Question

What was Wegener's continental-drift hypothesis? (A)

Short answer

Wegener's continental-drift hypothesis, proposed by German meteorologist and geophysicist Alfred Wegener in 1912, is the theory that the Earth's continents were once part of a single large landmass called Pangaea, which broke apart and drifted to their current positions due to forces related to tectonic plate movements. Wegener collected evidence such as similar continental coastlines, identical fossils and rock types across oceans, and paleoclimatic data to support his hypothesis. Although initially met with skepticism, it later became the foundation for the modern theory of plate tectonics.

Step-by-step solution

Introduction of Wegener's Continental-Drift Hypothesis

Alfred Wegener was a German meteorologist and geophysicist who introduced the concept of continental drift in his book "The Origin of Continents and Oceans" in 1912. He proposed that the continents were once a single, large landmass that broke apart and drifted to their current positions over millions of years.

Explanation of the Hypothesis

Wegener's continental-drift hypothesis is the theory that the Earth's continents were once joined together as a single landmass called Pangaea, which later broke apart and drifted to their current positions on the Earth's surface. This process is driven by forces related to the movements of tectonic plates, which include convection currents in the Earth's mantle and the spreading of the seafloor.

Evidence Banked by the Hypothesis

In support of his hypothesis, Wegener collected various lines of evidence, including: 1. The similarity of the continental coastlines: Particularly the matching of the east coast of South America with the west coast of Africa. 2. The presence of identical fossils and rock types across oceans: He found that fossils of the same species and age could be found on continents that are now thousands of miles apart, suggesting that those areas were once contiguous. 3. Paleoclimatic evidence: He discovered evidence of glaciations in regions close to the equator, which indicated that the continents had shifted and changed their positions relative to the Earth's poles. While Wegener's continental-drift hypothesis was initially met with skepticism, it later became the foundation for the modern theory of plate tectonics.

Key concepts

Pangaea

Alfred Wegener's groundbreaking hypothesis began with the idea of a supercontinent named Pangaea. This immense landmass is believed to have included all the Earth's continents, which were connected as a single entity approximately 300 million years ago. The term 'Pangaea' itself comes from Ancient Greek, meaning 'all lands', a fitting name for this monolithic prehistoric continent.

The concept of Pangaea is crucial to understanding the geological history of our planet. It's like the pieces of a jigsaw puzzle that fit together to form a complete picture. Over time, due to the movement of tectonic plates, Pangaea started to break apart. This division led to the formation of the continents as we know them today. Studying the shapes of the continents, geologists observed how South America and Africa, for example, appear to be complementary pieces that were once joined together.

Tectonic Plates

Imagine the Earth's surface is like a cracked shell of a hard-boiled egg. The large pieces of this shell represent 'tectonic plates.' These plates are massive slabs of the Earth's lithosphere, floating on a semi-fluid layer beneath them called the asthenosphere.

The lithosphere includes the crust and the upper mantle, and it's broken into multiple plates, including both continental and oceanic ones. Tectonic plates are constantly moving, albeit at a very slow rate - about the same rate that fingernails grow! The movement of these plates is responsible for a number of geological phenomena, such as earthquakes, volcanic activity, and the creation of mountain ranges.

Convection Currents in the Earth's Mantle

So, what causes the tectonic plates to move? The answer lies deep beneath the Earth's crust, in the swirling motion of molten rock in the Earth's mantle. These movements are known as 'convection currents.' Heat from the core of the Earth causes this hot magma to rise toward the crust where it cools, then sinks back down to be reheated and rise again.

These currents create a churning action that acts like a conveyor belt for the tectonic plates sitting atop the mantle. The constant upwelling and sinking of material in the mantle exert a force on the plates, pushing and pulling them in different directions.

Seafloor Spreading

The Mid-Ocean Ridges

Seafloor spreading is a process intimately connected with the movement of tectonic plates. At the mid-ocean ridges — underwater mountain ranges at the borders of oceanic plates — magma from the mantle upwells to create new oceanic crust. As this magma cools and solidifies, it adds fresh material to the edges of the tectonic plates.

Imagine a conveyor belt coming out from the ridge, adding more belt material at the center and pushing the older material aside. This continual addition of new crust causes the ocean floors to 'spread,' moving apart at these ridges, and allowing for the creation of new oceanic crust. This is the fundamental process that Wegener believed contributed to continental drift.

Fossil Distribution

Wegener's hypothesis gained substantial evidence from the distribution of fossils across continents. The existence of similar fossils on widely separated lands suggests that these areas were once connected. For example, identical fossil species of the plant Glossopteris and the freshwater reptile Mesosaurus have been found in South America, Africa, and Antarctica.

This indicates that these organisms once lived on a single landmass where they could roam freely, which supports the idea of the continents being united as Pangaea. Wegener used this fossil distribution as a pivotal part of his argument, showing that it was unlikely for the same species to develop independently on different continents without any physical connection between them.