Plate Tectonics

Who proposed the Continental Drift hypothesis?

Alfred Wegener.

What was the name of the supercontinent proposed by Wegener?

Pangaea.

What type of scientist was Alfred Wegener?

A German meteorologist and geophysicist.

What was the first piece of evidence for continental drift?

The continents fit together like a jigsaw puzzle.

Which two continents showed an almost perfect fit in Wegener’s theory?

South America and Africa.

What kind of fossil evidence supported the continental drift?

Fossils of similar plants and animals found on continents now separated by oceans.

Name two fossil species that supported continental drift.

Mesosaurus and Glossopteris.

Why couldn’t Mesosaurus or Glossopteris have crossed the oceans?

Mesosaurus lived only in freshwater, and Glossopteris seeds were too heavy to be wind-dispersed across oceans.

What geological features matched across different continents?

Similar rock types and mountain belts, such as the Appalachian and Caledonian mountains.

What was the climate-based evidence for continental drift?

Glacial deposits found in now-tropical regions like Africa and Australia.

Why was Wegener’s hypothesis initially rejected?

He could not explain how the continents drifted.

Why did Wegener believe the matching coastlines of continents supported the idea of a supercontinent?

Because the fit of the continents suggested they were once joined together as one landmass—Pangaea—and later drifted apart.

How did fossil distribution support continental drift?

Fossils of species that could not have crossed oceans were found on separate continents, implying those continents were once connected.

How do similar rock formations across continents support Wegener's hypothesis?

Mountain belts and rock types of similar age and composition appearing on different continents suggest those landmasses were once joined.

How did ancient climate evidence contradict current climate zones?

Glacial deposits were found in present-day tropical regions, which only makes sense if the continents had moved from polar regions.

Why wasn't the continental drift hypothesis accepted during Wegener’s time?

The mechanism for how the continents moved was missing; Wegener couldn’t explain the driving force behind the drift.

A student finds a diagram showing glacial evidence on both South America and Africa. What does this imply about the past positions of these continents?

That the continents were once joined and located closer to the South Pole where glaciers formed

If a fossil of the same plant species is found in both India and Antarctica, what does this suggest in terms of continental configuration?

That these landmasses were once connected, allowing the plant species to grow in both areas.

A geologist notices that rock layers in North America and Europe show the same age and composition. Which hypothesis does this observation support?

The Continental Drift hypothesis.

If a current desert region contains coal deposits from ancient tropical swamps, what does this suggest about past continental positions?

The region was once located in a more tropical climate zone due to continental drift.

What major geological feature was discovered after World War II through ocean exploration?

The global oceanic ridge system.

How long is the global oceanic ridge system?

Around 80,000 kilometers.

Where does new oceanic crust form?

At the axis of the oceanic ridge system.

What happens to oceanic crust as you move away from the ridge axis?

It becomes older and thicker with more sediment.

What is the term for the process of oceanic crust formation and movement away from the ridge?

Seafloor spreading.

Who proposed the concept of seafloor spreading?

Harry Hess and Robert Dietz.

What was the maximum age of the oceanic crust discovered through dredging?

No more than 180 million years old

What theory explains the constant creation and recycling of oceanic crust?

The plate tectonics theory.

According to the plate tectonics model, what is the Earth’s lithosphere broken into?

Rigid slabs called tectonic plates.

What layer do tectonic plates overlie?

The ductile asthenosphere.

What are the seven major tectonic plates?

African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American plates.

What are some examples of minor tectonic plates?

Philippine Sea, Juan de Fuca, Cocos, Nazca, Scotia, and Arabian plates.

What are the regions where tectonic plates interact called?

Plate boundaries.

What are divergent plate boundaries?

Boundaries where two plates move apart relative to each other.

What is another name for divergent plate boundaries?

Constructive margins.

Why are divergent boundaries called "constructive margins"?

Because molten material rises from the mantle and creates new crust as the plates pull apart.

Where can divergent boundaries occur?

Both on the ocean floor and within continents.

What geological feature forms at divergent boundaries on the ocean floor?

The oceanic ridge system.

What process generates new seafloor at oceanic divergent boundaries?

Seafloor spreading.

What happens to oceanic crust as it moves away from the ridge?

It becomes older, denser, and moves toward the plate’s edge.

How does the density of oceanic crust change over time?

Younger crust is hot and less dense; older crust is cooler and more dense.

What is the range of spreading rates along the oceanic ridge system?

From as slow as 2 cm/year to as fast as 15 cm/year.

What feature is formed by divergent boundaries within a continent?

A continental rift.

How is a continental rift formed?

By the stretching and thinning of the lithosphere.

What eventually forms if a continental rift continues to grow?

A new ocean basin.

What is a modern-day example of a continental rift?

The Red Sea in the East African Rift in Eastern Africa.

How does the formation of new crust occur at divergent boundaries?

