Composition of Oceanic Crust Part 2: Pillow Lava, Dikes, Gabbro, and Peridotite
Summary
TLDRThis script delves into the composition of oceanic crust, starting with pillow lavas formed by rapidly cooling lava at mid-ocean ridges. It explains the formation of sheeted dikes, a magma conduit system, and the underlying gabbro layer, which is the crystallized magma chamber. The crust's upper layers consist mainly of calcium-rich plagioclase feldspar, augite, and magnesium-rich olivine. The script also describes the layered peridotite at the lithosphere's base, including wehrlite and the process of magma differentiation. Ophiolites, sequences of oceanic crust rocks, are highlighted with the Semali ophiolite in Oman as a notable example, used for seismic studies of oceanic crust.
Takeaways
- 🌊 Oceanic crust is covered by sediment and consists of multiple layers of rock.
- 🔥 The uppermost layer is pillow lava, formed by rapidly cooling lava from mid-ocean ridge eruptions.
- 🌌 Pillow lavas are also known as pillow basalts, characterized by a mafic composition and fine grain size.
- 💧 The rapid cooling of lava in contact with cold ocean water creates the pillow shapes.
- 📚 Beneath the pillow basalt is a layer of sheeted dikes, which are magma conduits formed by crystallization within fractures.
- 🏗️ Sheeted dikes resemble a row of dominos, indicating the direction of magma flow towards the ocean floor.
- 🗻 Underneath the dikes lies gabbro, a coarser-grained igneous rock with larger crystals due to slower cooling.
- 🌋 Gabbro represents the crystallized remnants of the magma chamber feeding the mid-ocean ridge volcanism.
- 📈 The upper oceanic crust is primarily composed of calcium-rich plagioclase feldspar, augite, and magnesium-rich olivine.
- 🌍 At the base of the lithosphere is layered peridotite, with wehrlite being the crystallized ultramafic partial melts from the mantle.
- ⚗️ The process of magma cooling and crystallization leads to differentiation, where the magma becomes more felsic or silica-rich.
- 🏞️ Ophiolites, such as the Semali ophiolite in Oman, are sequences of oceanic crust rocks exposed on land, providing valuable geological insights.
Q & A
What is the first layer of rock found beneath the sediment on the ocean floor?
-The first layer of rock beneath the sediment is called pillow lava, which forms when molten lava erupts from cracks in the crust and rapidly cools upon contact with cold ocean water.
Why are pillow lavas also known as pillow basalts?
-Pillow lavas are called pillow basalts due to their mafic composition and fine grain size, with 'basalt' referring to the type of lava.
What is the difference between pillow lavas and the layer of rock called sheeted dikes?
-Pillow lavas are formed at the ocean floor's surface, while sheeted dikes are underground formations created by magma crystallizing within fractures that act as conduits for magma from the mantle to the ocean floor.
How are sheeted dikes formed?
-Sheeted dikes are formed when rising magma crystallizes within fracture-conduits instead of extruding onto the ocean floor to form pillow lava, resulting in clusters of basalt columns that create a complex resembling a row of dominos.
What is the composition of the gabbro layer found beneath the sheeted dike complex?
-The gabbro layer is composed of coarser-grained igneous rock and has the same composition as the pillow basalt above it, with larger crystals due to slower cooling away from the ocean water.
Why do the crystals in gabbro form larger than those in pillow basalt?
-Gabbro forms larger crystals because it cools much more slowly as it is not in direct contact with the ocean water, allowing for more time for crystal growth.
What minerals are primarily found in the upper oceanic crust?
-The upper oceanic crust is mainly composed of calcium-rich plagioclase feldspar (CaAl2Si2O8), augite (CaMgSi2O6), and magnesium-rich olivine (Mg2SiO4).
What is the layered peridotite and its significance in the composition of the oceanic crust?
-Layered peridotite is the rock at the very bottom of the lithosphere, with its upper portion composed of crystallized ultramafic partial melts of the mantle called wehrlite, representing the first and most undifferentiated magma created at the ridge.
