SOIL, WEATHERING, STRATIGRAPHY &
PLATE TECTONICS
WEATHERING REVIEW
• Weathering: the gradual breaking down of rock into smaller pieces • There are 3 mains types of weathering: ① _Physical _______: crumbled or cracked mechanically ② Chemical _________: chemical change occurs (acid rain) ③ Biological __________: plant roots fissure rock
• When water seeps into cracks in rock, then freezes, __________________ has Frost wedging occurred & the rock will weather & erode
EXAM QUESTION
1. How can water be an agent for physical weathering? A. By absorbing gases from the atmosphere & ground to chemically react with minerals B. By seeping into the soil & dissolving the minerals in rocks C. By absorbing sulfur oxides & creaGng acid precipitaGon D. By seeping into the cracks of rocks & freezing
STRATIGRAPHY
• StraGgraphy is a branch of geology which studies rock layers (strata) & layering
• According to the law of superposi@on, the _oldest ______ layer is on the boAom & the youngest layer is on the top
STRATIGRAPHY
• Geologists establish the age of rocks in two ways: numerical da@ng & rela@ve da@ng • Numerical da@ng determines the actual ages of rocks through the study of radioac@ve decay • Rela@ve da@ng can’t establish absolute age, but it can establish whether one rock is older or younger than another
RELATIVE DATING
• RelaGve daGng requires an extensive knowledge of stra@graphic succession, a fancy term for the way rock strata are built up & changed by geologic processes • There are a few helpful principles/laws that help geologists determine the relaGve age of rocks....
RELATIVE DATING
• The Principle of Original Horizontality: All rock layers were originally horizontal
RELATIVE DATING
• Law of Superposi@on: The oldest rock layers are furthest toward the boOom, & the youngest rock layers are closest to the top
RELATIVE DATING
• Law of Cross-‐CuOng Rela@onships: Rock formaGons that cut across other rocks must be younger than the rocks that they cut across
RELATIVE DATING
• Law of Inclusions aka the law of included fragments: Any rock fragment found inside a layer of rock must be older that the rock it is in
REVIEW
• RelaGve daGng cannot give an exact age or Gme period, but it can tell us how old one strata is compared to another • Law of horizontal originality: All sediments were deposited __________________ Horizontally • Law of superposi@on: The more recent layers are on _top ___ & the older are @ the bottom ___________ • Law of cross-‐cuOng rela@onships: An intrusion is younger _______than the rock it intrudes • Law of inclusions: A fragment is ________ older than the rock it is in
Plate Tectonics
• The movement of the lithosphere, called plate tectonics, is the reason behind a lot of Earth's most dramaGc geologic events
• When one plate moves beneath another, or when two plates rub together, they can create earthquakes & volcanoes
Plate Tectonics
• 1915: Alfred Wegener, a German scienGst, proposed his theory of con@nental driR • Wegener’s con@nental driR hypothesis stated that the conGnents had once been joined to form a single superconGnent • He called this superconGnent Pangaea, meaning all land
Evidence for drift
• Evidence: The Con@nental Puzzle – Wegener noGced the similarity between the coastlines on opposite sides of the South AtlanGc Ocean – He showed how the shore lines fit together – CriGcs argued that erosion changes the shorelines over Gme
Evidence for drift • Evidence: Matching Fossils – Fossil evidence for conGnental driZ includes several fossil organisms found on different landmasses – Wegener reasoned that these organisms could not have crossed the vast oceans presently separaGng the conGnents – Mesosaurus (aquaGc repGle) is an example eastern (South America & Southern Africa)
Evidence for drift • Evidence: Matching Fossils – However, criGcs argues that land bridges existed & allowed animals to travel from place to place...leaving their fossils behind – BUT, no signs of such land bridges have ever been found in the AtlanGc Ocean
Evidence for drift
Evidence for drift • Evidence: Rock Types & Structures – In the conGnental driZ puzzle, matching rock types & mountain belts fit together – If the conGnents existed as Pangaea, the rocks found in a parGcular region on one conGnent should closely match in age and type those in adjacent posiGons on the adjoining conGnent
Evidence for drift • Evidence: Rock Types & Structures – Rock evidence for conGnental driZ exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean
Evidence for drift • Evidence: Rock Types & Structures – The Appalachian mountain belt runs northeastward through eastern US ending off the coast of Newfoundland – rocks of similar age are found in the BriGsh Isles & Scandinavia
Evidence for drift • Evidence: Ancient Climates – Wegener was a meteorologist & was interested in understanding the ancient climate to support his theory – He found evidence for dramaGc global climate changes – Wegener found glacial deposits showing between 220 millions & 300 million years ago, ice sheets covered large areas of the Southern Hemisphere
Evidence for drift • Evidence: Ancient Climates – Layers of glacial @ll were found in southern African & South America as well as in India & Australia – The grooves suggested that the ice had moved from the sea onto land, which is unusual – Many of the glaciers were found near the Equator, a now tropical locaGon?
