (Possible causes of movement)

Possible Causes of Plate Movement

 Convection in the Mantle

-        Convection is the term used to describe the heat transfer that occurs when magma moves. It is thought that the Earth's radioactive element decay serves as the convection's heat source.

Ridge Push

-        It is essential to the movement of plates. Magma rises and forces the plates apart at mid-ocean ridges, where new crust is forming. New oceanic crust is created when plates are pushed away from the ridge by ridge-push forces.

Slab Pull

-        Slab-pull forces are the result of subduction, or the sinking of ancient oceanic crust at trenches. The motion downward is a part of the plate motion. The remainder of the plate is pulled down as one plate falls beneath another. 

 

1.5 Evidence of plate movement

Evidence that Support the Movement of Tectonic Plates

Complementary Coastlines:

-        Certain continents imply that they were formerly connected. For example, the western coast of Africa and the eastern coast of South America complement each other nicely. The hypothesis that these landmasses were formerly a part of a larger supercontinent is supported by this alignment. The theory of continental drift developed in large part because of this evidence.

Paleomagnetic Evidence:

-        Paleomagnetism is the study of ancient magnetic fields preserved in rocks. The magnetic minerals in rocks align with the Earth's magnetic field during their formation. Researchers found that the magnetic orientations of rocks from various continents did not correspond with their present locations. Rather, they lined up with opposing magnetic poles. For instance, polar wandering curves were clearly visible in the rocks of North America and Europe, indicating that these continents had moved in relation to one another over millions of years.

Distribution of Earthquake and Volcanoes:

-        The concentration of earthquakes and volcanoes along plate boundaries, such as mid-ocean ridges, subduction zones, and continental faults, lends credence to the theory of plate tectonics. These geological phenomena' alignment with plate boundaries suggests that tectonic plates are moving dynamically, which is strong evidence in favor of the theory of continental drift.

Ocean Floor Features:

-        Seafloor spreading, deep ocean trenches, and mid-ocean ridges form a characteristic pattern on the ocean floor. Underwater mountain ranges known as the mid-ocean ridges were created at divergent boundaries. when the older crust is pushed away from the ridge by a newly formed oceanic crust. Deep ocean trenches, meanwhile, are created where two plates converge and subduct beneath one another. The ocean's deepest regions are these trenches.

Plate Movements and Mountain Building:

-        Subduction, or the pushing of one plate beneath another, occurs when plates converge, causing severe deformation and uplift. Over millions of years, the tectonic plate movement gradually modifies the Earth's topography. Mountain ranges created by plate tectonics include the Himalayas, Andes, and Rockies. 

References:

Dastrup, A. (n.d.). The Composition and Structure of Earth | Physical Geography. https://courses.lumenlearning.com/suny-geophysical/chapter/the-composition-and-structure-of-earth/

Decker, B. B., & Decker, R. W. (2024, March 25). Volcano | Definition, Types, & Facts. Encyclopedia Britannica. https://www.britannica.com/science/volcano

Cinder Cones (U.S. National Park Service). (n.d.). https://www.nps.gov/articles/000/cinder-cones.htm

Composite volcanoes (Stratovolcanoes) (U.S. National Park Service). (n.d.). https://www.nps.gov/articles/000/composite-volcanoes.htm

Shield Volcanoes (U.S. National Park Service). (n.d.). https://www.nps.gov/articles/000/shield-volcanoes.htm

Types of volcanoes - Volcanoes, craters & lava flows (U.S. National Park Service). (n.d.). https://www.nps.gov/subjects/volcanoes/types-of-volcanoes.htm#:~:text=Each%20volcano%20is%20somewhat%20unique,stratovolcanoes)%2C%20and%20shield%20volcanoes.