Evidence of Plate Motions – Plate Tectonics and Theories

Evidence of Plate Motions

In this article, we’ll learn about evidence of plate motions with plate tectonics and theories. Let’s begin

Introduction

You have studied in previous lessons that lithospheric plates, or the solid part of the Earth, slowly move to form various geologic activities such as earthquakes, mountain structures, and volcanoes. But what is causing these plates to shift? What is the proof that the Earth’s plates are shifting?

These questions stimulated our scientists’ interest, resulting in the creation of the following concepts: Continental Drift theory, seafloor spreading, and magnetic reversal on Earth.

Do you ever wonder why Earth looks the way it does when you look at a globe? Why are large bands of land separated by oceans? Why is there not a single large land mass? Why are there different continents?

You will look for answers to these questions in this module. You will learn about Earth and how it has changed over time. You’ll also get a glimpse of how it might look in the future. One thing you will undoubtedly discover is that the Earth is constantly changing.

The Earth’s crust is made up of about fifteen massive slabs or plates. Some of the plates are separating. Others are slipping past one another. The plates can be found beneath both land masses and oceans. Continental plates are those found beneath the Earth’s surface.

Oceanic plates are those found beneath the oceans. One plate may disappear, and another may appear over time, causing the number of plates to change. However, the figure will not change during your lifetime.

These alterations take millions of years to happen.

Plate Tectonics

Plate tectonics is the formation and movement of plates. The term tectonic is derived from a Greek word that means “to construct.”

Plate tectonics refers to the fact that the Earth is made up of rock plates. These plates are very likely to have formed very early in Earth’s five-billion-year history.

One of the first ideas geologists had about how continents moved over time was continental drift. The science of plate tectonics has largely replaced the theory of continental drift.

Plate Movement Evidence: Continental Drift Theory

Continental Drift Theory

• In the 1920s, Alfred Wegener suggested the continental drift theory.
• According to continental drift theory, there was a single large landmass called Pangaea which means “all lands” in Greek, that was surrounded by a single large ocean called Panthalassa.
• Tethys Sea separated Pangaea into two massive landmasses: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys.

• Drift began around 200 million years ago (Mesozoic Era, Triassic Period, Late Triassic Epoch) when the continents began to separate and drift apart.
• Wegener became interested in the concept of drifting continents after noticing that the edges of South America and Africa in a World Map might be fitted together like a jigsaw puzzle.

Is the fitting of continent edges sufficient evidence that the continents are drifting? What other evidence has Wegener gathered to assist his continental drift theory?

Evidence for Continental Drift

• Alfred Wegener accumulated evidence to back up his theory, including geological “fit” and fossil evidence. It is important to note that Wegener did not present the following specific fossil evidence to support his theory. Wegener did not collect the fossils, but he did draw attention to the idea of using scientific documents stating that there were fossils of species present on different continents to support his claim.
• The matching of large-scale geological features on different continents is referred to as geological “fit” evidence.

It has been acknowledged that the coastlines of South America and West Africa appear to coincide, but the rocky terrains of the two continents confirm this as well.

The Appalachian Mountains of eastern North America are linked to the Scottish Highlands, the recognizable rock strata of South Africa’s Karroo system are correctly matched with the Santa Catarina system in Brazil, and the mountain ranges of Brazil and Ghana agree over the Atlantic Ocean.

Glaciers carve and leave marks on rocks as they move. Scientists can use this evidence to assess the direction of motion of each continent.

Furthermore, the presence of coal deposits in Antarctica indicated that it was previously located near an area of the planet where the climate is suitable for complex life forms such as plants and tall trees. Notice the direction of movement of the continents outlined by the arrows during Pangaea time in the figure below.

Take a look at the map below. It demonstrates how Alfred Wegener used biogeographic proof for continental drift and land bridging to map out the distributions of the four Permian and Triassic fossil groups.

Distribution of Fossils

• The fossil relevance is an important piece of evidence in the continental drift theory. There are numerous examples of fossils discovered on different continents and in no other regions. This suggests that these continents were once joined together because the vast oceans between them act as a type of barrier for fossil transfer. Mesosaurus, Cynognathus, Lystrosaurus, and Glossopteris (plant fossil) are four fossil examples.
• The discovery of Lemurs in India, Madagascar, and Africa prompted some to propose a contiguous landmass, “Lemuria,” linking these three continents.
• The Mesosaurus was a type of reptile, similar to the modern crocodile, that moved through the water with its long hind legs and limber tail. It lived between 286 and 258 million years ago, and its remains have only been discovered in South Africa and Eastern South America. Because the Mesosaurus was a coastal animal, it would be impossible for it to swim across such a large body of water as the Atlantic if the continents remained in their current positions.

Cynognathus, which translates to “dog jaw,” was a mammal-like reptile that is now extinct. The Cynognathus was as large as a modern wolf when it roamed the landscapes during the Triassic period (250 to 240 million years ago).

Its fossils can only be found in South Africa and South America. The Cynognathus, as a land-dominant species, would not have been able to migrate across the Atlantic.

The Lystrosaurus, which translates to “shovel reptile,” was thought to be a herbivore with a pig-like build. It grew to about one meter in length and was fairly dominant on land during the early Triassic period (250 million years ago).