Evidence for drift • Evidence: Ancient Climates – When considering his theory, it made sense that South Africa was once centered over the South Pole, creaGng condiGons necessary to form large areas of glacial ice over much of the Southern Hemisphere
Rejection • Wegener’s driZ hypothesis faced a lot of criGcism • One objecGon was that Wegener could not describe a mechanism that was capable of moving the con@nent across the globe • Most scienGsts rejected his hypothesis
New theory emerges • During the years that followed Wegener’s hypothesis, major strides in technology allowed mapping of the ocean floor • Extensive data on earthquake acGvity & Earth’s magneGc field also became available • By 1968, these findings led to a new theory plate tectonics • This theory provided a framework for understanding most geologic processes, such as the formaGon of mountains
The Earth’s Plates
• Today there are seven major plates: North America, South America, Eurasia, Africa, India, the Pacific, & AntarcGca • The number & the shape of the plates have changed through Gme • Some plates being destroyed at subducGon zones & other plates, carrying low-‐density conGnents, colliding to form 'superconGntents' • The most recent 'superconGnent' was Pangea
The Earth’s Plates (continued) Major Plates
Minor Plates
1. Pacific 2. North American 3. South American 4. African 5. Eurasian 6. Indian-‐Australian 7. AntarcGc
1. Juan De Fuca 2. Cocos 3. Nazca 4. Caribbean 5. ScoGa 6. Arabian 7. Philippine
The Earth’s Plates • Earth's crust is divided into 7 major tectonic plates & a few smaller ones
The Earth’s Plates (continued)
• The Earth's CRUST is cracked into 7 Major and 7+ Minor Plates, & they are all moving around: colliding, separaGng, or sliding past one another
PLATE TECTONICS
• Plate tectonics is the study of the dynamics of Earth’s lithosphere – that is, the movements of the plates
How Plates Move • The very slow movement of Earth’s plates is driven by the unequal distribu@on of heat within Earth • Hot material found deep in the mantle moves slowly upward as part of Earth’s internal convecGon systems • At the same Gme, cooler, denser slabs of oceanic lithosphere descend into the mantle, selng Eath’s rigid outer shell into moGon
How & Why Plates Move • Remember, Alfred Wegner couldn’t explain why conGnents move, & he didn’t take into account ocean movement • With advancements in technology such as SONAR in the 1950s, we developed a beOer understanding of why the plates move • Basically, ConGnental Plates move due to Sea-‐Floor Spreading and because of Convec@on Currents in the mantle
A little history & Important Peeps • In 1952, Marie Tharp developed the first chart of the Mid-‐Atlan@c Ridge System • Later, with Bruce Hoezen, they complied a map of the Global Ridge System
A little history & Important Peeps • Harry Hess, Princeton Geologist & CO of USS Cape Johnson during WWII – Conducted sonar mapping of sea floor while transiGng from baOle to baOle during the island-‐hopping campaign in the Pacific – Developed theory that new ocean crust is forming and spreading laterally at mid-‐ ocean
Why is the sea floor spreading apart?
• ConvecGon current in the mantle are causing oceanic crust to separate
Types of Plate Boundaries • All major interacGons among individual plates occur along their boundaries • The three main types of boundaries are convergent, divergent, & transform fault boundaries
transform
convergent
divergent
Divergent • Divergent boundaries (also called spreading centers) occur when two plates move apart • This process results in upwelling of material from the mantle to create new seafloor
Is Earth getting bigger? • Although new lithosphere is constantly being added at the oceanic ridges, our planet is not growing larger • Earth’s total surface area remains the same • HOW IS THIS POSSIBLE? • To accommodate the newly created lithosphere, older porGons of oceanic plates return to the mantle along convergent plate boundaries
Convergent • Convergent boundaries form where two plates move together (also called destrucGve plate margins) • This process results in oceanic lithosphere plunging beneath an overriding plate, & descending into the mantle
Convergent (types) • Oceanic-‐Con@nental: Denser oceanic crust sinks below the less dense conGnental crust (volcanoes erupt from rising magma) • Oceanic-‐Oceanic: One will descend under the other, forming deep trenches and/or underwater volcanoes/maybe volcanic islands • Con@nental-‐Con@nental: Forms mountains, conGnental crust is buoyant, which keeps it from sinking to any great depth
Continent-Continent
– Con@nent-‐con@nent: forms mountains like the Himalayas
Continent-Ocean
– Con@nent-‐oceanic: creates a subducGon zone (oceanic sinks below conGnental) – Subduc@on is a way of recycling the oceanic crust.
Ocean-Ocean
– Ocean-‐ocean: creates a subducGons zone and a deep ocean trench (Mariana Trench) – Trenches are the deepest parts of the ocean and remain largely unexplored – Manned or unmanned submersible vehicles have explored small parts of trenches discovering new species like the fish shown & amazing ecosystems
Transform Fault • Transform fault boundaries are margins where 2 plates grind past each other without the producGon or destrucGon of lithosphere (known as faults) • The San Andreas fault zone in Cali is an example of a transform fault boundary
Plates: Overview