Inside Earth Geology
by Owen Borville
January 30, 2021
Learning, Geology, Science
The Earth's Internal Layers:
The Crust is the outermost layer of the Earth, consisting of a thicker and less dense continental crust and a thinner and denser oceanic crust. The uppermost 15 to 35 kilometers of the crust is brittle enough to produce earthquakes.
The continental crust is the rigid, outer layer of relatively low density rock that makes up the continents. The density of continental crust is approximately 2.7 g/cm3 (2.7 grams per cubic centimeter). The depth of the continental crust ranges from 25 to 85 kilometers. Oceanic crust is located beneath the oceans, is thinner and denser than the continental crust, and is composed mainly of basalt, iron, and magnesium rich minerals. The density of oceanic crust is approximately 3.0 g/cm3. The depth of the oceanic crust ranges from five to seven kilometers.
The Mantle is the layer of the Earth below the crust and above the core. The uppermost part of the mantle is rigid and along with the crust forms the tectonic plates of the Earth. The mantle is made up of dense iron and magnesium rich (ultramafic) rock such as dunite and peridotite. The density of the mantle is 3.3 to 5.8 g/cm3. The mantle extends to a depth of 2,900 kilometers into the Earth.
The Core is the innermost layer of the Earth and is made up of mostly of iron and nickel. The core begins just below the mantle at the depth of 2,900 kilometers and extends to the center of the Earth at the depth of 6,370 kilometers from the surface. The core is divided into a liquid outer core and a solid inner core. The core is the most dense of the Earth's layers. The density of the outer core is approximately 10 to 12 g/cm3. The density of the inner core is approximately 12 to 13 g/cm3.
Reflection and Refraction
Reflection of seismic waves occurs as the waves travel through the Earth and strike a new layer of different density, bouncing off in the opposite direction. By studying the travel times of reflected seismic waves following earthquakes, geologists can gradually construct a picture of the general structure of the Earth's interior.
Refraction of seismic waves occurs as the waves travel through the Earth and are bent or shift in direction at an angle as they strike a new layer of different density. By studying the travel times of refracted seismic waves following earthquakes, geologists can gradually construct a picture of the general structure throughout Earth's interior. A seismic discontinuity is a point inside the Earth where seismic waves change velocity and indicate a change in density of the medium in which the wave is travelling.
The P-wave shadow zone is the area of the Earth’s surface where p-waves from an earthquake are unrecorded because the waves are refracted as they pass through the boundary between the mantle and the core. The s-wave shadow zone is the area of the Earth’s surface that does not receive any s-waves and is caused by s-waves being stopped entirely as they enter the liquid core.
The Moho is the boundary between the crust and the mantle inside the Earth. The Moho is the depth where seismic waves change velocity and a change in chemical composition occurs. The Moho, also termed the Mohorovicic discontinuity, is located between 25 and 60 kilometers deep beneath the continents and between five and eight kilometers deep beneath the ocean floor.
Seismic tomography is a technique used to create an image of the interior of the Earth by studying the velocity and travel times of seismic waves that are generated by earthquakes. The velocity of the seismic waves helps geologists identify what type of material the waves travelled through. The velocity of seismic waves is affected by density, chemical composition, and thermal structure of the material. Seismic wave velocity is recorded by seismometers at locations across the Earth’s surface. Installing more seismometers allows geologists to collect higher resolution tomographic images. Seismic tomography is currently being used to study tectonic plate subduction, mantle plumes, and magma chambers beneath volcanoes.
Gravity is the attraction between two masses, such as the Earth and an object on its surface. Changes in the gravity field can be used to infer information about the structure of the Earth's lithosphere and upper mantle. A gravimeter is an instrument used to measure the local gravity and can be used to locate gravity anomalies. A gravity anomaly is a location where the local gravity measurement is higher or lower than expected or normal. Metallic ore deposits would have a positive gravity anomaly because of high density while sedimentary rock deposits such as salt domes would have a negative gravity anomaly because of low density.
