Investigating Seafloors and Oceans

From Mud Volcanoes to Giant Squid

Antony Joseph

• Publisher: Elsevier
• Year: 2016
• Print ISBN: 9780128093573
• eBook ISBN: 9780128093634
• Topic: Physical Sciences
• Subtopic: Ecology

Investigating Seafloors and Oceans: From Mud Volcanoes to Giant Squid offers a bottom-to-top tour of the world's oceans, exposing the secrets hidden therein from a variety of scientific perspectives. Opening with a discussion of the earth's formation, hot spots, ridges, plate tectonics, submarine trenches, and cold seeps, the text goes on to address such topics as the role of oceans in the origin of life, tidal bore, thermal effects, ecosystem services, marine creatures, and nutraceutical and pharmaceutical resources. This unique reference provides insight into a wide array of questions that researchers continue to ask about the vast study of oceans and the seafloor. It is a one-of-a-kind examination of oceans that offers important perspectives for researchers, practitioners, and academics in all marine-related fields. - Includes chapters addressing various scientific disciplines, offering the opportunity for readers to gain insights on diverse topics in the study of oceans - Provides scientific discussion on thermo-tolerant microbial life in sub-seafloor hot sediments and vent fields, as well as the origin of life debates and the puzzles revolving around how life originated - Includes detailed information on the origin of dreaded episodes, such as volcanic eruptions, earthquakes, tsunamis, internal waves and tidal bores - Contains information on the contribution of the oceans in terms of providing useful nutraceutical and pharmaceutical products

Chapter 1

The Chaotic and Vibrant Seafloor

Abstract

This chapter begins with an introduction touching on the overall composition of planet Earth, covering the reversal of its magnetic field and the consequences of such repeated events. This is followed by a brief description of the discovery of formation of different continents from one big land mass and the clues put forward by several thoughtful minds, which finally led to the development of plate tectonics theories that succeeded in unifying continental drift and seafloor spreading notions. Various kinds of triple junctions on the seafloor are given adequate attention. Rotating microplates and last year’s discovery of the first oceanic microplate in the Indian Ocean is brought to light. Formation of submarine mountain chains, seamounts, island arcs, mud volcanoes, canyons, and trenches are briefly addressed. Finally, the chapter ends with a discussion of submarine canyons, subduction zones, and submarine trenches, including the latest finding based on analysis of results from an ocean drilling experiment.

Lists of references, bibliography, figure captions, and tables are provided.

Keywords

Magnetic field reversal; Plate tectonics; Continental drift; Seafloor spreading; Triple junction; Oceanic microplate; Ridge; Mud volcano; Canyon; Subduction zone; Trench

1.1 Introduction

Some of the marvels of the oceans cannot be adequately brought out and appreciated without first making a brief mention of the following: earth’s formation, interior, and triple plate junctions; earthquakes and volcanic eruptions; oceanic ridges and seamounts; seafloor spreading; plate tectonics; submarine trenches (long, narrow, very deep, curved, and usually bow-shaped basins in the seafloor); canyons (a class of narrow, steep-sided major incisions into the continental slope and rise that originate on the continental shelf of the oceans and continue to be carved and modified by a mix of flowing water and sediments); hot spots (fixed points on the earth’s surface defined by long-lived volcanism); submarine hot fluid-spewing chimneys; and cold seeps. The author believes this approach useful in providing a framework to evaluate the evolution of viable ecologic niches on this planet. Thus this book begins with a glimpse of the geological history of early Earth and the gradual evolution from its initially hot and entirely gaseous lifeless state to its present lively state.

The Big Bang event (see Chapter 5 for details) took place approximately 13.8 billion years ago (BYA). At that time, all of space was contained in a “single point,” which Dr. (Fr.) Georges Lemaitre who propounded the Bing Bang theory termed the “cosmic egg.” Following the Big Bang, which gave birth to the Universe in an explosion, the heaviest elements were born in the explosions of supernovae. In first-generation stars, heavier elements such as carbon, nitrogen, and oxygen were formed. Aging first-generation stars then expelled them out into space. The forces of gravity subsequently allowed for the formation of newer stars and of planets. The formation of the Solar System began approximately 4.6 BYA with the collapse of a small part of a giant nebula (an interstellar cloud of dust, hydrogen, helium, and other ionized gases). Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

The surface of a fire ball of hot gases consisting of rock vapor clouds (a Venus-sized proto-Earth), presumably formed at or close to the same time as the rest of the Solar System (approximately 4.6 BYA), started cooling from birth. The proto-Earth collided with a Mars-sized projectile some time before 4.5 BYA. Vaporized rock, mainly from the projectile, condensed in orbit to form the Moon. It is believed that the tenuous top surface layer (photosphere) of the rock vapor clouds radiated heat to space at approximately 2300 Kelvin (K), cooling the proto-Earth planet to liquid rock in approximately 1000 years. Thus early Earth was fully molten. Further cooling resulted in its present form of a flattened sphere called Earth whose polar diameter is about 12,714 kilometers (km) and equatorial diameter is about 12,756 km (see Sleep et al., 2011).

Water clouds condensed at the top of atmosphere. Heat radiated to space at cloud-top temperatures of approximately 250 K (see Zahnle et al.’s, 2010 review of the Earth’s earliest atmospheres). From 4 to 3.8 BYA, Earth underwent a period of heavy asteroidal bombardment. Steam escaped from the crust while more gases were released by volcanoes. Clouds formed as the planet cooled. Rain gave rise to the oceans about 3.8 BYA, or even earlier. A solar-heated greenhouse with approximately 100 bars CO₂ above a liquid water ocean at approximately 500 K remained (see Kasting and Ackerman, 1986; Sleep et al., 2001). The interior of the early Earth was hotter than at present only by a few hundred K, and the magnesium oxide (MgO)-rich lava, called komatiite, erupted under these hot conditions. Earth’s surface could not have cooled to habitable conditions without the eventual sequestration (seclusion) of atmospheric CO₂ (a greenhouse gas) as carbonates in hot rocks. The Earth’s surface was pleasant (approximately 30°C) only when approximately 1 bar of atmospheric CO₂ remained.

