The Earth's Interior

To get some understanding of what happens below the surface, one might have to use indirect means.

Geophysics - a branch of geology that applies the laws & principles of physics to study of earth.

Seismic waves - Reflection (usually at a rock boundary); Refraction (changing of direction, bending as waves run from one layer to another layer of different density)

Minimum distance from epicenter for refracted wave to arrive before direct wave

The Earth's Internal Structure

Crust - (Granite, Schist, Gneiss, and Sedimentary rocks)

30-50 km thickness

density 2.7 gm/cm³

P wave 6 km/sec

Continental Sial

Oceanic Sima

(7 km) marginal

D = 3.0 gm/cm²

composition... Basalt (gabbros)

P-wave 7 km/sec

Mantle

P wave 8 km/sec

Ultramafic rocks (e.g. peridotitie)

Lithosphere - crust and part of upper mantle - make up the plates of the plate tectonic theory (70 - 125 km)

Asthenosphere with lower p- wave velocity is probably a partially melted rock zone. This may the zone of magma generation.

P-wave shadow zone in region between 103⁰ and 143⁰

Composition of the core

Iron center? This is based on the average density of the earth which (which) is 5.5 gm/cm³

Crust and Mantle make up ~ 85% of earth's volume and here avg. density is lower than 5.5 gm/cm³

(1) Seismic activity

(2) density and assumptions

(3) based on meteorites composition

Existence of earth's magnetic field suggests metallic core

Isostasy - a balance or equilibrium between adjacent blocks of brittle crust floating on the plastic upper mantle

Gravity measurements

Force of gravity between A & B = constant

Gravity meter - measures the force of attraction of the earth to a mass over the meter. Also used to explore for ore or salt domes (positive and negative) gravity anomaly of ready is higher than the normal regional gravity

The Earth's Magnetic Field

Temp at the earth's center is too high for magnetism to hold as heat discharges magnetism

Magnetic field - a region of magnetic force dipolar - fluid metallic interior. Magnetic north and geographic north pole

Magnetic reversal - time interval during which the North magnetic pole and the south magnetic pole exchange positions

Magnetic meters - magnetic anomalies

Heat within the Earth

Geothermal gradient ~ 25C/km (75F/Mile) Problem with the gradient? This gradient cannot go on for ever. Core temp is between 4000⁰C to 6900⁰C

The Sea Floor (Ocean Floor)

Methods of studying the sea floor

rock dredge

cover

sea-floor drilling

submersibles

echo sounders

seismic profiler (similar to echo sounder but better mig louder noise at lower frequency)

magnetic, gravity & seismic refractor

Features of the Sea Floor

Shelf, Slope, Rift valley, Slope Shelf


sketch of the sea floor.....(diagram)

Rise, Abyssal Plain, 5 km Mid-oceanic Ridge, Sea Mounts, Trench

Continental shelf are topographic features that are defined by their depth, flatness and gentle seaward tilt slope <1

Topographic Oceanic Basin is divided into 3 major units

1. Continental Margins (2 types- active and inactive)

Continental Shelf

Continental Slope

Continental Rise

2. Oceanic Floor

3. Mid ocean ridges

Continental Slope is relatively steep slope that extends from a depth of 100-200 m at the edge of the continental shelf. Angle of depth ~ 4-5

Submarine canyons - V-shaped valleys that cut across continental shelves and down continental slope

Abyssal fans - found at the base of many submarine canyons

Canyons are erosional features and turbidity current is a great factor.

Passive continental margins - include a continental shelf, slope and rise. These develop on geologically quiet coasts (No earthquakes, volcanoes or young mountain ranges) found in Atlantic, Arctic, and Indian Oceans.

Continental rise has slope ~ 0.5 rests upon oceanic crust

Types of deposition

Sediments are deposited two ways - through turbidity current and contour current

Abyssal plains - very flat regions

Active Continental Margins - characterized by Earthquake

Oceanic trench - Average depth 8-10 km. Associated with oceanic trenches are earthquakes of the Benioff seismic zones - these areas are marked by abnormally low heat flow compared to normal ocean crust.

Mid-Oceanic Ridges - giant undersea mountain range > 80,000 km long, 1500-2500 km wide and 2-3 km high above the ocean floor

Fracture zones - 'major' lines of weakness in the earth's crust that cross the mid-oceanic ridge at approximately right angle.

Sea mounts, Guyots and Aseismic Ridges

. Sea mounts - conical undersea mountain 1000 m or more above sea floor

. Guyots - flat-topped sea mounts

. Aseismic Ridges - the alignment of sea mounts and or guyots with some ridges which have no earthquake activities

Reefs - wave-resistant ridges of coral, algae & other calcereous organisms and warm, shallow sunlit water with low suspended sediments

3 types of reefs

i. fringing reefs - flat and attached directly to shore

ii. Barrier reefs - parallel to shore separated by wide deep lagoons

iii. Atolls - circular reefs that rim lagoons

Sediments of the floor - Terrigenous (land derived), Pelagic Sediment (clay skeleton)

Terrigenous

Biogenous

Hydrogenous

Limestone

Manganese nodules

(economic protection)

Oceanic crust and ophiolites

Age of sea floor not older than 200 million years old as opposed to 4.5 billion years of earth

Geologic Riches in the sea

Oil & gas

Phospherite

Gold, Diamonds

Heavy black sands

Manganese nodules