Only Lystrosaurus fossils have been discovered in Antarctica, India, and South Africa. The Lystrosaurus, like the land-dwelling Cynognathus, would not have been able to swim across any ocean.

Glossopteris is the plant with the most significant fossil evidence. Glossopteris is the largest genus of extinct seed ferns and is named after the Greek word for tongue due to its tongue-shaped leaves.

Glossopteris first appeared during the early Permian period (299 million years ago) and remained the dominant land plant until the end of the Permian.

Glossopteris fossils can be found on all of the southern continents, as well as Australia, Antarctica, India, South Africa, and South America. Since the Glossopteris seed is big and bulky, it could not have floated or flown across the oceans to various continents.

Plate Movement Evidence: Seafloor Spreading Theory

The seafloor spreading theory will provide strong support for the idea that continents are drifting and will pinpoint the origin of plate movements.

Until World War II, when sounding equipment known as SONAR produced new evidence of what the seafloor looked like, the concept of continental drift circulated in scientific circles. The 1930s-era equipment bounced sound waves off the seafloor to assess its depth and characteristics.

Harry Hammond Hess, a geologist from Princeton University, was given command of one of the attack transport ships, the USS Cape Johnson. Hess, who was in his late thirties at the time, wanted to continue his scientific research even while at war.

As a result, he left his ship’s sounding gear on at all times, not just when approaching port or navigating a difficult landing. What Hess discovered took him by surprise.

What did Harry Hess and his men discover by chance while exploring the ocean floor? Were they able to pinpoint the origin of all movements on the Earth’s surface? Furthermore, did the Harry Hess team collect a lot of strong evidence to back up their claim that continents are moving?

Exploration of the ocean floor kept going, and by the 1950s, other researchers had discovered that a massive rift ran along the top of the Mid-Atlantic Ridge. Hess was able to understand his ocean floor profiles in the Pacific as a result of this.

He revealed that the seafloor was not as smooth as expected but was filled with canyons, trenches, and volcanic sea mountains. He noticed that the Earth’s crust had been moving further away on each side of oceanic ridges, down the long and volcanically active Atlantic and Pacific oceans.

Harry Hess discovered that the rate of creation of new seafloor at the mid-ocean ridge is not always the same as the speed of destruction of old seafloor at the subduction zone. This tries to explain why the Pacific Ocean is shrinking and the Atlantic Ocean is growing wider.

The ocean keeps shrinking if the subduction zone moves faster than the seafloor spreads. He published his theory in the journal History of Ocean Basins in 1962, and it became known as “seafloor spreading.”

Dating of ocean-core samples in the early 1960s revealed that the ocean floor was younger at the Mid-Atlantic Ridge but progressively older in either direction, proving the actual fact of seafloor spreading.

Magnetic Reversal as Evidence of Plate Movements

A portion of the continental drift theory is contradicted by the seafloor spreading theory. The seafloor spreading theory certainly suggests that plate movements occur at the mid-oceanic ridge.

By 1963, geophysicists had discovered that the Earth’s magnetic field had reversed polarity numerous times, with each reversal lasting around less than 200,000 years. At the time that part of the seafloor crust formed, rocks of the same age would have acquired magnetic polarity. Surveys on either side of the Mid-Atlantic Ridge revealed a symmetrical pattern of alternating polarity stripes.

The discovery of magnetic rocks near the ridge that follows a pattern, in addition to the fact that the rocks close to the ridge are younger than those further away from the ridge, strengthened seafloor spreading.

The magnetic reversal has occurred numerous times in the past. The magnetic patterns in magnetic rocks can explain the occurrence of magnetic reversal. Our scientists can use these magnetic patterns to determine the ages and rates of movement of materials from the mid-oceanic ridge.

The magnetic reversal of the Earth, also known as the “magnetic flip,” occurs when the North Pole transforms into the South Pole, and the South Pole transforms into the North Pole. This occurs as a result of the changing direction of material flow in the Earth’s liquid outer core.

There have been an average of 4 to 5 reversals per million years over the last 10 million years. At the ridge, new rocks are added to the ocean floor in roughly equal quantities on both sides of the oceanic ridge.

By the 1970s, geologists had come to an agreement to use the term “plate tectonics” to describe what had become their discipline’s central paradigm. They used the word “plates” because they discovered evidence that not only continents but entire plates of the Earth’s crust move.

A plate may contain a continent, parts of a continent, and/or undersea crustal portions. Together with Harry Hess’s Seafloor Spreading, Alfred Wegener’s concept of continental drift was developed and refined.

Most modifications on the Earth’s surface happen so slowly that humans don’t notice them right away. The concept that the Earth’s landmasses have broken off from each other, rejoined, and moved to different parts of the world is part of Plate tectonic theory.

Drawbacks of Continental Drift Theory

• Wegener did not explain why the drift started only in the Mesozoic era (252 million years ago to about 66 million years ago) and not earlier.
• Oceans are not taken into account in the theory.
• Proofs rely heavily on generalizable assumptions.
• Buoyancy, tidal currents, and gravity are all too weak to move continents.
• Modern theories (Plate Tectonics) acknowledge the existence of Pangaea and related landmasses but explain the causes of drift in a very different way.