Geology: Chap3: Drifting Continents and Spreading Seas

Chapter 3: Drifting Continents and Spreading Seas

German meteorologist that proposed continental drift theory and hypothesized a former supercontinent called Pangea.

Alfred Wegener

Wegner’s evidence for continental drift

Past glacial striations/till; Climatic belts; Continental fit of 86%; Fossil distribution; matching geological units

Means “all-land” or “one land”

Pangea

True or False: Glaciers are the most powerful erosive force on earth.

True

Scratches glaciers leave behind

Striations

Sediment a melting glacier leaves behind. It’s also evidence that glaciers are moving.

Till

Glacial evidence of past _________ Age glaciers were found on four different continents.

Permian

True or False: Glaciers move across the Earth in the direction of North and South.

False. Glaciers don’t move in any certain direction as a rule. They move from high to low just like water…it flows downhill.

_____ ______ form along the equator in warm water.

Coral reefs

Created when tropical vegetation decays and layer upon layer gets pushed down under pressure.

Coal

Identical fossils of creatures and organisms that could NOT have crossed the ocean were found on widely separated land masses. This is called fossil distribution and is a direct explanation of ________.

Pangea

__________ are rocks found in the dead sea.

Evaporites

Wegner’s idea of a once existing “supercontinent” was widely criticized because he couldn’t explain _____ or______ the continents moved.

How or Why

The flow in the ____________creates the Earth’s magnetic field. Which is similar to the field produced by a ________ magnet.

Liquid outer core; bar

The magnetic pole is different from the __________ pole.

Geographic

Polar reversals have occurred in the past, but right now the North Pole is ________ and the South Pole is _________.

Positive NP and Negative SP

_______ minerals in rock preserve information about the magnetic field at the time the rocks formed.

Iron (Fe)

Lava turns into magma, then cools. The igneous rock “_______ “ only, will have paleomagnetism.

In situ

When iron minerals in very hot magma finally cool, they archive the magnetic signal at formation. This is called _____________.

Paleomagnetism

Before WWII we knew little about the sea floor until a team led by ______ and _______ mapped it out using sonar.

Deitz and Harry Hess

Huge fracture zones segment this area and it’s where plates are splitting apart.

(MOR) Mid-ocean ridges

Deep-sea trenches occur near _______________.

Volcanic Island Chains

A mid ocean mountain range runs through every __________?

Ocean

MOR, deep ocean trenches that occur near volcanic island chains, submarine volcanoes, and fracture zones can all be explained by _______ ________.

Plate Tectonics

Oceanic crust is covered by sediment, therefore it is ___________ near the continents.

Thickest

New oceanic crust is _________ at the mid-ocean ridge

thinnest or absent

_________ rock is higher up around the mid-ocean ridge axis. While _________ rock is closer to the continents.

Younger; Older

Sea-floor spreading occurs at _____________.

MOR

Oceanic crust rocks are mafic and made of _________ and _________.

Basalt and Gabbro

Continental crust is more Felsic and made of ________.

Granite

What is the term for the difference between geographic north and magnetic north?

Magnetic declination

What does the property of inclination refer to in terms of magnetic field lines?

Tilt of a magnetic needle

What phenomenon is observed in layered basalts that record magnetic changes over time?

Apparent polar wander

Who discovered that oceanic crust is covered by sediment and is thickest near continents?

Harry Hess

Where do earthquakes occur in concentrated belts related to crustal movement?

Mid-ocean ridge axes

What do some mid-ocean ridges resemble and potentially indicate?

Rift valleys

What is the summary of the "new" data discussed by Hess in his essay?

Seafloor spreading

What is the mechanism proposed by sea-floor spreading

Upwelling of the mantle

What do the magnetic anomaly stripes on the sea floor represent

Magnetic polarity reversals

What is the term for the geologically rapid flip of Earth's magnetic field?

Magnetic reversal

What type of rocks reveal reversals in magnetic polarity?

Igneous

What is the term for the time markers provided by magnetic reversals?

Subchrons

How are magnetic anomalies used to determine rates of sea-floor spreading?

Width of anomalies

What did deep-sea drilling in the 1960s reveal about the age of oceanic crust?

The youngest crust is near mid-ocean ridges

How has the reversal chronology been extended in deep-sea drilling studies?

To 170 Ma

What are the three main features revealed by today's view of the ocean floor?

Abyssal plains, mid-ocean ridges, and deep-ocean trenches

What is the term for the concept that continents passively move apart due to seafloor spreading

Plate tectonics

What is the term for the concept that continents passively move together as a result of seafloor subduction?

Plate tectonics

Where does seafloor spreading primarily occur?

Mid-ocean ridges

How do marine magnetic anomaly belts run in relation to mid-ocean ridges?

Parallel to ridges

What best describes the distribution of earthquakes on the ocean floor?

