Earth's Layers, Tectonic Plates and Continental Drift

• The Earth is composed of four layers: the crust, mantle, outer core, and inner core.

Earth's Layers

• The crust is the Earth's outer layer; it is thinner under the ocean and thicker on land.
• The mantle consists of soft rock due to high temperatures, but it moves slowly due to convection currents.
• The outer core, made of iron and nickel, is the Earth's only liquid layer and is magnetic and molten.
• The inner core is at the Earth's center, remains solid due to immense pressure, and doesn't melt.
• The Earth's core has a thickness of 3520 km.

Tectonic Plate Theory

• The Earth's surface is divided into a series of moving tectonic plates.
• The crust is broken into tectonic plates that float on a semiliquid magma atop the mantle and move at a rate of 1–10 cm annually.

Continental Drift Theory

• Continents are in continuous motion over millions of years.
• Alfred Wegener proposed that all continents were once joined in a supercontinent called Pangaea.

Evidence of Continental Drift

• Coastlines fit together.
• Similar fossils and rocks appear across widely spread continents.
• Glacial landforms are shared across continents.
• Rocks near ocean ridges are younger than those further away.

Seafloor Spreading Theory

• New oceanic crust forms as the middle of the ocean spreads apart.

Evidence of Seafloor Spreading

• Marie Tharp discovered the mid-Atlantic ridge, an underwater mountain range in the Atlantic Ocean's center.

Convection Currents

• Hot magma rises in the Earth's mantle, creating a ridge in the crust, and new rising magma pushes the crust to the side.
• Convection currents involve fluid movement (liquid or gas) due to temperature and density differences.

Tectonic Plate Boundaries

• The three types of tectonic plate boundaries are convergent, divergent, and transform.

Types of Convergent Plate Boundaries

• Ocean/ocean.
• Oceanic/continental.
• Continental/continental.

Types of Divergent Plate Boundaries

• Divergent ocean/ocean.
• Divergent continental/continental.

Plate Boundary Actions

• Convergent boundaries involve plates moving into each other.
• Divergent boundaries involve plates moving away from each other.
• Transform boundaries involve plates moving side to side against each other.

Convergent Boundaries

• At a convergent ocean/ocean boundary, one plate subducts under the other.
  • The subducted plate melts, and the melted material rises, forming volcanoes on the non-subducted plate, also forming a deep ocean trench.
• At a convergent continental/continental boundary, neither plate subducts; instead, they push against each other, forming mountains.
• At a convergent ocean/continental boundary, the denser oceanic plate subducts under the continental plate.
  • Volcanoes form on the continental plate, and an ocean trench forms.

Divergent Boundaries

• At a divergent ocean/ocean boundary, plates move apart, magma rises from the mantle, cools, and forms new igneous rock, creating a mid-ocean ridge.
• At a divergent continental/continental boundary, plates move apart, forming a rift valley where volcanoes can also form, eventually creating an inland sea.

Transform Boundaries

• At transform boundaries, pressure builds up and is suddenly released as an earthquake.

Earthquakes in California vs. Australia

• California experiences more earthquakes than Australia due to its location near the transform boundary of the Pacific Plate and the North American Plate, specifically the San Andreas Fault.
• Australia is located in the center of the Indo-Australian Plate, far from major active fault lines, and has less frequent plate interaction.