Oceanography Midterm #1 (Plate Tectonics)

Plate Tectonics

• Plate tectonics affects all of us, whether in relation to the
  destruction caused by volcanic eruptions and
  earthquakes, or politically and economically, due to the
  formation and distribution of valuable natural resources.

• Plate tectonics is the unifying theory of geology, tying
  together many seemingly unrelated geologic
  phenomena and illustrating why Earth is a dynamic
  planet of interacting subsystems and cycles.

• Explains why we have mountains and volcanoes in certain places and why Earthquakes happen

Protoearth

• Larger than Earth today

• Homogeneous composition

• Bombarded by meteorites

• The Moon formed from a collision with a large asteroid

• Radioactive heat

• Spontaneous disintegration of atoms

• Fusion reactions

• Heat from contraction (protoplanet shrinks due to gravity)

• Protoearth partially melts

• Density Stratification (layered Earth)

Earth’s Interior (Chemical and Physical Comp)

Chemical Composition

• Crust

• Mantle

• Core

Physical Composition

• Lithosphere

• Asthenosphere

• Mesosphere

• Outer core

• Inner Core

Crust

The outermost layer, the crust, is low-density, mainly composed of silicate minerals.

1. continental crust: granite, thicker

2. oceanic crust: made of basalt, thinner crust, higher density

Mantle

The mantle surrounds the core and is divided into:

• Lithosphere: Ridged(solid), brittle plates that move at the surface

• an asthenosphere that behaves plastically and flows slowly.

• Mainly iron (Fe) and magnesium (Mg) silicate minerals.

• Mesosphere (ridged)

• more magnesium

• very rare to have this type of rock reach the surface

• Silicate materials

Core

The core consists of:

• A small, solid inner region

A larger, liquid, outer portion

• Flow generates Earth’s magnetic field
  ❑ High-density, mainly iron (Fe) and nickel (Ni)

Alfred Wegener (1912)

First Proposed Continental Drift

The Theory of Plate Tectonics is a fundamental foundation for the geosciences - explains the surface features on the Earth

• Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana.

• As Pangaea broke up, the various continents moved to their present-day locations.

• Concluded that the crusts move 

• Came up with Pangea 

• First to come up with the idea that the lithosphere is moving and that all continents were together (Pangaea)

Evidence of Continental Drift

1. Fit of continents (the outline of the continents)

2. Matching geological units ( look at rock types and mountain chains)

• Look at the layers, dates, and ages of rocks

• Continents once fitted together – Pangaea, one large continent, existed 200 million years ago

• Panthalassa – one large ocean

• Noted puzzle-like fit of modern continents corroborated in the 1960s

• Sir Edward Bullard used computer models to fit continents.

• There is a close fit between the continents off the coast at a
  depth of about 2000 m.

3. Glacial ages ( find certain details on rocks that can determine directions of ice)

• Find that polar regions are colder during periods when the Earth is tilted a certain way- 

• Glacial cycles

• Suggests that Antarctica was connected to South America, India, and Australia  

• When ice moves, it’s like a bulldozer

• Pulverizes rock through abrasion, it will streak the hard rock, revealing the direction of the ice 

• Those plates at one point moved 

4. Distribution of fossils ( looking at specific fossils that can only be in certain locations, only found in certain regions, but are now split apart) 

• Glossopteris fern

• Mesosaurus, freshwater reptile

Objections to Continental Drift

• Hostile criticism and open ridicule (The Origins of Continents and Oceans, 1915)

• Continents cannot plow through ocean basins

• Tidal gravitational attractions are too small

• The hypothesis was correct in principle; however, The proposed mechanism defies the laws of physics

• Erosion is happening, so they don't fit perfectly now

Paleomagnetism

Researchers refined new techniques (radiometric dating), developed new instruments (sonar), and discovered new fossils, which strengthened Wegener’s argument;
However, it was the research obtained from paleomagnetism and the exploration of the sea floor, which provided evidence of plate movements and the ocean floor spreading

• Liquid core and it rotates, making Earth a magnet

• Measure the movement and intensity of magnetic fields

• Paleomagnetism is the remnant magnetism in ancient rocks recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation.

• Magnetic polarity recorded in igneous rocks

• Magnetite in basalt, a natural magnetic mineral, when lava cools, it locks in place, which allows us to measure its intensity

• Geographic North- Stays the Same

• Magnetic North - Keeps Moving

Earth’s Magnetic Field

• Earth’s present magnetic field is considered normal

• Normal – with the north and south magnetic poles located approximately at the north and south geographic poles.