As plates pull apart, molten material rises from the mantle and solidifies at the surface, forming new crust.

Why does older oceanic crust move toward the edge of the plate?

It becomes denser over time and is gradually pushed outward by the formation of new crust at the ridge.

How do continental rifts demonstrate the early stages of ocean formation?

They show how stretching lithosphere creates depressions that can eventually split continents and form ocean basins.

A geologist discovers a long, underwater mountain range with symmetrical patterns of magnetic minerals on either side. What type of plate boundary is she likely observing?

A divergent plate boundary, specifically an oceanic ridge.

A newly discovered region is experiencing thinning crust and formation of large depressions inland. What geological process might be occurring?

Continental rifting due to a divergent plate boundary.

A student claims that older oceanic crust is always found at the center of a mid-ocean ridge. What is wrong with this claim?

The center of the ridge contains the youngest crust; older crust is found farther away as it spreads out.

What are convergent plate boundaries?

Boundaries where two tectonic plates move towards each other.

Why are convergent boundaries also called “destructive margins”?

Because crust is destroyed or consumed during subduction or collision.

What are the three types of plate convergence?

Oceanic-Continental, Oceanic-Oceanic, and Continental-Continental convergence.

What happens when an oceanic plate and a continental plate converge?

The denser oceanic plate subducts beneath the lighter continental plate.

What is the process called when one plate sinks beneath another?

Subduction.

What is formed in the ocean at the subduction zone?

A deep-ocean trench.

What is the world’s deepest ocean trench, and where is it located?

The Mariana Trench in the western Pacific Ocean.

What geological feature is formed on land due to partial melting in the overriding plate?

A continental volcanic arc.

What type of crust is denser, basaltic or granitic?

Basaltic (oceanic crust).

In oceanic-oceanic convergence, which plate subducts?

The older and denser oceanic plate.

What volcanic feature forms from oceanic-oceanic convergence?

A volcanic island arc (or island arc).

Where are most volcanic island arcs found?

In the western Pacific Ocean, with a few in the Atlantic Ocean.

What happens when two continental plates collide?

The crust is too buoyant to subduct, so the material is uplifted, forming mountains.

What is the process of mountain formation from plate collision called?

Orogeny.

What is the most famous example of continental-continental convergence?

The formation of the Himalayas from the collision of the Indian and Eurasian plates.

How long ago did the Indian and Eurasian plates begin colliding?

Nearly 50 million years ago.

Why does oceanic crust always subduct beneath continental crust?

Because oceanic crust is denser (basaltic) compared to the lighter continental crust (granitic).

How does subduction cause volcanic activity?

Subduction induces partial melting in the overriding plate, leading to magma formation and volcanic eruptions.

Why don’t continental plates subduct when they collide?

Because continental crust is thick and buoyant, resisting subduction and instead pushing upward.

What is the difference between a volcanic arc and an island arc?

A volcanic arc forms on land (continental crust), while an island arc forms in the ocean (from oceanic crust)

A deep-ocean trench and a line of volcanoes parallel to a continental coast are observed. What type of plate boundary is this and what plates are involved?

Oceanic-Continental convergence with the oceanic plate subducting beneath the continental plate.

A series of volcanic islands are discovered forming a curve in the western Pacific. What geological process likely caused their formation?

Subduction of one oceanic plate beneath another, creating a volcanic island arc.

A geologist studying high mountain ranges with intense folding and thick crust finds no sign of volcanic activity. What type of plate boundary is involved?

Continental-Continental convergence, involving orogeny, not volcanism.

What are transform plate boundaries?

Boundaries where two tectonic plates slide past each other.

What is another name for transform plate boundaries?

Transform faults.

Do transform boundaries create or destroy crustal material?

No, they neither create nor destroy crustal material.

Where are transform faults usually found?

In fracture zones on the ocean floor.

What are fracture zones?

Linear breaks on the ocean floor that run perpendicular to oceanic ridges.

What lies between two offset oceanic ridges in a fracture zone?

An active transform fault.

What are the areas beyond the oceanic ridge zones in a fracture zone called?

Inactive zones.

Why are transform boundaries also called “conservative margins”?

Because the plates move horizontally past each other without creating or consuming crust.

How are fracture zones related to transform boundaries?

Transform faults occur within fracture zones that offset oceanic ridges, with only the section between ridges being active.

How do transform faults affect mid-ocean ridges?

They offset and segment the mid-ocean ridges, allowing for differential motion along the spreading centers.

A geologist detects horizontal plate movement along a linear ocean-floor break that offsets a mid-ocean ridge. Which type of boundary is this, and what is it called?

It is a transform plate boundary, also known as a transform fault.

A satellite image shows two tectonic plates sliding past one another with no new crust forming. What type of plate boundary is this, and what geologic feature is likely present?

A transform plate boundary with a transform fault, likely located within a fracture zone.