Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Evidence for Sea-Floor Spreading

Evidence for Sea-Floor Spreading 

For a hypothesis to become a theory, researchers must demonstrate that the idea really works. During the 1960s, geologists found that the sea-floor spreading hypothesis successfully explains several previously baffling observations. Here we discuss two: (1) the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies; and (2) the variation in sediment thickness on the ocean crust, as measured by drilling.

Marine Magnetic Anomalies

Recognizing anomalies 

Geologists can measure the strength of Earth’s magnetic field with an instrument called a magnetometer. At any given location on the surface of the Earth, the magnetic field that you measure includes two parts: one produced by the main dipole of the Earth generated by circulation of molten iron in the outer core, and another produced by the magnetism of near-surface rock. A magnetic anomaly is the difference between the expected strength of the Earth’s main dipole field at a certain location and the actual measured strength of the magnetic field at that location. Places where the field strength is stronger than expected are positive anomalies, and places where the field strength is weaker than expected are negative anomalies.

The Discovery of Sea-Floor Spreading

The Discovery of Sea-Floor Spreading

New Images of Sea-Floor Bathymetry 

Bathymetry of mid-ocean ridges and abyssal plains.
Military needs during World War II gave a boost to sea-floor exploration, for as submarine fleets grew, navies required detailed information about bathymetry, or depth variations. The invention of echo sounding (sonar) permitted such information to be gathered quickly. Echo sounding works on the same principle that a bat uses to navigate and find insects. A sound pulse emitted from a ship travels down through the water, bounces off the sea floor, and returns up as an echo through the water to a receiver on the ship. Since sound waves travel at a known velocity, the time between the sound emission and the echo detection indicates the distance between the ship and the sea floor. (Recall that  velocity distance/time, so distance velocity s time.) As the ship travels, observers can obtain a continuous record of the depth of the sea floor. The resulting cross section showing depth plotted against location is called a bathymetric profile (figure above a, b). By cruising back and forth across the ocean many times, investigators obtained a series of bathymetric profiles and from these constructed maps of the sea floor. (Geologists can now produce such maps much more rapidly using satellite data.) Bathymetric maps reveal several important features.

Paleomagnetism and the Proof of Continental Drift

Paleomagnetism and the Proof of Continental Drift

More than 1,500 years ago, Chinese sailors discovered that a piece of lodestone, when suspended from a thread, points in a northerly direction and can help guide a voyage. Lodestone exhibits this behaviour because it consists of magnetite, an iron rich mineral that, like a compass needle, aligns with Earth’s magnetic field lines. While not as magnetic as lodestone, several other rock types contain tiny crystals of magnetite, or other magnetic minerals, and thus behave overall like weak magnets. In this section, we explain how the study of such magnetic behaviour led to the realization that rocks preserve paleomagnetism, a record of Earth’s magnetic field in the past. An understanding of paleomagnetism provided proof of continental drift and, contributed to the development of plate tectonics theory. As a foundation for introducing paleomagnetism, we first provide additional detail about the basic nature of the Earth’s magnetic field.

Earth’s Magnetic Field 

Features of Earth’s magnetic field.
Circulation of liquid iron alloy in the outer core of the Earth generates a magnetic field. (A similar phenomenon happens in an electrical dynamo at a power plant.) Earth’s magnetic field resembles the field produced by a bar magnet, in that it has two ends of opposite polarity. Thus, we can represent Earth’s field by a magnetic dipole, an imaginary arrow (figure above a). Earth’s dipole intersects the surface of the planet at two points, known as the magnetic poles. By convention, the north magnetic pole is at the end of the Earth nearest the north geographic pole (the point where the northern end of the spin axis intersects the surface). The north-seeking (red) end of a compass needle points to the north magnetic pole.

Wegener’s Evidence for Continental Drift

Wegener’s Evidence for Continental Drift 

Wegener suggested that a vast supercontinent, Pangaea, existed until near the end of the Mesozoic Era (the interval of geologic time that lasted from 251 to 65 million years ago). He suggested that Pangaea then broke apart, and the landmasses moved away from each other to form the continents we see today. Let’s look at some of Wegener’s arguments and see what led him to formulate this hypothesis of continental drift.

The Fit of the Continents 

Almost as soon as maps of the Atlantic coastlines became available in the 1500s, scholars noticed the fit of the continents. The northwestern coast of Africa could tuck in against the eastern coast of North America, and the bulge of eastern South America could nestle cozily into the indentation of southwestern Africa. Australia, Antarctica, and India could all connect to the southeast of Africa, while Greenland, Europe, and Asia could pack against the northeastern margin of North America. In fact, all the continents could be joined, with remarkably few overlaps or gaps, to create Pangaea. Wegener concluded that the fit was too good to be coincidence and thus that the continents once did fit together.