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The University of Southern Mississippi
Department of Marine Science
Ocean Crust
1. Structure
a. Introduction. Layers 1-3 are defined based on seismic studies of reflection, refraction,
and speed of seismic waves. The structure revealed by seismic studies has a close
relationship to the composition and structure of rocks found in these layers. In this lecture,
we will inspect this relationship.
i. Layer 1 is equivalent to the sediment cover on the ocean crust.
ii. Layer 2 is equivalent to basalt1 and is further subdivided into 2A and 2B (sometimes a
2C)based on a slight increase in velocity of seismic waves.
iii. Layer 3 is equivalent to the gabbro2 layer above mantle.
b. Seismic evidence (describe starting from the top of the section)
i. Layer 2
(1) ~1.7 km thick, the thickness is variable.
(2) acoustic velocities are variable, ranging from 3.4-6.3 km/s.
(a) velocities are variable due to faulted cracks and subsequent sealing during
hydrothermal alteration (2A slow due to cracks, 2B fast due to filling of cracks,
2C is a deeper layer.
(b) three velocity sublayers, 2A, 2B, 2C with velocities of 3.64, 5.19, and 6.09 km/s
respectively.
(3) Layer 2 rocks are strongly magnetized relative to deeper layers.
(4) The slowest velocities (2A) occur near ORs due to great porosity and fracturing of
the rock. Velocities increase with increasing age, due to sealing of pores and
cracks. The slowest velocities of sublayer 2A disappear by about 30 Ma in the
Pacific Ocean and 60 Ma in the Atlantic Ocean.
(5) The base of Layer 2 is marked by an increase in acoustic velocity.
ii. Layer 3
(1) ~4.7 km thick
(2) velocities not as variable as those of layer 2; they range from 6.65 to 6.83 km/s
(3) The base of Layer 3 is the Mohorovičić discontinuity (Moho).
2. Seismology studies indicate that there are distinct layers in the crust. Now we want to learn
what the layers are made of, their structure, and their history. How are the different parts of
1
Basalt is a dark colored igneous rock made of Fe- and Mg-rich minerals and plagioclase feldspar.
2
Gabbro is a course-grained equivalent of basalt, with crystals that are larger because the rock has
cooled more slowly and in a hotter environment than basalt.
MAR 581, Geological Oceanography
Chapter 7, Oceanic Crust p. 1
The University of Southern Mississippi
Department of Marine Science
the crust formed?
a. The evidence comes from several sources.
b. Dredging on ridges and fracture zones, where lower curst and even uppermost mantle
is exposed at some extreme faults
c. Ophiolites - igneous rock formations on land that consist of ocean crust that was
uplifted (obducted) onto the continent in a tectonic “accident.”
d. Submersibles
e. Drilling, especially hard-rock legs of the Deep Sea Drilling Project/Ocean Drilling
Program/Integrated Ocean Drilling Program (DSDP/ODP/IODP)
f. Evidence from ophiolites
i. examples
(1) Oman ophiolite, which contains the Moho and the pillow basalt-dike transition and
the dyke to massive-gabbro transition near the Layer 2-3 boundary
(2) Troodos complex in Cyprus
ii. Formation of ophiolites - Pieces of sea floor crust are obducted (as opposed to
subducted!) onto continental rock through various tectonic accidents related to
compressional forces at subduction zones or in contracting backarc basins. The
problem with ophiolites is that the basalt may be formed in regions that are comparable
but not entirely like ocean ridges, so the crust might not be representative of normal
ocean crust. Furthermore, the process of obduction is rough and can disrupt and
distort the crust. However, ophiolites are accessible.
iii. Structure of the ocean crust inferred from ophiolites
(1) associated with deep-water sediments
(2) basalt
(a) pillow basalts
(i) ~1 km thick
(ii) zeolite alteration facies
(b) sheeted dikes
(i) 1.5 km thick
(ii) Greenschist metamorphic (alteration) facies
(3) gabbro: 3-5 km, amphibolite metamorphic facies
(a) massive gabbro above
(b) underlain by layered gabbro sills and metagabbros (amphibolite metamorphic
[alteration] facies)
(4) transition to mantle of layered gabbros intruded into dunite-peridotite rocks typical
of the mantle.
g. Evidence from dredging, submersible observations, and DSDP/ODP drilling at rifts3 and
fracture zones, where crust is thin or faulting has brought the mantle near the surface [Why
3Rift
zone = rift valley that contains the spreading center of an oceanic ridge (OR).
MAR 581, Geological Oceanography
Chapter 7, Oceanic Crust p. 2
The University of Southern Mississippi
Department of Marine Science
is this the case?]. These locales were typically known from extensive submersible and
dredging studies prior to drilling, which is very costly and not done without extensive
pre-cruise surveys to insure the likelihood of interesting results (a stripped-down cruise
with no fancy technology costs 6.2 million dollars. Drilling crust takes fancy technology,
so this is a very expensive undertaking).
i. magmatic events at ocean ridges
(1) are local in extent, 45- 60 m thick, <100 m in diameter
(2) episodic lasting <100 y with ~5000 y quiescence.
ii. important locales that expose the crust or where thin crust was drilled.