What is the process of magma changing composition as it rises called?
-The process of magma changing composition as it rises, becoming more felsic or rich in silica, is called differentiation.
What is an ophiolite and where can ophiolite outcrops be found?
-An ophiolite is the entire sequence of rocks that comprise oceanic crust, from layered peridotite at the bottom to sediment on top. Ophiolite outcrops can be found in various locations, including Greece and notably Oman.
Why are ophiolites important for scientists studying the oceanic crust?
-Ophiolites are important for scientists as they provide a point of reference for seismic measurements of oceanic crust, offering insights into the composition and structure of the ocean floor.
Outlines
🌊 Formation of Oceanic Crust
This paragraph delves into the formation of oceanic crust, starting with the creation of pillow lava at mid-ocean ridges where new crust is formed. Molten lava erupts and rapidly cools in the cold ocean water, forming pillow-shaped structures. These pillow lavas, also known as pillow basalts, are characterized by their mafic composition and lack of visible crystals. Beneath the pillow basalt lies the sheeted dike complex, a system of fractures that serve as conduits for magma from the mantle. These dikes crystallize within the fractures, forming clusters of basalt columns. The gabbro layer, which is coarser-grained and composed of the same material as the pillow basalt, lies beneath the dike complex. Gabbro forms larger crystals due to slower cooling away from the ocean water. The upper portion of the gabbro has a uniform structure, while the bottom shows a layered or cumulate structure due to crystal settling. The composition of the upper oceanic crust includes calcium-rich plagioclase feldspar, augite, and magnesium-rich olivine. The layered peridotite at the base of the lithosphere is composed of wehrlite, a product of undifferentiated magma, which becomes more felsic as it rises and cools. The entire sequence of rocks is known as an ophiolite, with the Semali ophiolite in Oman being a notable example. Ophiolites provide valuable insights into the structure of oceanic crust and are used for seismic measurements.
🔍 Exploring the Composition of Rocks
The second paragraph shifts focus from the large-scale geological processes to the fundamental components of rocks. It sets the stage for a series of tutorials aimed at understanding what rocks are made of. This exploration into the composition of rocks is intended to provide a more familiar and observable perspective on geology, encouraging a deeper dive into the materials that constitute the Earth's crust and beyond.
Mindmap
Keywords
💡Sediment
💡Pillow Lava
💡Basalt
💡Sheeted Dikes
💡Gabbro
💡Plagioclase Feldspar
💡Augite
💡Olivine
💡Peridotite
💡Differentiation
💡Ophiolite
Highlights
Oceanic crust is covered by a layer of sediment, beneath which lies pillow lava formed by rapidly cooling lava at mid-ocean ridges.
Pillow lavas are bulbous bodies of basalt, formed by continuous lava eruptions, cooling, and stacking.
Pillow basalts are mafic in composition, with fine grain size and no visible crystals.
Below the pillow basalts, sheeted dikes act as magma conduits, feeding lava to the ocean floor.
Sheeted dikes are formed by magma crystallizing within fracture-conduits, creating a row of basalt columns.
Underneath the sheeted dikes is a layer of gabbro, a coarse-grained rock with the same composition as basalt but larger crystals due to slower cooling.
The gabbro layer represents the crystallized remnants of magma chambers that feed mid-ocean ridge volcanism.
The bottom portion of the gabbro layer has a cumulate structure, formed by dense crystals sinking in the magma chamber.
Basalt and gabbro are primarily composed of calcium-rich plagioclase feldspar, augite, and magnesium-rich olivine.
Layered peridotite, the lowest layer of oceanic crust, is composed of ultramafic partial melts of the mantle called wehrlite.
Wehrlite, rich in olivine and augite, represents the earliest magma created at mid-ocean ridges and undergoes differentiation as it rises.
Beneath wehrlite is another cumulate layer of olivine crystals, followed by unmelted mantle residuum composed of harzburgite and dunite.
The entire sequence of oceanic crust rocks, from sediment to peridotite, is known as an ophiolite.