Geophysical Logs: See Inside the Earth
Wells are holes that are drilled in the ground to extract oil, water, or other natural resources. Well cores are long tube shaped core samples of rock sediment recovered after wells are drilled. The cores are used by geologists to study the stratigraphy and geology of the area where the core was drilled.
Geophysical logging is a method of measuring properties of the subsurface rock by applying instruments inside a borehole. A variety of techniques exist which use electric currents, gamma rays, and sound waves to measure the porosity or density of the surrounding rock from the borehole. Porosity is the percentage of pore space in a particular rock.
Resistivity is a measure of how strongly a material opposes or resists the flow of an electric current. The unit of measure is the ohm (Ω). Geologists use instruments called resistivity logs which measure the resistivity of an electric current projected through the subsurface to determine properties of the underlying rock and sediment. Resistivity logs are a geophysical method of measuring the ability of rocks in the subsurface to conduct an electric current and are measured from inside a borehole.
Induced polarization (IP) is a method used to identify subsurface materials by producing electronic images (commonly of ore) in which an electric current is induced into the subsurface using electrodes. Induction logging is a type of geophysical log in which an electric current is produced inside a borehole and travels through the subsurface to a receiver and allows properties of the rock to be measured. Spontaneous potential logging (SP) (or self-potential log) is a technique used by geologists to identify and characterize subsurface rocks by measuring the natural electrical potentials (in millivolts) between depths inside a borehole and at the surface.
Gamma ray logs are a technique of measuring the natural gamma radiation of a rock unit (from a borehole) to obtain information about the rock unit such as the composition or type of rock. Gamma ray logs are commonly used in oil exploration. Porosity logging is a technique used to measure the porosity of the subsurface in a particular area by using one or several measurements including the density log or the neutron log. Density logs use gamma rays to detect the density of the surrounding rock. Neutron logging is used when neutrons are emitted to the surrounding wall rock and results in gamma rays being released, which allows for the calculation of the porosity of the rock.
Sonic logging is a technique used to measure the ability of a geologic rock unit to transmit sound waves and is used to measure the porosity of a rock unit. Acoustic velocity logs record the velocity of pulsed sonic waves generated in a probe and transmitted into rocks surrounding the probe in a drill hole. The sonic waves are reflected to receivers in the probe. This log can reveal the porosity and density of the rock.
Ground penetrating radar (GPR) is an instrument or device used to detect subsurface features and creates images of the subsurface. Seismic waves are emitted into the subsurface and reflected back to the surface, which allows images to be created. Magnetic logs are geophysical instruments that measure the magnetic field inside a borehole to collect information about the properties of the subsurface rock.
Earth's Magnetic Field
A magnetic field surrounds the Earth and changes with time and with location. The magnetic field resembles, in general, the field generated by a dipole magnet (a straight or linear magnet with a North and South Pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately eleven degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth's magnetic field is characterized by a direction and intensity which can be measured.
The Earth’s magnetic field is not actually generated by a magnet. The Earth’s outer liquid core is mostly made up of iron that convects very rapidly, acting as a dynamo that generates a magnetic field.
The magnetic needle in a compass is attracted by the magnetism of the Earth, and therefore always points to the constantly shifting magnetic North Pole. The geographic North Pole is static and is located about 1,200 miles north of the Magnetic North Pole. Maps and directions are usually oriented toward the Geographic North Pole, also referred to as True North.
Magnetic declination is the direction and amount of variation between the Magnetic North Pole and True North. The amount and direction of declination depends upon how those two poles align relative to a given point on Earth. When the two poles align, declination is zero, and the line of zero declination is termed the agonic line. At points west of the agonic line, a magnetic needle will point east of true north (positive declination). At points east of the agonic line, a magnetic needle will point west of true north (negative declination). Isogonic lines are like magnetic contour lines in that they trace a path of constant magnetic declination.
