On June 21 (= summer solstice), midday sun directly over city of Syene (Aswan)

On June 21: Angle of sun at midday in Alexandria was 7.2o If the world was flat than there should be no difference in the angle. (500 miles)(360o /7.2o ) = x = 25,000 miles

worked out simple method for determining la)tude: • determine the angle between the horizon and the North Star (Polaris) 

Lat across, long up and down. Grid system used to precisely describe locations on the Earth’s surface. Based on angular relationships to Earth’s center. Expressed in degrees (o); each degree is divided into 60 minutes (‘), each minute into 60 seconds (“)  

• “parallels” = grid lines that are parallel to the Equator

 • 0o at Equator, 90o at poles • 0o -90o N = N. Hem.

• 0o -90o S = S. Hem.

Everywhere on Earth’s surface: 1o lat. = 60 nautical miles, 1’ lat. = 1 nautical mile.

• “meridians” = grid lines that intersect at poles

• 0o = “PrimeMeridian” (Greenwich, England)

• 180o = halfway around Earth

• 0o-180o to west of Greenwich = W. Hem.

• 0o-180o to east of Greenwich = E. Hem.

sextant is a tool to measure the angle of the sun to the horizon to get lat and chronometer compares your time to Greenwich time. (The difference between your time and GMT) X (15o Longitude per hour) = Your Longitude. If you are ahead of GMT you are in the Eastern Hemisphere (E is for Earlier).

determine latitude by measuring the angle of the north star from the horizon. Use southern cross in the southern hemisphere.

Sound source & receiver (hydrophone) on hull of ship. High frequency sound waves travel through the water, reflect off the seafloor, and are recorded by the hydrophone. Provide continuous 2-dimensional depth profiles along a ship’s track. d = (v)(t/2)

Like an echo sounder, but it images a 60 km swath of seafloor š Overlapping swaths = complete coverage (3-D)

First attempt at a bathymetric map by Matthew Fontaine Maury based on soundings. Showed vast relatively shoal area in Mid-Atlantic. 

Lower frequency energy source (stronger sound source with fewer sound waves per second). Deeper penetration into sedimentary layers and ocean crust

Sound source and hydrophone towed by ship. 2-D, continuous profile. Lower frequency energy source (stronger sound source with fewer sound waves per second). Deeper penetration into sedimentary layers and ocean crust

Precise altimeters (using microwaves) can map the relief of the ocean surface with centimeter-scale resolution š Bathymetric highs and lows on the seafloor, and differences in rock density, cause measureable gravitational distortion of the ocean surface

depict the topography, or relief of the seafloor; isobaths connect points of equal depth, like the contours lines on topographic maps

Oceaninc and Continenetal

Oceaninc crust, and lava part.

Thin region between the mantle and crust.

all crust portions

the upper mantle part that is lava right under oceanic crust.

less thick than continental and denser cause primarily basalt. Subducted at trenches during collision.

thicker and less dense than oceanic cause primarily granite. Preferentially preserved during collision.

Density=mass/volume.

the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density of underlying roots of the low density of the mountain.

offshore extension of the continent relatively shallow; 

underlain by thinned continental crust dissected by numerous submarine canyons

- thick accumulation of terrigenous sediments at base of the slope (= deep-sea fans)

deep, extensive, virtually flat plains

islands, atolls, guyots, seamounts, abyssal hills

fringed by coral reefs that continue to grow up as it subsides Makes steeps sided pillar.

– flat topped volcanic peak. Once at surface, but wave-cut then subsiding

deep, long, narrow, steep-sided troughs seismically active associated with volcanism landward of the trench

convection of rising hotter lava in mantle drives upward creating cracks that drive coninents outward from it on both sides. 

edge of shelf that is pushing against another due to being side furthest from spreading center.

edge of shelf that is not getting pushed against another due to being the side closest to the spreading center.

- the perimeter around the pacific plate that is exerting force almost all the way around resulting in a circle of volcanoes and earthquakes.

results in island arcs

results in volcanic mountain range near the edge of a continent

results in high mountain range since thick buoyant continental lithosphere cannot be subducted.

where upwelling Asthenosphere results in melting and new ocean crust (basalt) is formed; the Mid-Atlantic Ridge is a slow spreading center, whereas the East Pacific Rise is a fast spreading center. Spreading centers are characterized by shallow earthquakes. Ocean ridges.

occur at off-sets of oceanic ridges (spreading centers); they represent strike-slip plate boundaries and are characterized by shallow earthquakes.

(= volcanoes in the ocean or on land) forms landward of the trench on the overriding plate.

are volcanoes formed by stationary mantle plumes from deep in the mantle, unrelated to convection in the Asthenosphere (i.e., they are not related to plate tectonic processes such as subduction or seafloor spreading); hot spots produce linear island chains (e.g., Hawaiian Island-Emperor Seamount Chain) .

The profile of the ridge is related to the size of the magma chambers and the rate of seafloor spreading

Fast Spreading Rates - large magma chambers - poorly-defined central rift valley - broad oceanic ridge due to high heat flow - e.g., East Pacific Rise

Slow Spreading Rates - small, discontinuous magma chambers - well-defined central rift valley - relatively narrow and steep oceanic ridge - e.g., Mid-Atlantic Ridge