Lecture 2: Sea-floor depth, age, and heat flow

• Why do we have ocean basins?
• Mid ocean ridges and the topography of the sea-floor
• Heat transport in the Earth

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What are ocean basins?

"We can only sense that in the deep and turbulent recesses of the sea are hidden mysteries far greater than any we have solved." -Rachel Carson, The Sea Around Us, 1957

Ask the class what we know about oceans, list ideas and topics on the board. Move the discussion towards topography and the why of topography.

What are ocean basins?

• Why does this topography exist?
• Crustal thickness.
  • What is crust? Different rocks than mantle, peridotite vs basalt/andesite
  • often defined by Distance to Mohorovičić discontinuity

Mohorovičić discontinuity

What are the differences between lithosphere and asthenosphere and crust and mantle?

• The Moho marks the transition in composition between the Earth's crust and the lithospheric mantle.
• Defined by wave speed:
  • above moho (P-waves) are consistent with those through basalt (6.7–7.2 km/s)
  • below they are similar to those through peridotite or dunite (7.6–8.6 km/s).
  • commonly accepted as the lower limit of the Earth's crust.
  • lith/astheno boundary at mid ocean ridge

Mohorovičić discontinuity

What are the differences between lithosphere and asthenosphere and crust and mantle?

Mohorovičić discontinuity

"Seismic evidence shows that the so-called crustal thickness-depth to the M discontinuity-is 6 km under oceans and 34 km under continents on the average." Gravity data prove that these two types of crustal columns have the same mass-the pressure at some arbitrary level beneath them, such as 40 km, would be the same. They are in hydrostatic equilibrium." -Harry Hess, History of Ocean Basins, 1962

Mohorovičić discontinuity

"Seismic evidence shows that the so-called crustal thickness-depth to the M discontinuity-is 6 km under oceans and 34 km under continents on the average. Gravity data prove that these two types of crustal columns have the same mass-the pressure at some arbitrary level beneath them, such as 40 km, would be the same. They are in hydrostatic equilibrium." -Harry Hess, History of Ocean Basins, 1962

Mohorovičić discontinuity

"Seismic evidence shows that the so-called crustal thickness-depth to the M discontinuity-is 6 km under oceans and 34 km under continents on the average. Gravity data prove that these two types of crustal columns have the same mass-the pressure at some arbitrary level beneath them, such as 40 km, would be the same. They are in hydrostatic equilibrium." -Harry Hess, History of Ocean Basins, 1962



How can we test this assumption?

Testing isostatic equilibrium

"Seismic evidence shows that the so-called crustal thickness-depth to the M discontinuity-is 6 km under oceans and 34 km under continents on the average. Gravity data prove that these two types of crustal columns have the same mass-the pressure at some arbitrary level beneath them, such as 40 km, would be the same. They are in hydrostatic equilibrium." -Harry Hess, History of Ocean Basins, 1962

• Continental crust:
• Mean elevation: 797 m
• Mean thickness: 34 km
• Andesite with density: 2.8 g/cm³

• Density of water: 1 g/cm³

• Oceanic crust:
• Mean elevation: -3686 m
• Mean thickness: 6 km
• Basalt with density: 2.9 g/cm³

What is the density of the mantle in g/cm³?

Testing isostatic equilibrium

You should get 3.46 g/cm³ (density of peridotite: 3.1–3.4 g/cm³) using the following mass balance:

$$ \mathrm{ \Delta H_{cc}\rho_{cc} = \Delta H_{w}\rho_{w} + \Delta H_{oc}\rho_{oc} + \Delta H_m\rho_m \\ ~\\ \Delta H_{cc}\rho_{cc} - \Delta H_{w}\rho_{w} - \Delta H_{oc}\rho_{oc} = \Delta H_m\rho_m \\ ~\\ \frac{\Delta H_{cc}\rho_{cc} - \Delta H_{w}\rho_{w} - \Delta H_{oc}\rho_{oc}}{\Delta H_m} = \rho_m \\ ~\\ \Delta H_m = \Delta H_{cc} - \Delta H_{oc} - E_{cc} - E_{oc}\\} $$

where $\Delta H$ is thickness, $E$ is elevation, $\rho$ is density, and the subscripts $w$, $cc$, $oc$, and $m$ correspond to the water in the ocean, the continental crust, the oceanic crust, and the mantle, respectively.

The topography of the sea-floor

The topography of the sea-floor

The topography of the sea-floor

Consider at least two ways that the topography of the sea-floor (mid-ocean ridges and the increase in depth away from ridges) can be in isostatic equilibrium. Draw a sketch for both.

The topography of the sea-floor

Consider at least two ways that the topography of the sea-floor (mid-ocean ridges and the increase in depth away from ridges) can be in isostatic equilibrium. Draw a sketch for both.

Which model is better at explaining sea-floor topography? Why?

• Heat flow highest at ridge (unlikely to be thicker crust)
• lowest density under the ridge somehow (thermal?)
  • how to make density go up with distance? cooling + transport (convection)

The topography of the sea-floor

"Nevertheless, mantle convection is considered a radical hypothesis not widely accepted by geologists and geophysicists. If it were accepted, a rather reasonable story could be constructed to describe the evolution of ocean basins and the waters within them. Whole realms of previously unrelated facts fall into a regular pattern, which suggests that close approach to satisfactory theory is being attained." -Harry Hess, History of Ocean Basins, 1962

What observations supported this radical hypothesis?

• Heat flow highest at ridge (unlikely to be thicker crust)
• sed accumulation rates suggest seafloor significantly younger than 4ga
• continents have moved
• lower density, viscous layer that crustal plates ride across