Mid-ocean ridges (MORs)

1. Mid-ocean ridges (MORs) • Stand 2.5-4 km above abyssal plains, and may be ~1000 kmacross • Axial valleysare 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. MORscontinued • 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 ofoldestsediments 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 faultsbetween 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 10-11 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 ismore deformableand more densethan 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 = 25-40 km • Thickness of oceanic crust = 5-7 km • Crustal density? • At surface, continental crustal rocks have low density (often about 2.6-2.7 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. 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