The world’s longest mountain chain is the mid-ocean ridge system, extending
25,000 miles (40,000 km) around the planet, representing
places where two plates are diverging, and new material is
upwelling from the mantle to form new oceanic crust and
lithosphere. These mid-ocean ridge systems are mature extensional
boundaries, many of which began as immature extensional
boundaries in continents, known as continental rifts.
Some continental rift systems are linked to the world rift system
in the oceans and are actively breaking continents into
pieces. An example is the Red Sea–East African rift system.
Other continental rifts are accommodating small amounts of
extension in the crust and may never evolve into oceanic rifts.
Examples of where this type of rifting occurs on a large scale
include the Basin and Range Province of the western United
States and Lake Baikal in Siberia.
Divergent Plate Boundaries in Continents
Rifts are elongate depressions formed where the entire thickness
of the lithosphere has ruptured in extension. These are
places where the continent is beginning to break apart and, if
successful, may form a new ocean basin. The general geomorphic
feature that initially forms is known as a rift valley. Rift
valleys have steep, fault-bounded sides, with rift shoulders
that typically tilt slightly away from the rift valley floor.
Drainage systems tend to be localized internal systems, with
streams forming on the steep sides of the rift, flowing along
the rift axis, and draining into deep narrow lakes within the
rift. If the rift is in an arid environment, such as much of East
Africa, the drainage may have no outlet and the water will
evaporate before it can reach the sea. Such evaporation leaves
distinctive deposits of salts and evaporite minerals, one of the
hallmark deposits of continental rift settings. Other types of
deposits in rifts include lake sediments in rift centers and conglomerates
derived from rocks exposed along the rift shoulders.
These sediments may be interleaved with volcanic rocks,
typically alkaline in character and bimodal in silica content
(i.e., basalts and rhyolites).
Modes of Extension
There are three main end-member models for the mechanisms
of extension and subsidence in continental rifts. These are the
pure shear (McKenzie) model, the simple shear (Wernicke)
model, and the dike injection (Royden and Sclater) model.
In the pure shear model, the lithosphere thins symmetrically
about the rift axis, with the base of the lithosphere
(defined by the 2,425°F (1,330°C) isotherm) rising to 10–20
miles (15–30 km) below the surface. This causes high heat
flow and high geothermal gradients in rifts and is consistent
with many gravity measurements that suggest an excess mass
at depth (this would correspond to the denser asthenosphere
near the surface). Stretching mechanisms in the pure shear
model include brittle accommodation of stretching on normal
faults near the surface. At about 4 miles (7 km) depth, the
brittle ductile transition occurs, and extension below this
depth is accommodated by shear on mylonite and ductile
shear zones.
In the simple shear model, an asymmetric detachment
fault penetrates thickness of the lithosphere, dipping a few
degrees forming a system of asymmetric structures across the
rift. A series of rotated fault blocks may form where the
detachment is close to the surface, whereas the opposite side
of the rift (where the lithosphere experiences the most thinning)
may be dominated by volcanics. Thermal effects of the
lithospheric thinning typically dome the detachment fault up.
This model explains differences on either side of rifts, such as
faulted and volcanic margins now on opposite continental
margins (conjugate margins) of former rifts that have evolved
into oceans. These asymmetric detachments have been
observed in seismic reflection profiles.
The dike injection model for rifts suggests that a large
number of dense mafic dikes (with basaltic composition)
intrude the continental lithosphere in rifts, causing the lithosphere
to become denser and to subside. This mechanism does
not really explain most aspects of rifts, but it may contribute
to the total amount of subsidence in the other two models.
In all of these models for initial extension of the rift, initial
geothermal gradients are raised, and the isotherms
become elevated and compressed beneath the rift axis. After
the initial stretching and subsidence phases, the rift either
becomes inactive or evolves into a mid-ocean ridge system. In
the latter case, the initial shoulders of the rift become passive
continental margins. Failed rifts and passive continental margins
both enter a second, slower phase of subsidence related
to the gradual recovery of the isotherms to their deeper, prerifting
levels. This process takes about 60 million years and
typically forms a broad basin over the initial rifts, characterized
by no active faults, no volcanism, and rare lakes. The
transition from initial stretching with coarse clastic sediments
and volcanics to the thermal subsidence phase is commonly
called the “rift to drift” transition on passive margins.
Divergent Margins in the Oceans:
The Mid-Ocean Ridge System
Some continental rifts may evolve into mid-ocean ridge
spreading centers. The world’s best example of where this
transition can be observed is in the Ethiopian Afar, where the
East African continental rift system meets juvenile oceanic
spreading centers in the Red Sea and Gulf of Aden. Three
plate boundaries meet in a wide plate boundary zone in the
Afar, including the African/Arabian boundary (Red Sea
spreading center), the Arabian/Somalian boundary (Gulf of
Aden spreading center), and the African/Somalian boundary
(East African rift). The boundary is a complex system known
as an RRR (rift-rift-rift) triple junction. The triple junction
has many complex extensional structures, with most of the
Afar near sea level, and isolated blocks of continental crust
such as the Danakil horst isolated from the rest of the continental
crust by normal faults.
