GEOL 101 Chapter 13
Seamounts
(Submarine volcanoes) 1. Over a million seamounts exist 2. Found in all ocean floors but most common in the Pacific 3. Many form near oceanic ridges or over a hot spot
Rift Valley
(a deep, down-faulted structure) exists on the axis of most ridges
What are the 4 main features of a deep ocean basin?
1. deep-ocean trenches 2. abyssal plains 3. seamounts and guyots 4. oceantic plateaus
Continental margins
Outer margins of the continents and the transition to oceanic crust
Oceanic Plautaes
Vast outpourings of basaltic lavas on the ocean floor (Resemble continental flood basalts)
A seamount may grow large enough to emerge as a _________ _________.
Volcanic Island
Oceanic ridges
A broad, linear swell at a divergent plate boundary
Modern bathymretric techniques
After World War II the US Navy developed sidescan sonar Sidescan sonar: images a horizontal region above the seafloor
Active Continental Margins
Where the oceanic lithosphere is being subducted beneath the continent (often associated with deep-ocean trenches)
Satellite radar altimeters can detect subtle changes (__ ____ ____) in elevation of the ocean surface
a few cm
Layer 1
consists of deep sea sediments and sedimentary rocks
Layer 4
consists of gabbro
Layer 3
consists of numerous interconnected dikes called a sheeted dike complex
Layer 2
consists of pillow lavas
5. Oceanic ridges with ______ spreading rates generally do not have a rift valley and have a shallow profile
fast
Newly formed melt (seafloor spreading)
from decompression melting of the mantle (slowly rises toward the surface)
Most melt solidifies in the ________ _________, but some escapes to the sea floor and erupts as lava
lower crust
the HMS Challenger expedition measured the depth of the seafloor by?
lowering weighted lines overboard - (Deepest spot measured is now called the Challenger Deep » 10,994 m deep, Measured in 1875)
Mapping the ocean floor from space using satellites equipped with?
radar altimeters
Ocanic crust forms as: Magma ascends through these fractures, cools, and solidifies to form a __________ ______ _________.
sheeted dike complex
3. Oceanic ridges with _______ spreading rates have well-developed rift valleys and rugged topography
slow
What is the evoulution of an ocean basin?
starts with the formation of a contineitail rift
subduction erosion is effective when the angle of desent is __________.
steep
Mixtures of sediment and water carried down ____________ __________.
submarine canyons
When the lithosphere is thick and cold
(rifts are narrow) Examples 1. East African Rift 2. Rio Grande Rift 3. Baikal Rift 4. Rhine Valley
When the lithosphere is thin and hot
(the rift can be very wide) Examples 1. Basin and Range in the western United States
Atlantic ocean
1. After tens of millions of years, the Red Sea will develop into a feature similar to the Atlantic Ocean 2. As new oceanic crust was added to the diverging plates, the rifted margins moved further from the region of upwelling 3. These margins cooled and subsided below sea level - Eventually become passive continental margins
How does Ocenaic crust form?
1. Basaltic magma originates from partially melted mantle peridotite 2. The magma rises through the upper mantle in tiny cracks until it reaches a lens-shaped magma chamber beneath the ridge crest 3. As the pressure in the chamber increases, the rock about the chamber periodically fractures
What are the 3 major areas of the ocean floor based on topography
1. Continental margins 2. Deep ocean basins 3. Oceanic ridges
East African Rift
1. Continental rift extending through eastern Africa 2. Consists of several interconnected rift valleys 3. Normal faulting led to grabens (down-faulted blocks) 4. Area has expensive basaltic flows and volcanic cones
what are some example of volcanic islands
1. Easter Island 2. Tahiti 3. Bora Bora 4. Galapagos Islands
Abyssal Plains
1. Flat features of the ocean floor (Likely the most level places on Earth) 2. Sites of thick accumulations of sediment (Fine sediments from turbidity currents) (Minerals precipitated from seawater) (Shells of marine plankton) 3. Found in all oceans (Most extensive in the Atlantic Ocean)
Red sea
1. Formed when the Arabian Peninsula rifted from Africa beginning about 30 million years ago 2. Fault scarps surrounding the Red Sea are similar to structures seen in the East African Rift 3. If spreading continues, the Red Sea will grow wider and develop an elongated mid-ocean ridge
Passive Continental Margins
1. Geologically inactive regions (not associated with plate boundaries) 2. Found along most coastal areas that surround the Atlantic Ocean 3. Experience little volcanism and few earthquakes
Characteristic of oceanic ridge, or mid-ocean ridge, or rise
1. The longest topographic feature on Earth 2. Width varies from 1000 to 4000 km 3. Occupy elevated positions 4. Segments are offset by transform faults 5. Extensive faulting and earthquakes
Characteristics of a continental shelf
1. Varies greatly in width 2. Gently sloping (average one-tenth of a degree slope) 3. Contains important mineral and oil deposits 4. Some areas contain extensive glacial deposits 5. Some areas contain shelf valleys 6. Important fishing grounds
What are two things associated with volcanic activity?
