Term 2 Prep III
The City School
Handout: Science 2.4.1
Topic: Evidence of Tectonic Plates
Name: Sec:
Learning Objective(s):
2.4.1 Explain why the jigsaw appearance of continental coasts, location of volcanoes and earthquakes, fossil record and alignment of
magnetic materials in the Earth's crust are all evidence for tectonic plates.
The theory of tectonic plates explains how the Earth's lithosphere is divided into large plates that float and move on the
semi-fluid asthenosphere beneath them. These movements shape the Earth's surface and lead to the formation of
continents, mountains, volcanoes, earthquakes, and ocean basins. Several lines of evidence strongly support the theory,
providing insight into the processes that have shaped the Earth over millions of years.
1. Jigsaw Appearance of Continental Coasts
One of the earliest observations that hinted at the movement of continents was the way their coastlines seemed to fit
together like pieces of a puzzle. This is most notable with South America and Africa, whose shapes align almost perfectly
along the Atlantic Ocean. This observation was first highlighted by Alfred Wegener in 1912 when he proposed the theory of
continental drift.
Wegener suggested that millions of years ago, the continents were joined together in a supercontinent called Pangaea. Over
time, tectonic forces caused the landmasses to break apart and drift to their current positions. This jigsaw-like fit provides a
visual clue that the continents were once connected, supporting the idea that the Earth's surface is dynamic.
2. Location of Volcanoes and Earthquakes
Volcanoes and earthquakes are not randomly distributed across the Earth; instead, they occur in narrow, well-defined
zones along the boundaries of tectonic plates. For example, the Ring of Fire around the Pacific Ocean is one of the most
seismically and volcanically active regions in the world.
Term 2 Prep III
At divergent boundaries, where plates move apart, magma rises to form new crust, resulting in volcanic activity, as seen
at the Mid-Atlantic Ridge. At convergent boundaries, where plates collide, one plate is forced beneath the other,
creating powerful earthquakes and explosive volcanoes, such as those in the Andes Mountains. At transform
boundaries, where plates slide past each other, friction causes earthquakes, like those along the San Andreas Fault in
California.
These patterns of activity align with tectonic plate boundaries, providing evidence of plate movement and interactions.
3. Fossil Record
Fossils of identical plants and animals have been found on continents now separated by oceans, providing compelling
evidence for tectonic plate movement. For instance, fossils of the freshwater reptile Mesosaurus have been discovered
in both South America and Africa. These creatures could not have crossed the vast Atlantic Ocean that separates the two
continents today.
Similarly, fossils of the plant Glossopteris have been found in South America, Africa, Antarctica, India, and Australia. The
widespread distribution of these fossils indicates that these continents were once connected, allowing the organisms to
inhabit a shared landmass. This supports the idea of continental drift and the existence of Pangaea.
Term 2 Prep III
4. Alignment of Magnetic Materials in the Earth's Crust
Rocks on the ocean floor provide evidence of tectonic plate movement through the alignment of magnetic minerals.
These rocks show patterns of magnetic alignment that are symmetrical on either side of mid-ocean ridges.
When magma emerges at mid-ocean ridges and solidifies, its magnetic minerals align with the Earth's magnetic field.
Over time, as the Earth's magnetic field reverses, the alignment of these minerals changes. This creates alternating
patterns of normal and reversed polarity, forming "magnetic stripes" on the seafloor. These stripes mirror each other on
both sides of the ridge, providing evidence of seafloor spreading and plate movement.
Conclusion
The evidence for tectonic plates is overwhelming and comes from diverse sources. The jigsaw-like fit of continental
coastlines demonstrates that continents were once joined together. The distribution of volcanoes and earthquakes
highlights the locations where plates interact. Fossil records provide proof of connected landmasses, while magnetic
alignment on the seafloor reveals the process of seafloor spreading. Together, these observations confirm that the
Earth's lithosphere is divided into moving plates, shaping the planet's surface over geological time.