• At various times in the geologic past, Earth’s magnetic field has completely reversed

• One day, we are going to have a compass; over time, the earth is going to switch, and north will become the opposite

• Will impact navigation immensely

Magnetic Reversal

• Earth’s magnetic polarity reverses periodically

• Recorded in ancient igneous rocks

• 176 reversals in the past 76 million years

• Unpredictable pattern

• Last occurrence – 780,000 yrs

• Sir Edward Bullard: Looked beyond the shoreline at the continental shelves, they started to fit identically

• Everyone in the Arctic is now looking at the continental shelves 

• A mountain chain that spits 

Sea Floor Mapping

• 1955 – deep water rock mapping

• Magnetic anomalies – regular pattern of north-south magnetism “stripes”

• Stripes were symmetrical about the long underwater mountain range

Sea Floor Spreading

• Harry Hess - World War II submarine captain and geologist

• Depth recordings show sea floor features

• History of Ocean Basins

• Proposed - Seafloor spreading

• Mantle convection cells as a driving mechanism

• The mid-ocean ridge is a continuous underwater mountain range.

• Wraps 1.5 times around the globe.

• Rises 2.5 km

Frederick Vine and Drummond Matthews (1963)

• Seafloor spreading was confirmed by the discovery of magnetic anomalies in the ocean crust that were both parallel to and symmetric around the ocean ridges.

• This indicates that new oceanic crust must be formed along the spreading ridges.

• The oldest ocean floor is only 180 million years old

• Normal reverse was the result of these plates 

• Deep-sea drilling and the confirmation of seafloor, younger rock first, then the more you go, the older the rock gets

• The Pacific is getting smaller, atlantic is getting bigger

• Further evidence confirming seafloor spreading came from the Deep Sea Drilling Project are the age and thickness of the sediments overlying the oceanic crust.

• Millions of earthquakes occur every year 

  • We can measure this quickly and effectively 

  • Friction between plates can locate earthquakes based onthe  outlines of rocks

Plate tectonic Acceptance

• Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970s.

• The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources.

• For these reasons, it is known as a unifying theory.

• Moves from warm convection cells within the mantle that drive them apart

• Much evidence supports the theory today

Supporting Evidence: Earthquakes

• Most large earthquakes occur at subduction zones.

• Earthquake activity mirrors tectonic plate boundaries.

Plate Boundaries

Divergent

• Mid-ocean ridge, plates move away from each other, the most common

Convergent

• run into each other, plate approach each other, granite and basalt if they collide with each other

Transform

• Plates slide past each other

Divergent Plate Boundaries

An Example of Ancient Rifting

• Characteristic features of ancient continental rifting include faulting, dikes,
  sills, lava flows, and thick sedimentary sequences within rift valleys. Pillow lavas and associated deep-sea sediments are evidence of ancient
  spreading ridges.

Convergent Plate Boundaries

Oceanic-continental Convergence:

• The ocean plate will be subducted due to the salt material being denser: Pushed into the Earth 

• Gets pulled back into the earth and is recycled

• Creates a very deep trench 

• Basalt and Granite

• Most interesting Landforms, making very deep trenches

  

Oceanic-oceanic Convergence:

• Both salted and unsalted material 

• The denser plate is subducted and is older 

• Even though it's the same material, the more dense, the more likely it will be subducted 

• Trenches will be found 

• Island arcs, made from material from plates (salt) 

• older

  

Continental-continental Convergence: 

• Granite and granite, light in density 

• No subduction!

• Too light and not dense enough 

• If granite and granite smash into each other, you will have high mountain chains, tall/uplifted mountains: Himalayan (still growing)

Transform Plate Boundaries

These are boundaries along which plates slide laterally past each other along transform faults.

• Oceanic Transform Fault- ocean floor only 

• Continental Transform Fault- Cuts across the continent 

• San Andreas Fault

Transform faults occur between, mid-ocean ridge segments

Nematath - Hotspot Track

Recorded Ancient Plate Motions

• Adding water to magma makes it go crazy, and adding weight leads to molten 

• Not close to a plate boundary, it spews out

• Mantle plume doesn't move, while the plates still move

• Magma turns to lava and builds up, dividing the plates, creating island chains 

• New islands come up as the present ones shift over, creating the chain

• All islands will disappear, but new ones will be made

• We can measure this with technology today; continents move at the rate of your fingernails

• The Pacific is getting smaller, the Atlantic is getting bigger

Past and Future Plate Movements

Satellites have detected and measured plate movements over time

Paleogeography

The study of historical changes of continental shapes and positions

Continental Accretion

Continental material added to edges of continents through plate motion

50 Million Years in the Future

Assume the same direction and rate of plate motions as now
◼ Atlantic will enlarge, Pacific will shrink
◼ New sea from the East African rift valleys
◼ Further Himalayan uplift
◼ Separation of North and South America
◼ Part of California in Alaska

Mineral Deposits

• Many metallic mineral deposits are related to igneous and associated hydrothermal activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits

• Many of the world’s major metallic ore deposits are associated with convergent and divergent plate boundaries. Copper, iron, lead, zinc, gold and silver ore
  Deposits are associated with plate boundaries.

Distribution of Life

Fossil evidence provided one of the first proofs for plate tectonics. Together, plate tectonics and evolution have changed the way we view our planet.

• the world’s plants and animals occupy biotic provinces controlled mostly by:
  • Climate
  • Geographic barriers
  • The location of these provinces is mostly controlled by plate movement.

The world's plants and animals occupy biotic provinces controlled mostly by:

• Climate change 

• Geographic barriers

• Genova's features, passive margins