Composition of Oceanic Crust Part 2: Pillow Lava, Dikes, Gabbro, and Peridotite

Summary

This 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

00:00

🌊 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.

05:04

🔍 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.

Keywords

💡Sediment

Sediment refers to solid particles that settle at the bottom of a liquid, such as the ocean, and can include organic matter, minerals, and rocks. In the context of the video, sediment covers the oceanic crust and is an essential part of the geological processes that shape the seafloor. The script mentions that beneath the sediment lies a layer of rock called pillow lava, indicating the sediment's role in the oceanic crust's formation and composition.

💡Pillow Lava

Pillow lava is a type of volcanic rock formed when lava erupts underwater and rapidly cools, creating a distinctive pillow-like shape. The video script describes how pillow lavas are formed at mid-ocean ridges, emphasizing their significance in the construction of oceanic crust. The term 'pillows' is used to illustrate the unique morphology of these rocks, which are a direct result of the interaction between molten lava and cold ocean water.

💡Basalt

Basalt is a common type of volcanic rock characterized by its fine grain size and mafic composition, which means it is rich in iron and magnesium. The script refers to 'pillow basalts' to describe the specific type of pillow lava that forms the uppermost layer of the oceanic crust. Basalt's fine grain size is due to its rapid cooling, which prevents the formation of large crystals.

💡Sheeted Dikes

Sheeted dikes are a geological feature that forms when magma intrudes into fractures in the Earth's crust and subsequently cools and solidifies. The script explains that they are part of an 'underground plumbing system' that brings magma from the mantle toward the ocean floor. The term 'sheeted' describes the appearance of these dikes as a series of parallel, elongated bodies, like a row of dominos, which is a visual representation of their formation process.

💡Gabbro

Gabbro is a coarse-grained, intrusive igneous rock that is compositionally similar to basalt but has larger crystals due to slower cooling. The script mentions gabbro as the layer beneath the sheeted dikes, indicating its formation away from the influence of ocean water, which allows for slower cooling and the growth of larger crystals. Gabbro represents the crystallized remnants of the magma chamber that fed the volcanic activity along the ridge.

💡Plagioclase Feldspar

Plagioclase feldspar is a group of rock-forming tectosilicate minerals that are rich in calcium and aluminum. The script identifies it as a key component of the upper oceanic crust, along with augite and magnesium-rich olivine. Plagioclase feldspar's presence is significant in the context of the video as it contributes to the overall composition and properties of the oceanic crust.

💡Augite

Augite is a common rock-forming pyroxene mineral that is rich in calcium and magnesium. The script describes augite as one of the main components of the basalt and gabbro in the oceanic crust. Its presence is crucial for understanding the chemical and physical properties of these rocks, as well as the overall structure of the crust.

💡Olivine

Olivine is a mineral composed of magnesium and iron silicates and is one of the main components of the Earth's upper mantle. The script mentions olivine as a significant part of the oceanic crust, particularly in the composition of basalt and gabbro. Its abundance in these rocks reflects the mantle's influence on the crust's formation.

💡Peridotite

Peridotite is an ultramafic rock, rich in olivine, that is found in the Earth's mantle. The script discusses layered peridotite as the rock at the very bottom of the lithosphere, with the upper portion being wehrlite. Peridotite's presence is indicative of the deep mantle materials that are brought to the surface through tectonic processes.

💡Differentiation

Differentiation refers to the process by which magma changes composition as it cools and crystallizes, with the most refractory minerals forming first. The script explains that as magma rises and cools, it undergoes differentiation, becoming more felsic or silica-rich. This process is key to understanding how the diverse layers of the oceanic crust are formed.

💡Ophiolite

An ophiolite is a sequence of rocks that includes all the layers of oceanic crust, from the layered peridotite at the bottom to the sediment on top. The script mentions ophiolite outcrops in Greece and Oman, specifically the Semali ophiolite, which provides a valuable reference for seismic measurements of oceanic crust. Ophiolites are significant in the study of the Earth's crust as they offer insights into the geological processes that occur at mid-ocean ridges.

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.