Magnetic inclination (at a given location) is the angle between the total magnetic field and the horizontal plane (the plane is tangent to the surface of the Earth at that point). A freely moving compass needle or ball will align itself to point directly at the magnetic pole. So in addition to an offset east or west, it will also point in the direction through the Earth to the magnetic pole location. This angle is called magnetic inclination or magnetic dip. The inclination is positive when the magnetic field points downward into the Earth and negative when the magnetic field points upward. For the north end of the needle, inclination is down in the Northern Hemisphere and up in the Southern Hemisphere. The inclination is greatest over the magnetic poles where the needle will be vertical.
Magnetic anomalies are variations in the magnetic field measured in a particular area or region as detected by a magnetometer. The anomaly can be either positive (iron presence) or negative (sedimentary rock presence). A magnetometer is an instrument used to measure the strength of the magnetic field in a particular location.
Paleomagnetism is the study of natural magnetism that is acquired by some rocks, commonly igneous rocks that are rich in iron as they solidify. Iron-rich minerals inside the cooling rocks align with the magnetic field lines and point toward magnetic north. Paleomagnetic analysis can determine the intensity and direction of the Earth's magnetic field during past geologic time by studying the remnant magnetism recorded in these rocks.
Magnetic polarity is the direction of magnetic poles (either normal or reversed) preserved in igneous rocks after they cool. A magnetic polarity reversal occurs when the Earth's magnetic field reverses polarity. This phenomenon has occurred many times in the Earth’s geologic history and is recorded in magnetized rocks, providing a historical record of magnetic reversals. A magnetic reversal would cause a compass to point south instead of north. Magnetic striping is produced from the generation of magma at mid-ocean ridges during alternating periods of normal and reversed magnetism. This magnetic striping gives evidence for the seafloor spreading of oceanic crust.
Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity change time scales to estimate geologic ages of the rocks. Rocks created along oceanic spreading ridges commonly preserve this pattern of polarity reversals as they cool and this pattern can be used to determine the rate of ocean ridge spreading.
by Owen Borville
January 30, 2021
Learning, Geology, Science
The Earth's Internal Layers:
The Crust is the outermost layer of the Earth, consisting of a thicker and less dense continental crust and a thinner and denser oceanic crust. The uppermost 15 to 35 kilometers of the crust is brittle enough to produce earthquakes.
The continental crust is the rigid, outer layer of relatively low density rock that makes up the continents. The density of continental crust is approximately 2.7 g/cm3 (2.7 grams per cubic centimeter). The depth of the continental crust ranges from 25 to 85 kilometers. Oceanic crust is located beneath the oceans, is thinner and denser than the continental crust, and is composed mainly of basalt, iron, and magnesium rich minerals. The density of oceanic crust is approximately 3.0 g/cm3. The depth of the oceanic crust ranges from five to seven kilometers.
The Mantle is the layer of the Earth below the crust and above the core. The uppermost part of the mantle is rigid and along with the crust forms the tectonic plates of the Earth. The mantle is made up of dense iron and magnesium rich (ultramafic) rock such as dunite and peridotite. The density of the mantle is 3.3 to 5.8 g/cm3. The mantle extends to a depth of 2,900 kilometers into the Earth.
The Core is the innermost layer of the Earth and is made up of mostly of iron and nickel. The core begins just below the mantle at the depth of 2,900 kilometers and extends to the center of the Earth at the depth of 6,370 kilometers from the surface. The core is divided into a liquid outer core and a solid inner core. The core is the most dense of the Earth's layers. The density of the outer core is approximately 10 to 12 g/cm3. The density of the inner core is approximately 12 to 13 g/cm3.
Reflection and Refraction
Reflection of seismic waves occurs as the waves travel through the Earth and strike a new layer of different density, bouncing off in the opposite direction. By studying the travel times of reflected seismic waves following earthquakes, geologists can gradually construct a picture of the general structure of the Earth's interior.
