Understanding the Rock Cycle: Processes and Structures
The Cycle of Rocks
On the Surface
It produces the weathered rocks and their transformation into sediments, transported by geological agents and accumulated in sedimentary basins.
Inside the Earth’s Crust
The materials are subjected to high temperatures, and the sediments become rocks.
Pressure
Due to the weight of rocks, pressure increases with depth.
Temperature
Temperature increases with depth.
Efforts of Understanding and Detente
Produced by movements in the mantle, compress and stretch the material of the crust.
Diagenesis
Transformation of sediments into sedimentary rocks due to pressure and temperature.
Circulation
Changes experienced by the rocks subjected to high temperatures without reaching the melting point form metamorphic rocks.
Magmatic Processes
Forming a fusion of rock and magma originates igneous rocks.
The Rock Cycle
The rock cycle is the set of processes that change sediments into new rocks.
Geothermal Gradient
The temperature rise that is produced inside the Earth is called the geothermal gradient.
Impacts of Meteorites
- The kinetic energy is transformed into heat energy.
- The disintegration of radioactive elements produces electrons and neutrons at high speeds.
- The settling of denser material.
Composition and Structure of Earth
Crust
- Continental Crust: Composed of granite, with variable thickness of sedimentary rocks and unconsolidated sediments covering many parts of its surface.
- Oceanic Crust: Composed of basalt and gabbro, forming the sea beds.
Mantle
- Upper Mantle: Extends from the base of the crust up to 670 km deep.
- Lower Mantle: Extends from 670 km to the outer core.
Core
- Outer Core: Liquid layer surrounding the inner core.
- Inner Core: A solid sphere of about 1220 km in radius.
The Lithosphere
The outer part of the upper mantle is firmly bonded to the surface, forming a rigid assembly.
Continental Lithosphere
Consists of continental crust and the upper part of the mantle.
Oceanic Lithosphere
Consists of part of the oceanic crust and the upper mantle.
Wegener’s Theory
In 1912, Alfred Wegener, in his book The Origin of Continents and Oceans, proposed that the continents could move and that they were united 300 million years ago, forming a single landmass called Pangaea. He suggested that the continents moved over the sea floor but could not explain the force that was able to push them, alluding to the rotation of the Earth as a potential cause.
Harry Hess’s Contribution
Harry Hess proposed that mid-ocean ridges were actually creating new areas of oceanic crust, which was pushed around by new material, causing the ocean to increase in size.
Magnetic Tape of the Seafloor
The Earth’s magnetic field is unstable, with its polarity reversing periodically, causing the south magnetic pole to take the place of the north magnetic pole and vice versa. These reversals are recorded in volcanic rocks containing minerals such as magnetite.
The Asthenosphere
Joseph Barrell suggested the existence of the asthenosphere to explain tectonic movements, which became central to the development of plate tectonics.
The Lithospheric Plates
Oceanic Plates
Composed of oceanic lithosphere, including the Pacific Plate, the Cocos Plate, and the Nazca Plate.
Continental Plates
Composed of continental lithosphere, such as the Arabian Plate.
Mixed Plates
Contain both continental and oceanic lithosphere, like the Eurasian Plate.
Major Tectonic Plates
Seven large mixed plates occupy almost the entire area: North American, South American, African, Eurasian, Pacific, Antarctic, and Australian plates.
Relative Movements at Plate Boundaries
Plate boundaries are zones where two or more plates come into contact and interact, leading to intense geological activity: volcanism, seismicity, strain, and material deformation. Types of movements between the plates include divergent, convergent, and transform.
Constructive Lips at Oceanic Ridges
Characteristics
- Fracture zones, thousands of kilometers long, where hot material rises from the mantle to the surface, causing intense fissural volcanic activity.
- Volcanism produces large volumes of basalt, generating new oceanic crust.
- Convection currents produce divergent extensional forces that tend to separate the two sides of the ridge, favoring the continuous output of basaltic magma.
- The pressure that causes the magma to rise comes from inside the two edges of the fracture, forming the topography of the ridge.
In the rift zone, the crust is thin and highly fractured. Ocean water, upon contact with rocks, is expelled at very high temperatures, forming hydrothermal vents.