GM (1,2)

Learning Objectives MD. 1

Define continental drift and explain its basic premise
Discuss some of the early evidence for continental drift and Alfred Wegener’s
Explain some of the other models that were used early in the 20th century to understand global geological features
Explore how the continental drift theory laid the groundwork for the later development of plate tectonics

Learning Objective MD. 2

Describe how the ideas behind plate tectonics started with Alfred Wegener’s hypothesis of continental drift
describe the physical and chemical layers of the Earth and how they affect plate movement
explain how movement at the three types of plate boundaries causes earth quakes, volcanoes, and mountain building
identify convergent boundaries including subduction and collisions, as places where plate come together
identify divergent boundaries, including rifts and mis - ocean ridges, as places where plates separate
explain transform boundaries as places where adjacent plates shear past each other
Describes the Wilson Cycle, beginning with continental rifting, ocean basin creation, plate subduction, and ending with ocean basin closure
explain how the tracks of hotspots places that have continually rising magma, is used to calculate plate motion

Slide Information (MD. 1)

Alternative explanations considered at the time

The first person to speculate the drift was Abrham Ortelius in 1596, second person was Theodor Christoph Lilienthal 1756, third person was Alexander Von Humboldt 1801 - 1845, fourth person was Antoino Sinder - Pellegrini 1858

Expanding Earth

1900 was believed that closed continent covered the entire surface of a smaller earth
Thermal expansion caused volcanic activity which broke the land mass into smaller continents
This explained the ocean but not the mountains

Contracting Earth:

Mountains and continents were considered products of a cooling and afterwards shrinking earth

Slide Information (MD. 2)

An idea before his time

main obstacle for Wegener was that people did not believe in his theory and because he could not explain how continents could move
earth’s crust was continuous not broken into plates
Geologist at the time were aware that continents were made of different rocks than the ocean crust
the ‘material’ making up the continents was less dense, so Wegener proposed that the continents were like icebergs
he suggested that the continents were moved by the effect of Earth’s of earth’s rotation pushing objects toward the equator, and by the lunar and solar tidal forces, which tend to push objects toward the west
this theory was proved wrong when they realized these forces were far too weak to move continents

Other popular ideas

At the beginning of the 20th century geologist had a good understanding of how most rocks were formed and understood their relative ages through interpretation of fossils
at the end of the 19th century views on the origin of mountains was the theory of constructionism
the idea that since earth is slowly cooling it must be shrinking

But it had its faults. And so, did others

this hypothesis helped to address the dilemma of the terrestrial fossils by explaining how continents once connected could now be separated by oceans
one problem was that earth wasn’t cooling fast enough to create the necessary amount of shrinking
second problem was that blocks of continental crust would not be able to sink under the denser oceanic crust in the way necessary for oceans to form
by 1960 scientist had enough evidence to support Wegener’s theory namely seafloor spreading

video notes

Arthur Holmes did not have direct evidence for mantle convection when he first proposed the idea. Instead, he used theoretical reasoning and indirect evidence based on geological observations and the principles of physics. Here are the key aspects of his approach:
1. Holmes was influenced by the discovery of radioactive decay as a source of heat within the Earth. He reasoned that this heat could create convection currents in the mantle, providing a mechanism for continental movement.
2. Holmes considered the thermal gradient within the Earth, suggesting that heat from the interior would create temperature differences that could drive convection currents.
3. Drawing on principles of fluid dynamics, Holmes theorized that the mantle, although solid, could behave like a viscous fluid over geological time scales, allowing for the slow movement of convection currents. Think of silly putty. When you apply a slow, steady force, these materials can stretch and deform, behaving like a viscous fluid. However, if you apply a sudden force, they break or snap like a solid.
While Holmes' ideas were largely theoretical at the time, they laid the groundwork for later research and discoveries that confirmed the existence of mantle convection and its role in plate tectonics. His work helped shift scientific thinking towards accepting dynamic processes within the Earth's interior as drivers of continental drift.
The distribution of earthquake epicenters played a crucial role in defining plate tectonic theory by revealing the boundaries and interactions of Earth's tectonic plates. Earthquake epicenters are concentrated along specific zones known as seismic belts, which correspond to tectonic plate boundaries. The patterns of seismic activity provided clear evidence of the dynamic nature of the Earth's lithosphere, supporting the concept of plates moving and interacting. By mapping these epicenters, scientists were able to delineate the edges of tectonic plates and later, understand the processes driving plate movements, such as subduction, seafloor spreading, and transform faulting, thereby solidifying the framework of plate tectonic theory.