Concentrated along mid-ocean ridges and deep-ocean trenches

What does the rate of seafloor spreading exactly match?

Subduction at deep-sea trenches

What drives seafloor spreading?

Mantle convection currents

What is the term for the process in which new crust moves away from mid-ocean ridges and gathers sediment

Sea-floor spreading

What does the width of magnetic anomaly stripes indicate?

Duration of chrons

What is the primary rock type found in the oceanic crust?

Basalt

What is the term for the process of drilling holes in the ocean floor to collect samples?

Deep-sea drilling

What is the mechanism proposed by Hess and Dietz to explain the origin of ocean basins and continents?

Seafloor spreading

What is the term for the pattern formed by marine magnetic anomalies on the ocean floor?

Magnetic stripe pattern

What causes the magnetic field to "flip" during magnetic reversals?

Unknown reasons

How do layered lava flows reveal reversals in magnetic polarity?

Changes in magnetic intensity

What does the reversal chronology provide that can be used as time markers?

Duration of chrons

What does deep-sea drilling reveal about the age of oceanic crust as one moves away from mid-ocean ridges?

Age increases

What is the main feature identified by the width of magnetic anomaly stripes on the ocean floor?

Rate of seafloor spreading

What is the term for the concentrated belts where earthquakes occur in the oceanic crust?

Transform faults

What is the primary mechanism proposed by Hess and Dietz to explain the creation of new oceanic crust

Seafloor spreading

What is the term for the geological time scale divisions used in the study of magnetic reversals?

Magnetostratigraphy

How do magnetic anomalies help determine the rates of seafloor spreading?

By examining the width of anomalies

What is the primary factor responsible for the thickness variation of sediment on the ocean floor?

Seafloor spreading rates

True/False: Earthquakes occur in concentrated belts and are happening on plate boundaries

True

What is the main contribution of marine magnetic anomalies in supporting the theory of seafloor spreading?

Symmetry across mid-ocean ridges

What is the term for the geological time intervals identified in the magnetic reversal time scale?

Subchrons

Layered lava flow reveals _________ in magnetic polarity.

Reversals

True/False. Pole reversals are named after the scientists who discovered them.

True. Ex: Brunhes; Matuyama; Gaus; Gilbert

When interpreting marine magnetic anomalies related to the spreading rate; _________ spreading = wider stripes and _________ spreading = narrow stripes.

Faster; slower

What type of rocks are absent in the oceanic crust according to new observations?

Granite

. What is the primary factor influencing the rate of sea-floor spreading?

Mantle convection currents

What was discovered to be thickest near the continents on the oceanic crust?

Sediment

According to the apparent polar wander, what do separate polar wandering paths for each continent suggest?

The lava flows themselves have moved

What causes the magnetic needle to tilt, a property known as inclination?

Curved field lines

What is the term for the rate of seafloor spreading at the Mid-Atlantic Ridge?

~2 cm/yr

What is the main feature identified by the width of magnetic anomaly stripes on the ocean floor?

Sea-floor spreading

What are the seismic belts limited to in the oceanic crust?

Mid-ocean ridge axis

What term is used for the region of crustal spreading that resembles a rift valley at some mid-ocean ridges?

Ridge axis

What did marine magnetic anomalies reveal about the Earth's magnetic field during reversals?

It flips direction

What was the unexpected finding revealed by echo-sounding (sonar) of the sea floor before World War II?

Mid-ocean mountain range

What led to a greater understanding of seafloor bathymetry and exploration during World War II?

Military Needs

What did the discovery of a mid-ocean mountain range contribute to our understanding of Earth's geology?

Evidence for continental drift

What is the primary reason for the absence of metamorphic rocks in the oceanic crust?

Rapid cooling

True/False: Seismic activity happens during subduction

False. Seismic activity occurs at plate boundaries

According to Hess's proposal of sea-floor spreading, what happens at mid-ocean ridges?

Upwelling mantle erupts

What is the primary factor influencing the formation of magnetic anomaly stripes on the ocean floor?

Magnetic reversals

What property of rock magnetism is used to determine the magnetic field at the time rocks formed?

Magnetization

What phenomenon causes the magnetic needle to tilt, creating inclination?

Curved magnetic field lines

True/False. Marine magnetic anomaly belts run perpendicular in relation to mid-ocean ridges?

False. They run parallel in relation to MORs

What did Deep-sea drilling in the 1960s reveal about the age of oceanic crust away from the Mid-Atlantic Ridge?

Older age with distance

How did echo-sounding (sonar) contribute to a greater understanding of the ocean floor before World War II?

Revealing MORs

What does the age-dating of rocks during deep-sea drilling reveal about the oceanic crust?

Younger rocks near mid-ocean ridges

According to the concept of sea-floor spreading, what happens at trenches?

Subduction occurs

How do magnetic anomaly belts appear on maps in relation to mid-ocean ridges?

Symmetric patterns