Refraction of seismic waves occurs as the waves travel through the Earth and are bent or shift in direction at an angle as they strike a new layer of different density. By studying the travel times of refracted seismic waves following earthquakes, geologists can gradually construct a picture of the general structure throughout Earth's interior. A seismic discontinuity is a point inside the Earth where seismic waves change velocity and indicate a change in density of the medium in which the wave is travelling.
The P-wave shadow zone is the area of the Earth’s surface where p-waves from an earthquake are unrecorded because the waves are refracted as they pass through the boundary between the mantle and the core. The s-wave shadow zone is the area of the Earth’s surface that does not receive any s-waves and is caused by s-waves being stopped entirely as they enter the liquid core.
The Moho is the boundary between the crust and the mantle inside the Earth. The Moho is the depth where seismic waves change velocity and a change in chemical composition occurs. The Moho, also termed the Mohorovicic discontinuity, is located between 25 and 60 kilometers deep beneath the continents and between five and eight kilometers deep beneath the ocean floor.
Seismic tomography is a technique used to create an image of the interior of the Earth by studying the velocity and travel times of seismic waves that are generated by earthquakes. The velocity of the seismic waves helps geologists identify what type of material the waves travelled through. The velocity of seismic waves is affected by density, chemical composition, and thermal structure of the material. Seismic wave velocity is recorded by seismometers at locations across the Earth’s surface. Installing more seismometers allows geologists to collect higher resolution tomographic images. Seismic tomography is currently being used to study tectonic plate subduction, mantle plumes, and magma chambers beneath volcanoes.
Gravity is the attraction between two masses, such as the Earth and an object on its surface. Changes in the gravity field can be used to infer information about the structure of the Earth's lithosphere and upper mantle. A gravimeter is an instrument used to measure the local gravity and can be used to locate gravity anomalies. A gravity anomaly is a location where the local gravity measurement is higher or lower than expected or normal. Metallic ore deposits would have a positive gravity anomaly because of high density while sedimentary rock deposits such as salt domes would have a negative gravity anomaly because of low density.
Geophysical Logs: See Inside the Earth
Wells are holes that are drilled in the ground to extract oil, water, or other natural resources. Well cores are long tube shaped core samples of rock sediment recovered after wells are drilled. The cores are used by geologists to study the stratigraphy and geology of the area where the core was drilled.
Geophysical logging is a method of measuring properties of the subsurface rock by applying instruments inside a borehole. A variety of techniques exist which use electric currents, gamma rays, and sound waves to measure the porosity or density of the surrounding rock from the borehole. Porosity is the percentage of pore space in a particular rock.
Resistivity is a measure of how strongly a material opposes or resists the flow of an electric current. The unit of measure is the ohm (Ω). Geologists use instruments called resistivity logs which measure the resistivity of an electric current projected through the subsurface to determine properties of the underlying rock and sediment. Resistivity logs are a geophysical method of measuring the ability of rocks in the subsurface to conduct an electric current and are measured from inside a borehole.
Induced polarization (IP) is a method used to identify subsurface materials by producing electronic images (commonly of ore) in which an electric current is induced into the subsurface using electrodes. Induction logging is a type of geophysical log in which an electric current is produced inside a borehole and travels through the subsurface to a receiver and allows properties of the rock to be measured. Spontaneous potential logging (SP) (or self-potential log) is a technique used by geologists to identify and characterize subsurface rocks by measuring the natural electrical potentials (in millivolts) between depths inside a borehole and at the surface.
Gamma ray logs are a technique of measuring the natural gamma radiation of a rock unit (from a borehole) to obtain information about the rock unit such as the composition or type of rock. Gamma ray logs are commonly used in oil exploration. Porosity logging is a technique used to measure the porosity of the subsurface in a particular area by using one or several measurements including the density log or the neutron log. Density logs use gamma rays to detect the density of the surrounding rock. Neutron logging is used when neutrons are emitted to the surrounding wall rock and results in gamma rays being released, which allows for the calculation of the porosity of the rock.
