The Dynamic Earth: A Journey Through Its Layers, Processes, and Hazards
The Geosphere: One Dynamic Earth
Layers of the Earth
Crust:
A thin, solid layer. There are two types: oceanic and continental.Mantle:
Composed of the rock peridotite. It is solid or rigid, but plastic.Core:
Has a metallic composition, formed by an Fe alloy with Ni-S and other elements. It is liquid.
Continental Crust
Consists mainly of granitic rocks but presents great diversity.
Types of Rocks:
- Sedimentary Rocks: Formed by the compaction and cementation of sediments. Usually occur in strata. Different types include:
- Carbonate rocks (limestone and dolomite)
- Detrital rocks (clay and sandstone)
- Evaporite rocks (salt)
- Organic rocks (coal, oil)
- Metamorphic Rocks: Formed within the crust at high pressures and temperatures that change the minerals without melting the rock (e.g., slate, marble).
- Magmatic Rocks: Originate from the cooling of molten rock (magma). They can be:
- Plutonic: Formed by the slow cooling of magma (e.g., granite)
- Volcanic: Formed by rapid cooling during volcanic eruptions (e.g., basalt)
Seismic Zones and Lithosphere
Seismic zones have a sudden change in the speed and direction of seismic wave propagation. These are seismic discontinuities and surfaces separating layers of the geosphere.
Lithosphere:
A solid, rigid, and fragile outer layer of the geosphere. It includes the crust and the upper 100 km of the mantle. It is fragmented into lithospheric plates that move laterally due to convection currents in the mantle.
Plate Tectonics
Convection Currents:
Move the plates, originating as columns of hot rock rising in a solid and plastic form called thermal plumes. When a thermal plume is located beneath the lithosphere, it causes a hot spot. In hot spots in continental lithosphere, rifting efforts occur, causing breakage (rifting).
Plate Movements at Plate Edges:
- Convergent Edges: Areas of high seismicity, often accompanied by magmatic processes and volcanism. Different types of convergent boundaries include:
- Oceanic-Oceanic: Results in an island arc (e.g., Japan)
- Oceanic-Continental: Results in a thermal orogenic belt (e.g., Andes)
- Continental-Continental: Forms a collisional orogenic belt (e.g., Himalayas)
- Divergent Edges: Mid-ocean ridges are divergent boundaries. A fissure formed in the lithosphere is a rift, a divergent boundary where there is intense basaltic volcanism. The lava forms new oceanic crust.
- Transform Edges: Spreading ridges are discontinuous structures, interrupted by transform faults. These are passive margins where magmatism and seismicity occur in areas remote from the ridge.
Energy Sources
Solar Energy:
- Drives the movement of the hydrosphere and atmosphere and supports the functioning of the biosphere.
- Along with gravity, shapes the landscape through geological agents.
- Provides the energy required for photosynthesis.
- Maintains the hydrological cycle.
Internal Energy of the Earth:
- Drives mantle convection and is manifested in seismicity, volcanism, and plate movement.
- Sources include residual heat from Earth’s formation, meteorite impacts, radioactive decay, and crystallization of iron in the inner core.
Volcanoes and Volcanic Activity
Volcanoes:
Volcanic activity is the outward expulsion of products formed from magma inside the Earth’s crust. Magma is composed of molten rock and dissolved gases.
Types of Magma:
- Basaltic Magma: Low in silica, poorly differentiated, high temperature, and very fluid. Produces Hawaiian and fissure volcanic activity.
- Andesitic Magma: Intermediate composition, higher silica content, more differentiated, medium temperature, and viscous. Originates Strombolian and Plinian activity.
- Granitic Magma: Acidic, high silica content, very low temperature, and highly viscous. Leads to explosive Plinian activity.
Types of Volcanic Activity:
- Hawaiian Activity: Located in volcanic islands on hot spots. The magma is fluid and degassing occurs without violence. The lava flows rapidly and forms extensive sheets. The volcano has a broad, shield-like shape.
- Strombolian Activity: Occurs in subduction zones. The magma is less viscous and its degassing produces jets of lava and pyroclastics. The volcano is steeper and more cone-shaped (stratovolcano).
- Plinian Activity: Occurs in subduction zones. The magma is ejected at low temperatures, forming a dome. The eruption produces a massive column of ash and gas, often leading to pyroclastic flows.
- Explosive Activity: If the magma chamber empties, it can produce a highly destructive blast.
Volcanic Hazards and Risk Management
Volcanic hazards include ash and aerosol injection, lava flows, pyroclastic flows, lahars, slope movements, burning clouds, and earthquakes.
Volcanic Risk Prevention and Prediction:
Involves hazard assessment, mapping, and implementing measures to minimize damage. This includes land management, construction regulations, evacuation plans, public information, and monitoring networks to detect precursors to eruptions (e.g., ground deformation, changes in water levels, tremors, gas emissions).