Alfred Wegener’s Continental Drift Hypothesis (2.1)

Alfred Wegener was a German scientist who specialized in meteorology and climatology
- He proposed that continents were once joined together in a supercontinent called Pangaea, which eventually split apart and drifted to their current positions. Wegener's ideas laid the groundwork for modern geological theories, illustrating the dynamic nature of Earth's surface
In 1910 he disagreed with the explanation that the Bering Land Bridge was formed by isostasy and that similar land bridges once connected the continents
after reviewing the scientific literature, he published a hypothesis stating the continents were originally connected and they drifted apart
he did not have the precise mechanism worked out; his hypothesis was backed up by a long list of evidence

Early Evidence for Continental Drift Hypothesis (2.1.1)

first piece of evidence was that the coastlines of some continents fit together like a puzzle
similarities were being noticed in the coastlines of south America and Africa on the first world maps
suggestions of the continents being ripped part were being speculating
He used true edges of the continents based on the shapes of the continental shelves
Wegener also used rocks, mountains, fossils, and glacial formations for evidence by comparing them
fossils of reptiles like the Mesosaurus were found on the coastlines of Africa and south America
fossils of another reptile lystrosaurus were found on Africa, India, and antarctica
Wagner claimed that these two reptiles were land - dwelling creatures and could not just have swum across an entire ocean
people who were against the continental drift theory insisted trans - oceanic land bridges allowed animals and plants move between continents
these land bridges eventually dissolved ‘disappear' leaving the continents permanently separated
the problem about this hypothesis is the improbability of a land bridge being tall and long enough to stretch across a broad, deep ocean
Wagner found more evidence to support his theory when glaciers once existed in normally very warm areas such as southern Africa, India, Australia, and Arabia
similar evidence was discovered tropical plant fossils in the frozen region of the arctic circle

By the end Wegner came to a conclusion when collecting all his data he realized the explanation that best fit all the climate, rock, and fossil observation involved moving continents.

Proposed Mechanism for Continental Drift (2.1.2)

Wegener theory was considered a fringe
one of the biggest flaws in his hypothesis was an inability to provide a mechanism for how the continents moved
the scientific community would require exceptional evidence that supported credible mechanism
Wegener used centrifugal forces and precession, but this model was proven wrong
he speculated about seafloor spreading with hints of convection but could not substantiate these proposals
scientific knowledge reveals convection is one of the major forces in driving plate movements along with gravity and density

Plate Tectonics

Introduction

Revolution is reserved for significant political or social changes
Several of these idea revolutions forced scientists to re-examine their entire field of study
Ex: Gregor Mendel’s discovery was influenced by Darwin’s book ‘On The Origin of Species’ that was published in 1859
Ex: James Watson, Francis Crick, and Rosalind Franklin’s model for the structure of DNA in 19532
Ex: Albert Einstein’s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics
Plate tectonics was just as revolutionary for geology

The theory of plate tectonics attributes the movement of massive sections of the earth’s outer layer

Development of Plate Tectonic Theory (2.1.3)

Wegener died in 1930 on an expedition in Greenland
Wegener was poorly respected in his lifetime his idea about moving continents seemed uninteresting at the time, but his groundbreaking concept laid the foundation for the later acceptance of plate tectonics
In 1950s evidence started to trickle in that made continental drift a more viable idea by the 1960 scientists had amassed enough evidence to support the missing mechanism
Wegener’s hypothesis of continental drift to be accepted as the theory of plate tectonics
GPS and earthquake data continue to support Wegener’s theory
In 1947 researchers used an adaptation of SONAR (SOund, Navigation, And Ranging) to map a region in the Atlantic Ocean
Bruce Heezen and Marie Tharp created the first detailed map of the ocean floor to reveal the mid - Atlantic ridge,
scientist thought the ridge was part of a mechanism that explained the expanding of earth or ocean basin growth
1959 Harry Hess proposed the hypothesis of seafloor spreading
the mid - ocean ridges represented tectonic plate factories
scientist thought the ridge was part of a mechanism that explained the expanding earth or ocean basin growth hypotheses
Hess proposed seafloor spreading the mid ocean ridges represented tectonic plate factories
new oceanic plate was issuing from these long volcanic ridges
contributing to the continuous renewal of the ocean floor