The Red Sea has a juvenile spreading center similar in
some aspects to the spreading center in the middle of the
Atlantic Ocean. Geologists recognize two main classes of
oceanic spreading centers, based on geomorphology and
topography. These types are found to be related to spreading
rate, with slow spreading rates, 0.2–0.8 inch per year (0.5–2
cm/yr), on Atlantic-type ridges, and faster rates, generally
1.5–3.5 inches per year (4–9 cm/yr), on Pacific-type ridges.
Atlantic-type ridges are characterized by a broad,
900–2,000-mile (1,500–3,000-km) wide swell in which the
seafloor rises 0.6–1.8 miles (1–3 km) from abyssal plains at
2.5 miles (4.0 km) below sea level to about 1.7 miles (2.8
km) below sea level along the ridge axis. Slopes on the ridge
are generally less than 1°. Slow or Atlantic-type ridges have a
median rift, typically about 20 miles (30 km) wide at the top
to 0.6–2.5 miles (1–4 km) wide at the bottom of the 1-kilometer
deep medial rift. Many constructional volcanoes are
located along the base and inner wall of the medial rift.
Rugged topography and many faults forming a strongly
block-faulted slope characterize the central part of Atlantictype
ridges.
Pacific-type ridges are generally 1,250–2,500 miles
(2,000–4,000 km) wide and rise 1.2–1.8 miles (2–3 km)
above the abyssal plains, with 0.1° slopes. Pacific-type ridges
have no median valley but have many shallow earthquakes,
high heat flow, and low gravity in the center of the ridge, suggesting
that magma may be present at shallow levels beneath
the surface. Pacific-type ridges have much smoother flanks
than Atlantic-type ridges.
The high topography of both types of ridges shows that
they are isostatically compensated, that is, they are underlain
by low-density material and are floating on this hot substrate.
New magma upwells beneath the ridges and forms small
magma chambers along the ridge axis. The magma in these
chambers crystallizes to form the rocks of the oceanic crust
that gets added (in approximately equal proportions), to both
diverging plates. The crust formed at the ridges is young, hot,
and relatively light, so it floats on the hot underlying asthenosphere.
As the crust ages and moves away from the ridge it
becomes thicker and denser, and subsides, explaining the
topographic profile of the ridges. The rate of thermal subsidence
is the same for fast- and slow-spreading ridges (a function
of the square root of the age of the crust), explaining why
slow-spreading ridges are narrower than fast-spreading ridges.
The centers of the mid-ocean ridges are characterized by
abundant volcanoes, with vast outpourings of basaltic lava.
The lavas are typically bulbous-shaped forms called pillows,
as well as tubes and other more massive flows. The ridge axes
are also characterized by very high heat flow, with many
thermal vents marking places where seawater has infiltrated
the oceanic crust, made its way to deeper levels where it is
heated by coming close to the magma, then risen again to
vent on the seafloor. Many of these vents precipitate sulfide
and other minerals in copious quantities, forming chimneys
called “black smokers” that may be many meters tall. These
chimneys have high-temperature metal- and nutrient-rich
water flowing out of them (at temperatures of several hundred
degrees Celsius), with the metals precipitating once the
temperature drops upon contact with the cold seawater outside
the vent. These systems may cover parts of the oceanic
crust with layers of sulfide minerals. Unusual primitive communities
of sulfide-reducing bacteria, tube worms, and crabs
have been found near several black smoker vents along midocean
ridges. Many scientists believe that similar settings may
have played an important role in the early appearance and
evolution of life on the planet.
Seismic refraction studies in the 1940s and 1950s established
that the oceanic crust exhibits seismic layering that is
similar in many places in the oceans. Seismic layer 1 consists
of sediments, layer 2 is interpreted to be a layer of basalt
0.6–1.5 miles (1–2.5 km) thick, and layer 3 is approximately
4 miles (6 km) thick and interpreted to be crystal cumulates,
underlain by the mantle. Some ridges and transform faults
expose deeper levels of the oceanic lithosphere, which can be
shown to typically include a diabase dike complex, thick sections
of gabbro, and ultramafic cumulates. In some places,
rocks of the mantle are exposed, typically consisting of
depleted harzburgite tectonites. Much of the detailed information
about the deep structure of oceanic crust comes from
the study of ophiolites, which are interpreted to be tectonically
emplaced on-land equivalents of oceanic crust. Studies of
ophiolites have confirmed the general structure of the oceanic
crust as inferred from the seismic reflection and refraction
studies and limited drilling. Numerous detailed studies of
ophiolites have allowed unprecedented detail about the structure
and chemistry of inferred oceanic crust and lithosphere
to be completed, and as many variations as similarities have
been discovered. The causes of these variations are numerous,
including differences in spreading rate, magma supply, temperature,
depth of melting, tectonic setting (arc, forearc, back
arc, mid-ocean ridge, etc.), and the presence or absence of
water. However, the ocean floor is still largely unexplored,
and we know more about many other planetary surfaces than
we know about the ocean floor of the Earth.
See also OPHIOLITES; PLATE TECTONICS.
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