Mid-ocean ridges (MORs) Stand 2.5-4 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

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

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

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

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

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

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

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?

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

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

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

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