(1) Atlantis-II-fracture-zone of the Southwest-Indian-Ridge, Leg-118
(2) Hess Deep rift valley, East-Pacific-Rise, ODP-Site-894, Leg 147. (Layered
gabbros of Layer 3, dikes of layer 2, pillows of layer 2)
(3) Costa-Rica-Rift; Site 504
iii. largely confirm results from various ophiolites and expand on important in situ details.
h. Summary of formation of the crust (from the bottom up)
i. Layer 3 gabbros
(1) Formation of the layered gabbros: “Mantle-derived magmas cooled in these sills,
where they crystallized from a few percent to 50 of their mass. Residual liquids
then rose to form upper gabbros, dikes and lavas. Sills may form beneath
permeability barriers created by the crystallization of cooling liquid migrating by
porous flow. Once permeability barriers are present, however, porous flow
becomes a less important mode of magma ascent, compared to ponding in sills,
gradual increase in magma pressure, and periodic ascent in hydrofractures. Thus,
gabbroic sills in the crust-mantle transition zone (MTZ) may represent the
transition in fast-spreading ridge environments from continuous porous flow in the
mantle to periodic diking in the crust.” (Kelemen-Peter-B; Koga-Ken;
Shimizu-Nobu 1997. Geochemistry of gabbro sills in the crust-mantle transition
zone of the Oman Ophiolite; implications for the origin of the oceanic lower crust.
Earth and Planetary Science Letters.146; 3-4, Pages 475-488.)
(2) Formation of the massive gabbro: “The upper gabbro section of the oceanic crust
crystallizes from a thin melt lens located just below the sheeted dike complex.”
(Boudier-Francoise; Nicolas-Adolphe; Ildefonse-Benoit, 1996. Magma chambers
in the Oman Ophiolite; fed from the top and the bottom. Earth and Planetary
Science Letters.144; 1-2, Pages 239-250.)
ii. Formation of Layer 2 basalts
(1) formation of the sheeted dikes of layer 2
(a) the residual melt from the massive gabbro sills of Layer 3 is identical in
composition to the sheet dikes of Layer 2, a fact that suggests that this is the
source of the melt that forms Layer 2 (Kelemen et al., 1997)
(b) The dikes are injected when zones of weakness form due to extensional
faulting.
MAR 581, Geological Oceanography
Chapter 7, Oceanic Crust p. 3
The University of Southern Mississippi
Department of Marine Science
(2) formation of the pillow basalts interlayered with sediment
(a) these are extrusive that erupt onto the floor of the spreading center and pile up
over time. The pillows form by contact with seawater, and their intimacy with
the seafloor is further supported by the frequent occurrences of sediment amid
the pillows.
(b) The pillows form from lava brought to the surface through dikes.
iii. Seismic boundaries and lithologic boundaries occur in proximity but do not coincide
(1) the base of seismic layer 2 occurs above the dike/ gabbro transition in Hole 504B.
(Detrick-R-S; Collins-J; Stephen-R; Swift-S, 1993. Eos, Transactions, American
Geophysical Union.74; 43, Suppl., Pages 645.)
(2) The seismic Moho is shallower than the petrologic boundary between the mantle
and crust. The Moho may coincide with the top of the transition zone of layered
gabbros and peridotites.
3. Source of magnetism in the crust - integrated over all of the crust, but
a. The pillow basalts are most strongly magnetized, but thin
b. The rest of the crust is weakly magnetized, but holds a lot of mass and magnetic signal.
4. The ocean crust (basalt and gabbro) forms the top (<10%) of the lithosphere, which is in total
~100 km thick. The lithosphere below the crust is made of the uppermost mantle (peridotite and
related rocks). The lithosphere floats on the asthenosphere, which in this class we define as upper
mantle from the base of the lithosphere (at about 100 km depth) to the 660-km discontinuity
following Keary et al. (2009). Some geophysicists might place the boundary at a shallower depth.
a. I want to emphasize that crust, whether it is oceanic or continental, is a thin layer seated
on the uppermost mantle. Many plates (=lithosphere) have both oceanic and continental
crust seated on the same uppermost mantle peridotite.
b. Continental crust has formed by differentiation and fractionation of the interior of the
planet (the mantle). It is made of the lightest and most volatile materials, lighter than
mantle rock or even ocean crust, so it does not subduct. It is too buoyant to subduct.
Hence, the continents have accreted (grown) through the ages, growing larger and larger as
the mantle has fractionated. In contrast, because ocean crust does subduct, none is
younger than a few hundred million years. The oldest seafloor crust in the present-day is
about 160 million years old.
MAR 581, Geological Oceanography
Chapter 7, Oceanic Crust p. 4