VOCAB

Appalachian: they are mountains in the east of North America
continental drift: large scale horizontal movements of continents
margins: submarine edge of the continental crust
Pangea: was a supercontinent that existed on earth millions of years ago
Mesosaurus: an extinct genus of reptile that lived during the early Permian periods (300 million years ago)
glaciation: the process condition or results of being covered by glaciers or ice sheets
rock formations: large units called groups and smaller units called members of mappable with similar physical
supercontinent: each of several large landmasses thought to have divided to form the present continents in the geological past

Key Terms (MD.1)

supercontinent: a single large continent formed when all continents merge
Swamp: Wetland ecosystems characterized by waterlogged soils and dense vegetation, often dominated by trees, found dominantly in the tropical regions
continental crust: the thicker outermost layer of the earth (differentiated based on composition) with a low density
Glossopteris: A genus of fossilized woody plants Wegener used to infer that South America, Africa, Antarctica, India, and Australia were once together
Coal: A carbon-rich sedimentary rock formed from the remains of plants that accumulated in swampy, waterlogged environments millions of years ago
Oceanic crust: The thinner outermost layer of the earth (differentiated based on composition) with a high density
Geological Time Scale: The timeline used to describe the timing and relationships of events in Earth's history
Mantle: a layer of the earth (differentiated based on composition) found below the crust that contains more iron and magnesium than the crust
Meteorologist: A scientist who studies the atmosphere and weather patterns. They analyze data related to temperature, humidity, wind, and atmospheric pressure to understand and predict weather conditions.
The Appalachians: A major mountain range in eastern North America that extend from Newfoundland and Labrador in Canada, through the eastern United States, down to central Alabama. The range is characterized by its ancient, eroded mountains
Pangea: A supercontinent existing from approximately 335 million years ago to 175 million years ago
Continental drift: The gradual movement of continents across the Earth's surface over geological time
Paleoclimatology: The study of past climates
Mesosaurus: A small, freshwater reptile whose fossils were found in both South America and Africa.
Crust: the outermost layer of the earth (differentiated based on composition) that ranges in thickness from 10-100 kilometers
Glacial Striations: Scratches or gouges cut into bedrock by glacial activity

Key Terms (MD.2)

Asthenosphere: Waxy layer of the earth where convection occurs; defined based on its physical properties not its composition
Compression: The act of pressing something together
Compressional stress: The stress that resultsin the compaction of materials
Continental crust: The thicker outermost layer of the earth (differentiated based on composition) with a low density
Convection: The method of heat transfer that occurs in the asthenosphere and drives plate tectonics
Convergent plate boundary: A plate boundary created when two tectonic plate move toward one another
Crust: The outermost layer off the earth (differentiated based on composition) that ranges in thickness from 10 - 100 kilometers
Divergent plate boundary: A plate boundary created when two tectonic plates move away from one another
Earthquake: The ground shaking that results from movement on a fault
Fault: A crack in the earth’s crust along which movement occurs/occured
Geology: The study of the earth
Inner Core: The solid innermost layer of the earth primarily made up of iron
Juan de Fuca Plate: Tectonic plate located off the West Coast of Northern California, Oregon, and Washington
Lithosphere: Solid outermost layer of the earth made up of the crust and solid upper mantle; defined based on it’s physical properties, not its composition
mantle: A layer of earth (differentiated based on composition) with a high density
North American Plate: The tectonic plate upon which most of California sits
Oceanic crust: The thinner outermost layer of the earth (differentiated based on composition) with a high density
Outer core: The layer of the earth primarily made up of liquid iron
Pacific Plate: The tectonic plate located off of the west coast of most of California
San Andreas Fault: The main fault that marks the existence of a transform plate boundary between the Pacific and North
Strike - Slip Fault: A fault created by shear stress were crust on one side moves laterally relative to the other
Subduction: The process by which one tectonic plate dives down under another tectonic plate
Tectonic Plate: A piece of the lithosphere that moves independently of the other pieces; some are made of oceanic crust and the underlying upper mantle; others are made of continental crust and underlying upper mantle
Tension: The act of pulling something apart
Tensional Stress: The stress that results in the pulling apart of materials
transform plate boundary: A plate boundary created when two tectonic plates slide past one another
volcano: An opening in the crust through which gases, lava, and ash