1. Mid-ocean ridges (MORs)
Stand km above abyssal plains, and may be ~1000 km across
Axial valleys are 600 m – 2 km lower than ridge mountains
Axial valley/ridge mountains abut oceanic fracture zones
Numerous volcanoes have quiescent eruptions
Axial valleys & ridge mountains composed of basalts in distinctive pillow forms

2. MORs continued
Near axial trough, sediments and sedimentary rocks are thin or absent
Sediment thickness increases with distance from axial trough, but never exceeds 1.3 km
Age of oldest sediments increases with distance from axial trough
Sedimentary rocks are cut by faults, but MORs are not like continental mountains

3. Oceanic fracture zones (FZ’s)
Rectilinear (straight), often for 1000’s of km across ocean basin
Narrow, usually <100 km and may be <10 km wide
Vertical relief across a FZ may be several km
Axial valleys offest as much as 1000’s of km across FZ’s
When FZ’s extend into abyssal plains, they are usually covered sedimentary rock

4. Earthquakes in MOR-FZ systems
Earthquakes occur in axial valleys, especially on faults between valleys and adjacent ridge mountains
Earthquakes occur along fracture zones between offset axial valley segments

5. Deep-ocean trenches (DOT’s)
Arcuate
>6 km and often ~10 km deep
Narrow, typically ~5 km wide at base
Have a asymmetric V shape, with a gentle seaward slope, narrow base, and steeper slope rising to volcanic islands or continental volcanoes
Volcanoes may explosive, i.e. Mt. St. Helens or Mt. Pinatubo

6. Earthquakes near DOTs
Earthquakes occur along an inclined, planar zone that extends into the earth beneath the landward side of the DOT

7. Why do continents and oceans differ so much?
To find the answer, need to consider the character of earth’s interior
In a first pass at examining Earth’s interior, we note:
Earth’s interior is warmer than its exterior
Rock at earth’s interior of earth is denser than rock at surface

8. Interior of earth is warmer than the exterior
In boreholes and tunnels, temperature rises with depth
Heat flows steadily from earth’s interior
Molten rock originates inside earth - how?

9. Rocks inside earth are denser than rock at surface
Density of typical crustal rock is low, 2.7 g/cm3
Average density of earth is moderate to high, 5.5 g/cm3
Interior of earth must contain high density material - estimated to be g/cm3
Measurements of Earth’s moment of inertia confirm that earth’s mass is concentrated near its center

10. These two observations led earth scientists to the hypothesis of isostacy
Isostacy = floating balance, in which rigid crust floats on an underlying mantle
Mantle is more deformable and more dense than crust
Crust floats on mantle by
Airy mechanism - all crust has similar density. Where elevation is higher, crust is thicker
Pratt mechanism - crustal density differs. Where elevation is higher, crust is less dense

11. What causes differences in average elevation of continents and oceans?
Crustal thickness?
Thickness of continental crust = km
Thickness of oceanic crust = 5-7 km
Crustal density?
At surface, continental crustal rocks have low density (often about g/cm3), but lower continental crust is more dense
Aggregate density of continental crust is 2.8 g/cm3
Aggregate density of oceaic crust is 2.9 g/cm3
Variation in crustal thickness is more significant factor in explaining two elevations

12. Earthquakes and seismology
Earth is opaque. How can I state confidently what is the character of rock at depth? How can one determine more information about earth’s crust and its relationship to earth’s interior?
Earthquakes and seismology