The structure of the Earth and the plate tectonics
STRUCTURE OF THE EARTH
-Mantle
-Crust
THE CRUST
-thick (10-70 km)
-buoyant (less dense than oceanic crust)
-mostly old
Oceanic Crust
-thin (~7 km)
-dense (sinks under continental crust)
-young
WHAT IS PLATE TECTONICS?
-Continental Drift
- The Earth is made up of 3 main layers
-Mantle
-Crust
THE CRUST
- This is where we live!
- The Earth's crust is made of:
-thick (10-70 km)
-buoyant (less dense than oceanic crust)
-mostly old
Oceanic Crust
-thin (~7 km)
-dense (sinks under continental crust)
-young
WHAT IS PLATE TECTONICS?
-Continental Drift
- Alfred Wegner in the early 1900's proposed the hypothesis that continents were once joined together in a single large land mass he called"Pangea" (meaning "all land" in Greek)
- He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift
- According to the hypothesis of continental drift, continents have moved slowly to their current locations.
- Pangea about 200 million years ago, before it began breaking up, Wegener named the southern portion of Pangea Gondwana, and the northern portion Laurasia
- The position of the continents today. The continents are still slowly moving, at about the speed your fingernails grow. Satellite measurements have confirmed that every year the Atlantic Ocean gets a few inches wider!
- Continents fit together like a puzzle...e.g. the Atlantic coastlines of Africa and South America
- The best fits include the continental shelves (the continental edges under water.)
- Fossils of plants and animals of the same species found on different continents
- Rock sequences (meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities
- Wegener showed that the same three layers occur at each of these places.
seafloor spreading
SEAFLOOR SPREADING
- On reason scientists had a hard time with Wegeners theory is that there was no mechanism for the continents motion.
- In the 1960s, a scientist named Henry Hess made a discovery that would vindicate Wegner
- Using technology, radar, he discovered that the seafloor has both trenches and id-ocean trenches
- Henry Hess proposed the sea-floor spreading theory
- Hess proposed that hot, less dense material below Earth's crust rises toward the surface at the mid-ocean ridges.
- Then, it flows sideways, carrying the seafloor away from the ridge in both directions
- As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created
- The older seafloor moves away from the ridge in opposite directions
- This helped explain how the crust could move-- something that the continental drift hypothesis could not do.
- in 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor.
- Scientists found that the youngest rocks are located at mid-ocean ridges
- Seafloor spreading provided insight to mechanism for how the continents moved
- The magma while pushes up at the mid-ocean ridge provides the new land pushing the plates, and the subduction zones gobble up the land on the other side of the plates
- Both Hess's discovery and Wegener's continental drift theory combined into what scientist now call the Plate tectonic theory
- The Earth's crust and part of the upper mantle are broken into sections, called plates which move on a plastic like layer of the mantle
- Plate tectonics- The earths crust is divided into 12 major plates which are moved in various directions
- This plate motion causes them to collide, pull apart, or scrape against each other
- Each type of interaction causes a characteristic set of Earths structures or "tectonic" features
- The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction
- Plates are made of rigid lithosphere
- The lithosphere is made up of the crust and the upper part of the mantle
- Below the lithosphere (which makes up the tectonic plates) is the the asthenosphere.
- "Plates" of lithosphere are moved around by the underlying hot mantle convection cells
what happens at tectonic plate boundaries?
Divergent- where two plates move away from each other
- Convergent- there are three styles of convergent plate boundaries
- Continent-continent collision- forms mountains, e.g. European Alps, Himalayas
- Continent-oceanic crust collision- called subduction
- Ocean-ocean collision- when two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone
- The sub ducting plate is bent downward to form a very deep depression in the ocean floor called a trench
- The worlds deepest parts of the ocean are found along trenches
- E.g. The Mariana Trench is 11 km deep
- Transform- where plates slide past each other
- Spreading ridges
- As plates move apart new material is erupted to fill the gap
- Iceland: an example of continental drifting, Iceland has a divergent plate boundary running through its middle
- Subduction- oceanic lithosphere sub ducts underneath the continental lithosphere
- Oceanic lithosphere heats and dehydrates as it subsides
- The melt rises forming volcanism
- E.g. The Andes
GEOLOGICAL PRINCIPLE
HOW ARE FOSSILS USEFUL TO US?
- Ancient fossils and rock data provide relative data of past events
- The rock record has contributed to our modern day geologic time scale, a record of Earth's hiistory from 4.6 bya to the present
- Eons= largest division (billions of years)
- Era= hundreds of millions of years; based on fossilized life forms found in rocks
- Cenozoic (recent life)
- Mesozoic (middle life)
- Paleozoic (ancient life)
- Periods= tens of millions of years; based on life forms that were abundant OR became extinct
- Epochs= smallest division (millions of years)
- ONLY in the Cenozoic Era because rocks and fossils are more easily accessed (not buried or destroyed)
- Principle of Uniformitarianism
- the processes occurring on Earth today have been occurring since it formed, BUT the rate and intensity have changed
- Ex: erosion, earthquakes, sea levels
- Principle of Original Horizontality
- Sedimentary rocks are deposited in nearly horizontal layers
- Principle of Superposition
- Oldest rocks are on the bottom, each successive layer is younger
- Principle of Faunal Successsion
- Oldest fossils are in the bottom layers, successive layers are younger
- Principle of Cross-Cutting Relationships
- An intrusion is younger than a rock it cuts across
- Uncomformities
- layers get shifted up or down
- Layers get eroded
- Correlation= matching rock regions in one area to another area
- Helps determine sequence of events in Earths history