The Earth has a layered structure like an onion (see Fig. 1.1). Geologists collect information about Earth’s remote interior from several different sources. Some rocks found at the Earth’s surface, known as kimberlite and ophiolite, originate deep in the crust and mantle. Some meteorites are also believed to be representative of the rocks of the Earth’s mantle and core. These rocks provide geologists with some idea of the composition of the interior. Another source of information, while more indirect, is perhaps more important. That source is the earthquake, or seismic waves. When an earthquake occurs anywhere on Earth, seismic waves travel outward from the earthquake’s center. The speed, motion, and direction of seismic waves change dramatically at different levels within Earth, known as seismic transition zones. Therefore scientists can make various assumptions about the Earth’s character above and below these transition zones through careful analysis of seismic data.

Fig. 1.1 Structure of the Earth. From: https://en.wikipedia.org/wiki/Structure_of_the_Earth

From a geophysical perspective, the present Earth is endowed with a thin surface layer of rock called the crust, a thin “shell” of rock that covers the globe, whose 70% area lies below the oceans. There are two types of crusts: the continental crust, which consists mostly of light-colored rock of granitic composition that underlies the Earth’s continents; and the oceanic crust, which is a dark-colored rock of basaltic composition that underlies the oceans. One of the most important differences between continental and oceanic crusts is their density. The lighter-colored continental crust is also lighter in weight, with an average density of 2.6 g/cm³ (grams per cubic centimeter), compared to the darker and heavier basaltic oceanic crust, which has an average density of 3.0 g/cm³. It is this difference in density that causes the continents to have an average elevation of about 600 m above sea level, while the average depth of the ocean bottom is 3000 m below sea level. The heavier oceanic crust sits lower on the Earth’s surface, creating the topographic depressions for the ocean basins, while the lighter continental crust rests higher on the Earth’s surface, causing the elevated and exposed continental land masses. Another difference between the oceanic crust and continental crust is thickness. The thickness of the crusts varies from about 8 km under the oceans to about 40 km under the continents, but can reach up to 70 km in certain sections, particularly those found under newly elevated and exposed mountain ranges such as the Himalayas. The average depth of the oceanic crust is 3730 m below the sea surface and the maximum depth is 11,033 m (Mariana Trench in the Pacific Ocean).

Basalt is the dominant rock type in ocean crust. Basalt typically has approximately 50% SiO₂, 15% Al₂O₃, 10% each of CaO, MgO, and FeO and minor Na₂O and TiO₂. It is very abundant on the modern Earth. Olivine [(Mg, Fe)₂SiO₄] is the first major mineral to crystallize from most basaltic magmas (ie, molten rock) at shallow depths (Sleep et al., 2011).

The base of the crusts (both the oceanic and continental varieties) is determined by a distinct seismic transition zone called the Mohorovičic discontinuity. The Mohorovičic discontinuity, commonly referred to as “the Moho,” is named after the Croatian seismologist Andrija Mohorovičic. The Moho is the transition or boundary zone between the bottom of the Earth’s crust and the underlying unit, which is the uppermost section of the mantle called the lithospheric mantle. Like the crust, the lithospheric mantle is solid, but it is considerably denser. Because the thickness of the Earth’s crust varies, the depth to the Moho also varies from 5–10 km under the oceans to 35–70 km under the continents. The Moho is defined by the level within Earth where P-wave velocity increases abruptly from an average speed of about 6.9 km/s to about 8.1 km/s.

The temperature of the crust’s deepest part may reach 870°C. By examining the records of earthquake waves (P-wave, S-wave), scientists have learned that the inside of the Earth below the crust is divided into three distinct parts; the mantle, the outer core, and the inner core.

Underlying the crust is the mantle. The uppermost section of the mantle, which is a rigid layer, is called the lithospheric mantle. This section extends to an average depth of about 70 km, although it fluctuates between 50 and 100 km. The density of this layer is greater than that of the crust and averages 3.3 g/cm³. But like the crust, this section is solid and brittle, and relatively cool compared to the material below. This rigid uppermost section of the mantle (the lithospheric mantle), combined with the overlying solid crust, is called the lithosphere, which is derived from the Greek word lithos, meaning rock. At the base of the lithosphere, a depth of about 70 km, there is another distinct seismic transition called the Gutenberg low velocity zone. At this level, the velocity of S-waves decreases dramatically, and all seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a softer zone of partially melted material (with ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Foreword
• Preface
• Acknowledgments
• Chapter 1: The Chaotic and Vibrant Seafloor
• Chapter 2: Secrets of Bermuda Triangle and Formation of Polymetallic Nodules
• Chapter 3: Complex Wave Motions and Thermal Structure of the Oceans
• Chapter 4: The Role of Oceans in the Origin of Life and in Biological Evolution
• Chapter 5: Astronomical and Biological Organizational Relationships
• Chapter 6: Seafloor Hot Chimneys and Cold Seeps: Mysterious Life Around Them
• Chapter 7: Citadel-Building: Tiny Creatures in the Oceans—Corals
• Chapter 8: Marine Creatures With a Difference
• Chapter 9: Oceans: Abode of Nutraceuticals, Pharmaceuticals, and Biotoxins
• Chapter 10: Magic With Colors—Sea Surface Changes
• Index