Earth's Layers, Plate Tectonics, and Seismic Evidence

Crust

Outermost and thinnest layer of the Earth, with an average thickness of 15 km (ranges 5-70 km).

Continental Crust

Makes up continents/landmasses, composed of granite rocks (aluminum silicates), with a density of 2.7 g/cm³, thickness of 20-70 km, age of 3.8-4 billion years old, covering 71% of Earth's surface.

Oceanic Crust

Underlies ocean floors, composed of basaltic rocks (magnesium silicates), with a density of 2.9 g/cm³, thickness of 7-10 km, age of less than 200 million years old, covering 29% of Earth's surface.

Mantle

Thickest layer of the Earth, semi-solid, rocky, and very hot, making up 80% of Earth's mass, composed of iron and magnesium silicate rocks, with temperatures ranging from 900°C to 3,700°C.

Lithosphere

Includes crust and uppermost mantle, rigid and solid, containing tectonic plates.

Asthenosphere

Located below lithosphere (upper mantle), semi-solid and flowing, allows plate movement through convection currents.

Mesosphere

Lower mantle, rigid due to high pressure, transfers heat and supports upper layers.

Core

Innermost part of the Earth, divided into Outer Core and Inner Core.

Outer Core

Liquid layer of the Earth, mainly composed of molten iron and nickel, with a thickness of 2,270 km.

Inner Core

Solid layer due to extreme pressure, composed of solid iron and nickel, with a radius of 1,220 km and a temperature of ~6,000°C.

Electric Current & Magnetic Field

Movement of molten iron in the outer core generates electric currents, creating Earth's magnetic field through a spiral/dynamo effect.

Interface

The boundary between two layers of the Earth where seismic waves behave differently.

Mohorovičić Discontinuity (Moho)

Boundary between crust and upper mantle, discovered by Andrija Mohorovičić in 1909, marking the transition from lighter crust to denser mantle.

Gutenberg Discontinuity

Boundary between lower mantle and outer core, named after Beno Gutenberg in 1913, identified from seismic wave patterns.

Lehmann Discontinuity

Boundary between liquid outer core and solid inner core, discovered by Inge Lehmann in 1936.

Primary Waves (P-waves)

Fastest seismic waves that arrive first at a seismic station, can travel through solids, liquids, and gases, with a push-pull (compressional) motion.

Secondary Waves (S-waves)

Slower seismic waves that arrive second, can travel only through solids, moving the ground side-to-side (shearing motion).

Seismic Waves

Waves that reveal Earth's interfaces; at certain depths, wave speeds change, indicating a boundary.

Continental Drift Theory

Proposed by Alfred Wegener in 1912, stating that continents once formed a single landmass (Pangaea) and drifted apart over millions of years.

Pangaea

The supercontinent that existed when Earth began, surrounded by a vast ocean called Panthalassa.

Jurassic period

A period in Earth's history occurring 145 million years ago.

Triassic period

A period in Earth's history occurring 200 million years ago.

Alfred Wegener

German geophysicist & meteorologist who proposed the Continental Drift Theory in 1912.

Apparent Fit of Coastlines

Evidence supporting Wegener's theory, where the coastlines of South America and Africa seem to match like puzzle pieces.

Fossil Evidence

Evidence supporting Wegener's theory, including fossils of Mesosaurus, Lystrosaurus, Cynognathus, and Glossopteris found on different continents.

Rock and Mountain Evidence

Evidence supporting Wegener's theory, showing similar rock layers and sequences in distant continents and matching mountain ranges.

Lack of Mechanism

Reason Wegener's theory was initially rejected; there was no clear explanation for what force moved the continents apart.

Contradicted Existing Theories

Reason Wegener's theory was initially rejected; scientists believed Earth's features came from cooling/shrinking or vertical movements, not sideways drift.

Insufficient Ocean Floor Evidence

Reason Wegener's theory was initially rejected; little was known about the ocean floor in the early 1900s.

Theory of Plate Tectonics

Theory that combines Continental Drift Theory and Seafloor Spreading Theory, stating Earth's surface is divided into tectonic plates.

Seafloor Spreading

Proposed by Harry Hess, it states that seafloor, not the continents, moves and carries continents along, forming oceanic crust at mid-ocean ridges.

Convergent Boundary

Type of plate boundary where plates collide or come together, can form mountains, cause volcanic activity, or trigger earthquakes.

Divergent Boundary

Type of plate boundary where plates move away from each other, causing magma to rise and form new oceanic crust.

Transform Boundary

Type of plate boundary where plates slide past each other horizontally, causing earthquakes as plates grind together.

Oceanic-Oceanic Boundary

Type of convergent boundary that creates subduction zones and forms volcanic island arcs.

Oceanic-Continental Boundary

Type of convergent boundary that creates subduction zones where the oceanic plate sinks beneath the continental plate.

Continental-Continental Boundary

Type of convergent boundary where no subduction occurs (same density) and mountain ranges are formed.

Dip-Slip Fault

Type of fault where movement occurs along an angle or slope.

Strike-Slip Fault

Type of fault where plates slide past each other horizontally in opposite directions.

Normal Fault

Type of fault where the hanging wall moves downward while the footwall stays in place.

Reverse Fault

Type of fault where the hanging wall moves upward while the footwall stays in place.

Thrust Fault

Type of fault similar to reverse fault, but with an angle less than 30°.