Earth's Structure and Plate Tectonics

The Earth is composed of four main layers: the crust, the upper mantle, the lower mantle, and the core.
The crust is the outermost layer and is divided into oceanic and continental crust.
The upper mantle is a solid layer that flows slowly due to convection currents.
The lower mantle is denser and hotter than the upper mantle with higher pressure.
The core is the Earth's innermost layer, composed of a solid inner core and a liquid outer core.

Fossil Distribution: Identical fossils of extinct species have been found on continents separated by vast oceans.
Continental Fit: The continents appear to fit together like puzzle pieces.
Seafloor Spreading: Mid-ocean ridges are spreading centers where new oceanic crust is formed, and magnetic patterns on the seafloor indicate this process.
Geological Features: The distribution of volcanoes, earthquakes, and mountain ranges along plate boundaries provides evidence for tectonic activity.

Convection Currents: The Earth's internal heat drives convection currents within the mantle. These currents cause the plates to move slowly over time.
Slab Pull: Dense oceanic plates sink at subduction zones, pulling the rest of the plate behind them.
Ridge Push: The force exerted by the rising magma at mid-ocean ridges pushes the plates away from each other.

Formation of New Crust: Molten rock, or magma, rises from the mantle at mid-ocean ridges, where it cools and solidifies, forming new oceanic crust.
Magnetic Stripes: As new crust forms, it records the Earth's magnetic field, resulting in alternating stripes of reversed and normal polarity.
Seafloor Spreading Rates: The rate of seafloor spreading varies at different locations.

Divergent Boundaries: Plates move apart, resulting in the creation of new crust.
- Features: Mid-ocean ridges, rift valleys, volcanic activity.
- Example: The Mid-Atlantic Ridge.
Convergent Boundaries: Plates collide, resulting in the destruction of crust.
- Features: Subduction zones, mountain ranges, trenches, volcanoes.
- Example: The Andes Mountains.
Transform Boundaries: Plates slide past each other horizontally.
- Features: Earthquakes, no creation or destruction of crust.
- Example: The San Andreas Fault.

Plate boundaries are responsible for most of the Earth's seismic (earthquake) and volcanic activity.
Earthquakes occur when plates shift or grind against each other, releasing energy.
Volcanic eruptions occur when magma rises to the surface, often at convergent or divergent boundaries.

Active Volcano: A volcano that has erupted recently and is likely to erupt again.
Dormant Volcano: A volcano that has not erupted recently but is expected to erupt in the future.
Extinct Volcano: A volcano that is unlikely to erupt again.

The Earth is composed of four major layers: the crust, mantle, outer core, and inner core.
The crust is the outermost layer, divided into oceanic crust (thinner and denser) and continental crust (thicker and less dense).
The mantle is the thickest layer, primarily composed of solid rock that can behave plastically over long periods.
The outer core is a liquid layer of iron and nickel, generating Earth's magnetic field.
The inner core is solid iron and nickel, with extreme pressure and temperature.

Seafloor Spreading: Mid-ocean ridges, magnetic striping patterns on the ocean floor, and the age of ocean crust increase away from ridges provide evidence for seafloor spreading.
Continental Drift: The fit of continents, similar geological structures on different continents, and fossil distribution supports the idea of continents moving apart.
Plate Boundaries: The presence of earthquakes and volcanoes concentrated along specific zones on the Earth's surface indicates the existence of plate boundaries.

Convection Currents in the Mantle: Heat from the Earth's core drives convection currents in the mantle.
Ridge Push: The elevated mid-ocean ridges exert a force pushing plates apart, resulting in seafloor spreading.
Slab Pull: The denser oceanic plates sink (subduct) beneath continental plates, pulling the rest of the plate along.

Mid-Ocean Ridges: Underwater mountain ranges where new oceanic crust is formed.
Magmatic Activity: Molten rock (magma) rises from the mantle, cools, and solidifies, forming new oceanic crust.
Magnetic Striping: As new crust forms, it records the Earth's magnetic field, creating symmetrical patterns of alternating magnetic polarities on either side of the ridge.

Divergent Boundaries: Plates move apart, creating new crust (e.g., mid-ocean ridges, rift valleys).
Convergent Boundaries: Plates collide, resulting in the formation of mountains, trenches, and volcanic arcs (e.g., subduction zones, continental collisions).
Transform Boundaries: Plates slide horizontally past each other, causing earthquakes (e.g., San Andreas Fault).

A simple diagram depicting two plates moving apart, creating a rift valley where magma rises and forms new crust.

A simple diagram depicting two plates colliding with one subducting beneath the other, leading to trench formation and volcanic arcs.

A simple diagram illustrating two plates sliding horizontally past each other, resulting in earthquakes along the boundary.

Earthquakes: Occur mainly at plate boundaries due to the release of built-up stress and friction caused by plate movements.
Volcanoes: Form at divergent and convergent boundaries, where magma rises from the mantle due to plate interactions.

Active Volcano: An active volcano has erupted recently and is likely to erupt again.
Dormant Volcano: A dormant volcano hasn't erupted recently but could erupt in the future.
Extinct Volcano: An extinct volcano is unlikely to erupt again, as its magma source has solidified.