History of Earth – Continental Shapes and Continental Drift Theory

In this article, we’ll learn about continental shapes and continental drift theory. Let’s begin

Introduction

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 the structure of the 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.

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 – Forces Behind the Drifting of Continents

• According to Wegener, the drift was in two directions: equatorial due to the interaction of gravity, pole-fleeing force (caused by the Earth’s rotation), and buoyancy (ship floats in water due to buoyant force offered by water), and westward due to tidal currents caused by the Earth’s motion (Earth rotates from west to east, so tidal currents act from east to west, according to Wegener).
• Wegener proposed that tidal force (the gravitational pull of the moon and, to a lesser extent, the sun) was also important.

The polar-fleeing force is associated with the rotation of the Earth. The Earth is not a perfect circle; it has a bulge at the equator. The rotation of the earth causes this bulge (greater centrifugal force at the equator).

• As we move from the poles to the equator, the centrifugal force increases. According to Wegener, the increase in centrifugal force has resulted in pole fleeing.
• Tidal force is caused by the moon and sun’s attraction, which causes tides to form in oceanic waters.
• These forces, according to Wegener, would become effective if applied over many million years, and the drift is continuing.

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.

1. Evident Affinity for Physical Features

• Brazil’s bulge (South America) seems to fit into the Gulf of Guinea (Africa).
• Greenland appears to blend in with Canada’s Ellesmere and Baffin islands.
• The west coasts of India, Madagascar, and Africa appear to have merged.
• North and South America fit on one side of the mid-Atlantic ridge, while Africa and Europe fit on the other.
• The Caledonian and Hercynian mountains of Europe, as well as the Appalachians of the United States, appear to be one continuous series.

2. Botanical Evidence

• Fossils give information regarding when and where plants and animals lived in the past. Some forms of life “rode” on diverging plates, becoming isolated and evolving into new species.
• Finding identical or similar fossils across vast distances was one of the first clues used by scientists to reconstruct past plate movement.
• This distribution of fossils has led to theories that the southern continents were once part of a supercontinent known as Gondwana.
• 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.

3. 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.

4. Rocks of the Same Age Across the Oceans

• The 2,000 million-year-old belt of rocks from Brazil’s coast matches up to those from western Africa. Rocks of the same age can be found in other areas of the world also.

5. Tillite Deposits

• Tillite deposits are sedimentary rocks formed by glacier deposits.
• The Gondwana sedimentary system can be found in India, Africa, the Falkland Islands, Madagascar, Antarctica, and Australia (all were previously part of Gondwana).
• The overall similarity shows that these landmasses had remarkably similar histories.

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.