Chapter 3 and 4 Geology study guide

What is Uniformitarianism?

The idea that the same geologic processes occurring today also occurred in the past.

What does “the present is the key to the past” mean?

Processes happening today help scientists understand how Earth’s features formed in the past.

What is the Continental Drift Hypothesis?

The hypothesis that continents move across Earth’s surface over geologic time.

Who proposed the Continental Drift Hypothesis?

Alfred Wegener in 1915.

What was Pangaea?

A supercontinent that contained all of Earth’s major landmasses.

When did Pangaea begin breaking apart?

About 225 million years ago.

What were Laurasia and Gondwana?

The two major landmasses formed when Pangaea began breaking apart.

What did Wegener notice about the continents?

Their coastlines fit together like pieces of a jigsaw puzzle.

Which continents have an especially good fit?

South America and Africa.

What was the biggest problem with Wegener’s hypothesis?

He could not provide a convincing mechanism for moving the continents.

What were the two alternative Earth hypotheses discussed?

The Contracting Earth Hypothesis and the Expanding Earth Hypothesis.

What was the Contracting Earth Hypothesis?

The idea that Earth cooled and contracted, causing mountains to form like wrinkles on a drying apple.

Why was the Contracting Earth Hypothesis rejected?

Earth had not cooled enough, gravity had not significantly increased, and it could not explain features caused by extension.

What would happen to gravity if Earth contracted?

Gravity would increase.

How does radioactive decay affect Earth’s cooling?

Radioactive decay produces heat and slows cooling.

What was the Expanding Earth Hypothesis?

The idea that Earth expanded, causing the lithosphere to break apart and oceans to form.

Why was the Expanding Earth Hypothesis rejected?

Earth is not expanding and the hypothesis could not explain continental drift or subduction.

What would happen to gravity if Earth expanded?

Gravity would decrease.

What was Wegener’s first major evidence for continental drift?

The jigsaw-puzzle fit of the continents.

What was Wegener’s second major evidence?

Fossils of identical organisms on continents now separated by oceans.

What was Wegener’s third major evidence?

Rocks and landforms of the same age and composition align across continents.

What was Wegener’s fourth major evidence?

Glacial deposits and glacial striations.

What was Wegener’s fifth major evidence?

Paleoclimate evidence.

What are glacial striations?

Scratches made by rock fragments at the base of moving glaciers.

What is paleoclimate evidence?

Rocks and geologic features that provide evidence of past climates.

What are possible alternative explanations for identical fossils on separated continents?

Rafting, isthmian links, and island stepping stones.

Why did scientists initially oppose Wegener’s hypothesis?

He lacked a convincing mechanism and proposed incorrect forces for moving continents.

What did Wegener incorrectly suggest moved the continents?

Gravitational forces from the Moon and Sun.

What did Wegener incorrectly suggest continents moved through?

Oceanic crust.

What helped continental drift develop into Plate Tectonic Theory?

New research and discoveries about the ocean floor.

What technology helped scientists map the seafloor?

Sonar.

When was the Atlantic Mid-Ocean Ridge discovered?

1925.

How is oceanic crust different from continental crust?

Oceanic crust is mostly basalt, while continental crust contains a wider variety of rocks.

What is the oldest oceanic crust?

About 180 million years; no oceanic crust is older than this.

Why is there no oceanic crust older than about 180 million years?

Older oceanic crust is recycled into the mantle.

Where is heat flow especially high?

At mid-ocean ridges.

What does high heat flow at mid-ocean ridges suggest?

Hot or molten material is rising beneath them.

Who proposed Seafloor Spreading?

Harry Hess.

When did Harry Hess propose Seafloor Spreading?

1960.

What is Seafloor Spreading?

New oceanic crust forms at mid-ocean ridges and spreads outward.

What happens to new oceanic crust at a mid-ocean ridge?

It moves away from the ridge on both sides.

When was the complete Plate Tectonic Theory developed?

By about 1968.

What does Plate Tectonic Theory explain?

Continental drift, seafloor spreading, subduction, and plate movement.

What is the lithosphere?

Earth’s strong, rigid outer layer consisting of the crust and uppermost mantle.

What is the asthenosphere?

A hotter, weaker layer beneath the lithosphere that can slowly flow.

What moves over the asthenosphere?

Lithospheric plates.

Are continents themselves tectonic plates?

