Lecture 4  Sea floor spreading and mountain building processes

 Focus Question:  What is the significance of mid-ocean ridges and trenches?

1.        Historically,

tains and the deep ocean floor were places of great mysteries

Some mountain chains are volcanic, some not

Marine fossils and marine sedimentary rocks are at the tops of some very high mountains, like the Alps and the Himalayas

The ocean floor was an unknown region and mysterious region

1.        HMS Challenger Expedition-1872-1876

2.       Meteor Expedition-1920’s

3.       Both expeditions led to discovery of the mid-ocean ridge mountain system

2.       Features of the sea floor:

Mid-Ocean Ridge-high heat flow, shallow earthquakes, volcanos, hydrothermal vents

Trenches-low heat flow, shallow-intermediate-deep earthquakes, deepest places in the ocean

3.        Observations about Earthquakes-they are not randomly distributed on the globe-form patterns

4.       Heat Flow is highest around central rift of mid-ocean ridge system

5.       How do these features connect to the whole Earth?

Earth is layered by density (core, mantle, crust)

Layers also described by their behavior

                Lithosphere (crust and upper mantle) brittle, least dense

                Asthenosphere (the rest of the upper mantle) soft, flowing, but more dense

                Mesosphere-lower mantle

                Outer Core-Liquid

                Inner Core-solid

6.        Lithosphere made up of many rigid, brittle plates floating on Asthenosphere

Plate boundaries-3 types

Convergent-colliding plates - examples-Japan, Philippines, Andes, Cascades Mountains

Divergent-plates pull away from eachother - mid-ocean ridges

Transform-plates slide past one another  - transform faults like the San Andreas Fault, California

7.        Subduction Zones-where trenches form and ocean floor is destroyed and recycled

Ocean-ocean plate collision-colder lithospheric plate is pushed below warmer one

Ocean-Continental plate collision-ocean lithospheric plate is pushed below continent

            In both cases, the slab of ocean floor pushed into the asthenosphere melts, this magma rises and

Forms volcanic island arcs (Japan, Philippines, Aleutian Island Chain) or volcanic mountain chains (Andes)

8.        Sea Floor spreading zones-along mid-ocean ridges-new sea floor is made and plate motion is divergent-the plates pull away from eachother (mid-ocean ridge)

9.       Many early observations led to ideas that the continents had once been linked together into a giant land mass

a.       Alfred Wegener-author of book on Continental Drift-early ideas

b.      No mechanism for how the continents could have moved was known until after WWII

10.    Plate tectonics now understood, but evidence came from WWII and Cold War defense technology

1.        Magnetic record of minerals in the ocean basalts

2.       Magnetic Field orientation has reversed (Flipped) through time

3.       Evidence is recorded in magnetic polarity of basalts on the ocean floor

11.    Ocean floor is younger than continents-think why is this?  Continents do not subduct.

12.   Ocean floor is youngest on and around the mid-ocean ridges, older with distance away from ridges, symmetrical ages around the ridges

Next Lecture, Fossils-preservation and paleoenvironments,