Rocks
Matching Rock Formations:
Rocks of similar type, age, and structure are found on continents that are now
separated by oceans (e.g., the Appalachian Mountains in North America and the
Caledonian Mountains in Europe). This suggests these continents were once
connected.
Deformation Patterns:
Rocks along plate boundaries often show folding, faulting, or metamorphism,
evidence of plate interactions such as collisions or subduction.
2. The Ocean Floor
Seafloor Spreading:
At mid-ocean ridges (e.g., the Mid-Atlantic Ridge), molten material rises from the
mantle and solidifies, forming new oceanic crust.
o Evidence includes symmetrical patterns of magnetic stripes (see below) and
age gradients in oceanic crust, which is youngest near ridges and older
farther away.
Deep Ocean Trenches:
Found at subduction zones, these trenches mark where one plate is being forced
beneath another, recycling the oceanic crust into the mantle.
3. Fossils
Identical Fossils on Separate Continents:
Fossils of the same species (e.g., Mesosaurus, a freshwater reptile, and Glossopteris,
a plant) have been found on continents that are now widely separated, such as
South America, Africa, and Antarctica.
o These species couldn’t have crossed vast oceans, implying that these
continents were once connected.
4. The Position of Volcanoes and Earthquakes
Plate Boundaries:
o Volcanoes and earthquakes occur predominantly along plate boundaries,
such as the "Ring of Fire" around the Pacific Ocean.
o Divergent boundaries (e.g., mid-ocean ridges) and subduction zones (e.g.,
around the Pacific Plate) produce volcanic activity.
oTransform boundaries (e.g., the San Andreas Fault) are marked by frequent
earthquakes.
Hot Spots:
Volcanic islands, such as Hawaii, are formed as tectonic plates move over stationary
mantle plumes, recording plate motion.
5. Alignment of Magnetic Materials
Paleomagnetism:
o When molten rock at mid-ocean ridges cools, magnetic minerals (like
magnetite) align with Earth’s magnetic field.
o Periodic reversals of Earth’s magnetic field are recorded in the rocks, creating
symmetrical "magnetic stripes" on either side of the ridge.
o This pattern provides direct evidence of seafloor spreading and plate
movement.
Polar Wander:
Studies of magnetic materials in rocks indicate changes in the position of the
magnetic poles, which can be explained by the movement of tectonic plates.
Evidence for Tectonic Plates
1. Fit of the Continents
The coastlines of continents, such as South America and Africa, appear to fit together
like pieces of a jigsaw puzzle, suggesting they were once joined in a supercontinent
called Pangaea.
. 2. Rocks
Matching Rocks Across Continents:
Rocks of the same type and age are found on continents that are now far apart (e.g.,
South America and Africa).
o This shows the continents were once joined.
3. Ocean Floor
Seafloor Spreading:
o At mid-ocean ridges, new crust forms as molten rock rises and cools.
o The ocean floor is younger near the ridge and older farther away.
Deep Trenches:
Found where one plate sinks under another (subduction zones).
4. Fossils
Identical Fossils on Different Continents:
Fossils of the same plants and animals, like Mesosaurus, are found on continents now
separated by oceans.
o This means the continents were connected long ago.
5. Volcanoes and Earthquakes
Plate Boundaries:
o Most volcanoes and earthquakes occur along the edges of tectonic plates.
o Example: The Pacific "Ring of Fire."
These show where plates are moving or colliding.
6. Magnetic Materials
Magnetic Stripes on the Ocean Floor:
o Rocks on either side of mid-ocean ridges show matching magnetic patterns.
o These patterns record Earth's magnetic field reversals and prove the seafloor is
spreading.