Sonic logging is a technique used to measure the ability of a geologic rock unit to transmit sound waves and is used to measure the porosity of a rock unit. Acoustic velocity logs record the velocity of pulsed sonic waves generated in a probe and transmitted into rocks surrounding the probe in a drill hole. The sonic waves are reflected to receivers in the probe. This log can reveal the porosity and density of the rock.
Ground penetrating radar (GPR) is an instrument or device used to detect subsurface features and creates images of the subsurface. Seismic waves are emitted into the subsurface and reflected back to the surface, which allows images to be created. Magnetic logs are geophysical instruments that measure the magnetic field inside a borehole to collect information about the properties of the subsurface rock.
Earth's Magnetic Field
A magnetic field surrounds the Earth and changes with time and with location. The magnetic field resembles, in general, the field generated by a dipole magnet (a straight or linear magnet with a North and South Pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately eleven degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth's magnetic field is characterized by a direction and intensity which can be measured.
The Earth’s magnetic field is not actually generated by a magnet. The Earth’s outer liquid core is mostly made up of iron that convects very rapidly, acting as a dynamo that generates a magnetic field.
The magnetic needle in a compass is attracted by the magnetism of the Earth, and therefore always points to the constantly shifting magnetic North Pole. The geographic North Pole is static and is located about 1,200 miles north of the Magnetic North Pole. Maps and directions are usually oriented toward the Geographic North Pole, also referred to as True North.
Magnetic declination is the direction and amount of variation between the Magnetic North Pole and True North. The amount and direction of declination depends upon how those two poles align relative to a given point on Earth. When the two poles align, declination is zero, and the line of zero declination is termed the agonic line. At points west of the agonic line, a magnetic needle will point east of true north (positive declination). At points east of the agonic line, a magnetic needle will point west of true north (negative declination). Isogonic lines are like magnetic contour lines in that they trace a path of constant magnetic declination.
Magnetic inclination (at a given location) is the angle between the total magnetic field and the horizontal plane (the plane is tangent to the surface of the Earth at that point). A freely moving compass needle or ball will align itself to point directly at the magnetic pole. So in addition to an offset east or west, it will also point in the direction through the Earth to the magnetic pole location. This angle is called magnetic inclination or magnetic dip. The inclination is positive when the magnetic field points downward into the Earth and negative when the magnetic field points upward. For the north end of the needle, inclination is down in the Northern Hemisphere and up in the Southern Hemisphere. The inclination is greatest over the magnetic poles where the needle will be vertical.
Magnetic anomalies are variations in the magnetic field measured in a particular area or region as detected by a magnetometer. The anomaly can be either positive (iron presence) or negative (sedimentary rock presence). A magnetometer is an instrument used to measure the strength of the magnetic field in a particular location.
Paleomagnetism is the study of natural magnetism that is acquired by some rocks, commonly igneous rocks that are rich in iron as they solidify. Iron-rich minerals inside the cooling rocks align with the magnetic field lines and point toward magnetic north. Paleomagnetic analysis can determine the intensity and direction of the Earth's magnetic field during past geologic time by studying the remnant magnetism recorded in these rocks.
Magnetic polarity is the direction of magnetic poles (either normal or reversed) preserved in igneous rocks after they cool. A magnetic polarity reversal occurs when the Earth's magnetic field reverses polarity. This phenomenon has occurred many times in the Earth’s geologic history and is recorded in magnetized rocks, providing a historical record of magnetic reversals. A magnetic reversal would cause a compass to point south instead of north. Magnetic striping is produced from the generation of magma at mid-ocean ridges during alternating periods of normal and reversed magnetism. This magnetic striping gives evidence for the seafloor spreading of oceanic crust.
Polarity reversals can be preserved in sequences of magnetized rocks and compared with standard polarity change time scales to estimate geologic ages of the rocks. Rocks created along oceanic spreading ridges commonly preserve this pattern of polarity reversals as they cool and this pattern can be used to determine the rate of ocean ridge spreading.
