3.5PLATE TECTONICS

In 1912, a German scientist named Alfred Wegener proposed a hypothesis stating the continents once formed a single landmass, called a supercontinent, then broke up and drifted to their present locations.

This hypothesis is now called continental drift and explains why the continents seem to fit together like the pieces of a jigsaw puzzle.

Wegener’s Hypothesis

Alfred Wegener

Wegener also found other evidence to support continental drift.He said if the continents were once

joined, fossils of the same plants and animals should be found in areas that were once connected.

Fossils of the Mesosaurus, a freshwater reptile, were found in both South America and western Africa.

Fossils of the ancient fern, Glossopteris, were also found on several southern continents.

In addition to fossils, similar types of rocks and evidence of the same climate were found on several continents.

Supporting Evidence

Mesosaurus fossil

Glossopteris fossil

Supporting EvidenceThe Appalachian

Mountains run along the eastern side of North America and end off the coast of Newfoundland.

Mountains that are similar in age and structure are found in the British Isles and Scandanavia.

When these landmasses were united as Pangaea, these ancient mountain chains formed a nearly continuous belt.

Despite several forms of evidence supporting the hypothesis of continental drift, Wegener’s ideas were strongly opposed by other scientists. They rejected his explanation of how

the continents moved as he stated the continents plowed through the rock of the ocean floor.

This idea was shown to be physically impossible.

Wegener spent the rest of his life searching for a mechanism that would gain consensus. He died in 1930 during a journey to

Greenland to study further evidence.

Missing Mechanisms

The evidence Wegener needed to support continental drift came about two decades after his death. In 1947, a group of scientists set

out to map the Mid-Atlantic Ridge.

Discovered it is part of a system of mid-ocean ridges, or chains of submerged mountains at the bottom of the oceans.

Noticed 2 trends:Sediment is thinner the closer you

get to the ridge.

Ocean floor is very young (radiometric dating showed no rocks older than 200 million years).

Mid-Ocean Ridges

In the 1950’s a geologist by the name of Harry Hess suggested a new hypothesis stating the valley at the center of the ridge was a crack, or rift, in Earth’s crust.

At the rift, magma comes up from deep inside of Earth.

This creates a process where new oceanic lithosphere forms as magma rises to Earth’s surface and solidifies at the mid-ocean ridge, known as sea-floor spreading.

Sea Floor Spreading

Harry Hess

Although Hess’s ideas were only hypothesized, more supporting evidence came in the mid-1960’s.

Paleomagnetism is the study of the magnetic properties of rocks. As magma/lava solidify to form rock,

iron-rich minerals align with the Earth’s magnetic field.

Geologic evidence shows Earth’s magnetic field has not always pointed north, as it does now. When the magnetic field points to

the geographic north it is called normal polarity.

When the magnetic field points to the geographic south it is called reverse polarity.

Sea Floor Spreading

Earth’s crust is the thin, rocky outermost portion of the Earth and is classified into two types:Oceanic

About 7 km. thickAverage density of 3.0 g/cm3

ContinentalAbout 40 km. thickAverage density of 2.7 g/cm3

Earth’s Interior Layers

In 1909, a Croatian scientist named Andrija Mohorovicic, discovered a boundary separating the Earth’s crust from the mantle.He did so by studying seismic

waves and found their velocity increased dramatically about 50 km. below the surface.

In honor of his discovery, this boundary is called the Mohorovicic Discontinuity, or “moho” for short.

Earth’s Interior Layers

Andrija Mohorovicic

By the 1960’s, evidence supporting continental drift and sea-floor spreading led to the development of a theory known as plate tectonics, which explains how the continents move and studies the formation of features in Earth’s crust. Earth’s crust and the rigid, upper part of the mantle form a layer

of Earth called the lithosphere, or the outer shell of the Earth. Broken into several blocks called tectonic plates (15 major).

These plates move on part of the mantle known as the asthenosphere, which is a plastic, putty-like layer of rock below the lithosphere.

How Continents Move

Getting Deeper Into EarthThe mantle contains the

majority of Earth’s mass and volume.Recall the process of convection!

Hot material rises, cool material sinks.

2 different parts:Upper mantle

LithosphereAsthenosphere

Lower mantle

Getting Deeper Into EarthThe core is the inner-most

sphere of the Earth. Iron (Fe) and Nickel (Ni)

composition.

Average density of 13 g/cm3.

Divided into 2 distinct parts:Outer Core

Liquid layerFlowing metallic Iron (Fe)

generates Earth’s magnetic field.

Inner CoreSolid, densest part of EarthDespite its higher temperature,

the material is compressed into a solid due to intense pressure.

Scientists identify plate boundaries primarily by studying data from earthquakes. Volcano data can help as well. The Pacific Ring of Fire is a zone of several earthquakes and

active volcanoes. Indicates the Pacific Ocean is surrounded by plate boundaries.

The 3 main types of plate boundaries include: divergent, convergent, and transform.

Plate Boundaries

When two tectonic plates separate, or move away from each other, the boundary between them is called a divergent boundary.

New sea floor forms at divergent boundaries; most common mid-ocean ridges.

Divergent Plate Boundaries

When two tectonic plates collide, or come together, the boundary between them is called a convergent boundary.

The region along a plate boundary where one plate moves under another plate is called a subduction zone.

Convergent Plate Boundaries

When two tectonic plates slide past each other horizontally, the boundary between them is called a transform boundary.

Good example San Andreas Fault, CA

Transform Plate Boundaries

Obviously, convection in the mantle is the driving force behind plate movement and tectonic activity.

Cooling rock coming up from mid-ocean ridges slides down the slope of the ridge and exerts a force on the rest of the plate known as ridge push. Pushes the rest of the plate away from

the ridge.

Where the lithosphere is dense enough, it begins to subduct (sink) into the asthenosphere.

As the leading edge of the plate sinks, a force known as slab pull pulls the rest of the plate along behind it.

Plate Motion

Ridge Push

Slab Pull

One way the continents change shape is by breaking apart.

Rifting is the process by which a continent breaks apart resulting in new smaller continents.

Continents can also gain material (convergent boundaries).A terrane is a piece of

lithosphere that has a unique geologic history and may be part of a larger piece of lithosphere, such as a continent.

Reshaping Earth’s Crust

Scientists believe at several points throughout Earth’s history, the continents were arranged into large landmasses known as supercontinents.The process by which

supercontinents form and break apart over millions of years is called the supercontinent cycle.

The collision of plate boundaries along with the formation of rifts creates this movement of the continents across the globe.

The Supercontinent Cycle

About 1 BYA, the supercontinent Rodinia, meaning “motherland”, existed.Most landmasses were gathered south of the equator.

The Supercontinent Cycle

About 450 MYA, after Rodinia broke up, Earth’s continents were separated as they are today.

The Supercontinent Cycle

The continents had joined back together around 200-225 MYA in the supercontinent Pangaea, meaning “all Earth”. Certain mountain ranges formed, including the Appalachians.

The Supercontinent Cycle

When Pangaea split apart, it eventually separated into a northern piece (Laurasia) and a southern piece (Gondwanaland) around 150 MYA.

The Supercontinent Cycle

About 50 MYA, the continents continued their separation, resembling their current-day locations. India collided with Eurasia Himalaya Mountain formation.

The Supercontinent Cycle

Believed if plate movements continue at current rates, in 150 million years, Africa will collide with Eurasia, closing off Mediterranean Sea. New subduction zones will form, closing off the Atlantic as well.

The Supercontinent Cycle

A new supercontinent, Pangaea Ultima, will form in approximately 250 million years.

The Supercontinent Cycle