Ophiolite outcrops can be found in Greece and Oman, with the Semali ophiolite being one of the largest and most exposed examples.
Scientists use the Semali ophiolite as a reference for seismic measurements of oceanic crust, highlighting its significance in understanding Earth's geology.
Transcripts
In the previous tutorial we discussed the types of sediment that cover oceanic crust.
Now it’s on to the crust itself. Beneath the sediment is a layer of rock called pillow lava.
At mid-ocean ridges where new oceanic crust is made, molten lava erupts from cracks in the crust,
where it almost instantly cools once in contact with the cold ocean water. The rapidly quenched
lava forms pillows or bulbous bodies which can then break open and exude more hot lava
from beneath the surface to form additional pillows. This process repeats over and over,
forming a thick layer of stacked pillow lavas. The pillow lavas are often called pillow basalts,
with the term basalt referring to the lava’s mafic composition and fine grain size. Basaltic
rocks typically lack visible crystals. Underneath the pillow basalt is a layer
of rock called sheeted dikes. Dikes are a sort of underground plumbing system that bring magma from
the mantle upward, toward the ocean floor. They are essentially fractures caused by extensional
faulting that act as magma conduits. Sheeted dikes are formed when the rising magma crystallizes
within the fracture-conduits, instead of extruding from the ocean floor to form pillow lava, and over
time, clusters of basalt columns form, creating a sheeted dike complex that looks like a row of
several-meter-long dominos standing side-by-side. Beneath the sheeted dike complex is a layer of
coarser grained igneous rock called gabbro, which has the same composition as the above
pillow basalt. Gabbro has larger crystals because it cools much more slowly, as it is not near
the ocean water. In fact, the gabbro layer is basically the crystallized remnants of the magma
chamber that was responsible for feeding all the volcanism along the ridge. The upper portion of
the gabbro has a uniform structure, while the bottom portion displays a layered or cumulate
structure, caused by dense crystals forming in the magma chamber and sinking down to the bottom.
The basalt and gabbro of the upper oceanic crust is composed mainly of calcium-rich
plagioclase feldspar, or CaAl2Si2O8, augite, or CaMgSi2O6, and magnesium-rich olivine, or Mg2SiO4.
Finally, at the very bottom of the lithosphere is rock called layered peridotite. The upper
portion of this layer is composed of crystallized ultramafic partial melts of the mantle below,
called wehrlite. This material represents the first and most undifferentiated magma created
at the ridge. As magma rises, it cools and crystallizes out the most refractory
minerals first, which causes the magma to change composition as it rises,
becoming more felsic, or rich in silica. This process is called differentiation.
Wehrlite is rock that is composed mainly of augite and olivine, and as it rises,
much of the olivine crystallizes out, leaving behind a more silica-rich melt that eventually
forms the basaltic, feldspar-rich pillow lavas of the ocean floor. Beneath the wehrlite is another
cumulate layer, or layer composed mainly of olivine crystals that formed in the wehrlite
magma chamber and sank to the bottom. Finally, at the very bottom is the unmelted residuum
of the original mantle which is composed mainly of the olivine-rich rocks harzburgite and dunite.
The entire sequence of rocks that comprise oceanic crust, from the layered peridotite on the bottom
to the sediment on top, is called an ophiolite. Ophiolite outcrops can be found in Greece,
and most notably, Oman. The Semali ophiolite, which was formed along a 95-million-year-old
mid-ocean ridge, is one of the largest and best exposed ophiolites in the world.
As the ocean basin closed 80 million years ago, the entire section of oceanic crust was thrust
onto the southeastern corner of the Arabian Peninsula during the collision of the Arabian and
Eurasian plates. Ophiolites are typically the last sliver of oceanic crust that evades subduction and
gets smushed between the two continents right before they collide. Today, scientists use the
Semali ophiolite as a point of reference for seismic measurements of oceanic crust.
So, with that we conclude a broad assessment of all the layers of the earth. Now it’s time to zoom
in from this grander scale down to things that are much more familiar and readily observable. What
are rocks themselves made of? Let’s spend the next few tutorials answering this important question.
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