No. Continents are parts of larger tectonic plates.

What are the three types of plate boundaries?

Divergent, convergent, and transform.

What happens at divergent boundaries?

Plates move apart.

What happens at convergent boundaries?

Plates move toward each other.

What happens at transform boundaries?

Plates slide past each other.

What are the three forces associated with plate movement?

Ridge push, slab pull, and mantle convection.

What is ridge push?

Gravity pulls elevated lithosphere at a mid-ocean ridge downward, pushing the plate away from the ridge.

What is slab pull?

A cold, dense subducting plate sinks and pulls the rest of the plate with it.

What is mantle convection?

The movement of hot, rising mantle and cooler, sinking mantle.

Why does hot mantle rise?

Hot material is less dense and more buoyant.

Why does cooler mantle sink?

Cooler material is denser.

What happens to oceanic lithosphere as it moves away from a ridge?

It cools, thickens, and becomes denser.

Why can oceanic lithosphere eventually subduct?

It becomes denser than the surrounding asthenosphere.

Which plate-driving force is more effective according to the lecture?

Slab pull.

What initiated the formation and movement of tectonic plates?

Mantle convection.

Which two mechanisms currently drive plate motion?

Ridge push and slab pull.

What is paleomagnetism?

The study of Earth’s ancient magnetic field recorded in rocks.

Where is Earth’s magnetic field generated?

In Earth’s core.

How do magnetic minerals record Earth’s magnetic field?

They align with the magnetic field as rocks form.

What are magnetic stripes on the seafloor?

Alternating bands of normal and reversed magnetic polarity.

Why are magnetic stripes symmetrical around mid-ocean ridges?

New crust forms at the ridge and spreads outward in both directions.

What do magnetic stripes provide evidence for?

Seafloor spreading and plate tectonics.

What does GPS show about tectonic plates?

Their present-day velocity and direction of movement.

How do earthquakes support Plate Tectonic Theory?

Earthquakes frequently occur along and outline plate boundaries.

Where do earthquakes commonly occur?

At subduction zones, mid-ocean ridges, and transform boundaries.

What are hot spots?

Locations where mantle plumes rise and produce volcanic activity.

How do volcanic hot spot chains form?

A plate moves over a relatively fixed mantle plume, creating a chain of volcanoes.

Which volcano in a hot spot chain is youngest?

The volcano currently located above the mantle plume.

What does the direction of a hot spot chain show?

The direction of plate movement.

What is the Wilson Cycle?

The repeated opening and closing of ocean basins through plate tectonic processes.

What is relative plate motion?

Plate movement measured relative to another reference point.

What is absolute plate motion?

Plate movement measured relative to a fixed mantle plume or hot spot.

What is a divergent boundary also called?

A constructive margin.

Why is a divergent boundary called constructive?

New crust and lithosphere are created there.

What feature is commonly found at divergent boundaries?

Mid-ocean ridges.

What type of stress occurs at divergent boundaries?

Extension.

What type of faults occur at divergent boundaries?

Normal faults.

What is a convergent boundary also called?

A destructive margin.

Why is a convergent boundary called destructive?

Oceanic lithosphere can be destroyed through subduction.

What is subduction?

The process where one tectonic plate sinks beneath another into the mantle.

What happens at an oceanic-continental convergent boundary?

Dense oceanic crust subducts beneath continental crust.

What happens to water from the subducting plate?

It enters the asthenosphere and helps produce magma.

What forms above an oceanic-continental subduction zone?

A continental volcanic arc.

What happens at an oceanic-oceanic convergent boundary?

One oceanic plate subducts beneath another.

What forms above an oceanic-oceanic subduction zone?

A volcanic island arc.

What happens at a continental-continental convergent boundary?

Two continental plates collide, causing folding, faulting, and mountain building.

What mountain range formed from the Indian and Eurasian plates?

The Himalayas.

What type of stress occurs at convergent boundaries?

Compression.

What is a transform boundary?

A boundary where two plates slide horizontally past each other.

Are transform boundaries constructive or destructive?

Neither.

What is a transform fault?

A fault where plates move horizontally past one another.

What do half-arrows on transform boundary diagrams show?

The relative direction of movement on each side of the fault.

What is the San Andreas Fault?

A transform boundary between the Pacific and North American plates.

What is the Alpine Fault?

A transform boundary in New Zealand between the Australian and Pacific plates.