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.