2. Marine Geology 1 (Tectonics)

Ocean bathymetry (bottom depth)

Revealing the Topography of the Sea Floor

Sonar

sound navigation and ranging

Single-Beam Sonar

Creates a trackline of bottom depth as the ship moves forward

Multi-Beam Sonar

Creates a swath of bottom depth as the ship moves forward

Towed Multi-Beam Sonar

Creates a high-resolution swath of bottom depth as the ship moves forward

Who proposed Theory of Continental Drift in 1915

Alfred Wegener

Was theory of continental drift excepted, and why (not)?

the evidence was overwhelming that continental drift had occurred, the forcing mechanism he proposed (i.e., tidal action of the moon) was certainly wrong

when was plate tectonic theory developed

when “Sea-Floor Spreading” and “Sea-Floor Subduction” were discovered and the boundaries of Large Tectonic Plates were delineated in Post-WWII exploration

Rigid outer crust

Thin and Rigid and floats on the higher density mantle below

Plastic upper mantle

Although solid, the high temperatures cause the material to be sufficiently ductile to flow on very long timescales

last two layers of earth below upper mantle

molten outer core and solid inner core

Oceanic crust

✦ thin (~5 km)

✦ relatively higher density

✦ relatively young

✦ consists mostly of basalt

thinner and only slightly less dense than the underlying mantle material so it floats somewhat deeper in the mantle and creates a topographic low spot for ocean water to rush in to form ocean basins

Continental crust

✦ thick (~70 km)

✦ relatively lower density

✦ relatively old

✦ consists mostly of granite

thicker and less dense than the underlying mantle material and so it floats much higher in the mantle and creates a topographic high spot (dry land)

fit of the continents

Many naturalists had noticed the apparent fit of the continents

Paleobiogeography

Alfred Wegener noted in 1915 that in addition to the apparent good fit of continental boundaries, the distribution of fossil and mineral belts made sense if continents were joined together in the past.

Mid-ocean ridges

A mature divergent boundary; Discovered with Post WWII Sonar by Marie Tharp

Extensional faulting

Some indication of seafloor spreading at mid-ocean ridges was inferred based purely on observed morphology of the faulting* patterns along the ridge axis

faulting

Movement which produces relative displacement of adjacent rock masses along a fracture in the rock

Earth’s magnetic field moving

provided strong Evidence of Seafloor Spreading

• The Earth’s Magnetic Field is Currently Moving 35 Miles Each Year!

• The Earth’s magnetic field has undergone many reversals over geologic time

Magnetic Anomaly Patterns

a proxy measure of geologic time - each anomaly can be assigned a specific geologic date. Consequently, observing symmetric magnetic banding is really a round-about way of observing the rock getting symmetrically older as you move away from the ridge axis - and this is strong evidence that the ridge is slowly spreading away from the ridge axis in opposite directions

Discovery of Deep Sea Trenches

…and Associated Seismic Activity, Helped to Explain The Eventual Loss Of Ocean Crust that was Initially Formed at Mid-Ocean Ridges. This really sealed the deal!

Seismic Activity Around the World Delineates Plate Boundaries

both spreading and subduction boundaries

the driving mechanism of plate techtonics

Convection in the Mantle and Slab Pull

Convection in the Mantle and Slab Pull

Hot molten rock rises to the surface at divergent boundaries via convection and solidifies to form new oceanic crust.

The lithosphere/oceanic crust spreads laterally, cools over time and gets more dense and subsequently sinks and is pulled by gravity back into the mantle at subduction zones

a single tectonic plate is composed of lithosphere

solid ridge upper-mantle that is topped by either oceanic crust or continental crust or both. The lithosphere/crust section moves as a unit (i.e. as tectonic plate motion)

Plate Tectonics and Continental Drift

1. The earth’s outer crust is composed of many individual lithospheric plates called tectonic plates that move relative to one another

2. Lithospheric plates can be topped with ocean crust and/or continental crust.

3. When a Tectonic Plate moves so too does the continent that is a part of that plate.

Wilson Rock Cycle

the continents have repeatedly broken apart and collided back together

age of earth crust —- at ridges and symmetrically —- off-axis

Age of Ocean Crust is Youngest at Ridges and Symmetrically Older Off-Axis

Transform boundary

a place where two tectonic plates slide horizontally past each other

divergent plate boundaries

Occur where two plates are moving apart.

Most of these boundaries are mid-oceans ridges

Less commonly they can be continental rift zones.

East Africa Rift Zone

A New Divergent Plate Boundary

Hydrothermal vent systems

found at mid-ocean ridge spreading centers

1. seawater is drawn down through cracks in the off-axis region

2. seawater is heated by, and reacts with, hot rock material

3. mineral elements are extracted from the rock and dissolved into the hot seawater

4. heated seawater that is rich in rock minerals rises to ocean floor along the ridge axis

5. rock minerals in the hot seawater precipitate when the hot water hits the surrounding cold seawater

6. dissolved rock minerals support microbial life that forms the base of large vent ecosystems