1. Volcanic islands arcs 2. Contenintal volcanic arcs (mostly found in pacific ocean)
Modern are the 2 bathymetric techniques
1. bathymetry 2. sonar
___-___ percent of the magma reaches the seafloor, where it quickly solidifies, forming large tube-shaped protuberances known as ________ ________
10-20% Pillow Basalts
From _____ to ____, the HMS Challenger expedition collected oceanographic data.
1872 to 1876
rift valleys range in width from ___ to ___ km and can have walls
30 to 50 km
How many layers make up the ophiolite complex?
4 layers
Only about ___% of the seafloor has been mapped in detail
5%
rift valleys tower from ____-_____ m abouve the valley floor
500-2500 m
Deep ocean basins
Between the continental margins and the oceanic ridge
What is the deepest spot in the ocean
Challenger Deep, in Mariana trench (10,994 meters below sea level) surface expression of a subduction zone
Seafloor spreding was formulated in the early 1960s by?
Harry Hess
Deep Ocean trench
Long narrow creases that represent the deepest part of the seafloor
Interactions between seawater and oceanic crust
Permeable and highly fractured crust allows seawater to penetrate the crust by 2-3 km
Massive underwater structures exert ________________ ______ _________ gravitational attraction (Water piles up over these features)
Stronger than normal
Guyots
Submerged, flat-topped seamounts 1. After the volcano is extinct, it eventually erodes to sea level where waves flatten the top of the structure 2. As plates carry the structure away, it eventually sinks into the ocean
Bathymetry
The topography (shape) of the ocean floor
(Active Continental Margins) Sediments and rocks can be scraped from the descending plate and accumulate on the continental plate as an_____________ ________.
accretionary wedge
Hot groundwater dissolves ions of various metals from the rock and precipitates them on the seafloor as particle-filled clouds called _______ _________
black smokers
When a turbidity current emerges onto the relatively flat ocean floor, the sediments spread out in a fan shape called a _______-____ _____. (the continental rise is composed of multiple deep-sea fans)
deep-sea fan
Early bathymetric profiles were created using _______ ________-, which bounce a sound off an object to determine the distance
echo sounders
Seawater is heated as it circulates through the crust, altering the basalt by ______________ __________________.
hydrothermal metamorphism
High-resolution multibeam
instruments send out a fan of sound and record reflections from various receivers to provide a more detailed view of the ocean floor (Produced first photograph-like images of seafloor)
4. Oceanic ridges with ___________ spreading rates have subdued rift valleys and topography
intermediate
Ophilite complex
is The sequence of four layers composing the oceanic crust is called an ophiolite complex
Continental shelf
is a gently sloping, flooded portion of the continent
Conternintrial slope
is a steep structure that marks the boundary between the continental and oceanic crust (Inclination average is 5 degrees) (The slope can reach 25 degrees)
Conternintial rise
is a thick accumulation of sediment from the continental slope
A continental rift
is an elongated depression where the lithosphere is stretched and thinned
Ocenaic ridge toporgraphy 1. Newly created Lithopshere-
is hot and less dense than surrounding rocks
2. As the newly formed crust moves away from the spreading center.
it cools and increases in density
An oceanic ridge, or mid-ocean ridge, or rise is a broad is a
linear swell along a divergent plate boundary
Subduction erosion
occurs when the subducting plate scrapes the bottom of the overriding plate
Where are most continental margins primarily located around?
the Pacific Ocean
Seaflor sreading accurs along?
the crest of ocenaic ridges
What are the two types of continental rifts? lithosphere that is
thick and cold thin and hot
sediments from continental rise are carried by?
turbiditary currents
Sonar
using sound energy, is now used to measure the depth to the ocean floor