Earth Structure and Plate Tectonics Notes
Inside Earth
Continental Drift
Sea Floor Spreading
Plate Tectonics
The Earth’s Layers
The Earth is composed of distinct layers. Deeper layers consist of heavier materials and are hotter, denser, and under greater pressure than outer layers. Natural forces interact with the Earth's crust, shaping landforms or natural features on the surface.
Before discussing the forces that contribute to landforms, it's important to understand the Earth's layers, which play a vital role in the formation of continents, mountains, and volcanoes.
Earth's Structure
The Earth's structure from the inside out:
Inner Core: approximately 1228 km
Outer Core
Mantle: approximately 3500 km
Crust: approximately 6340 km - 6378 km
Crust
The crust is the rigid, rocky outer surface of the Earth.
The crust consists of two types of rocks: granite, which primarily makes up the continental crust, and basalt, which makes up the oceanic crust. Basalt is denser than granite, causing the less dense continents to float on the denser oceanic plates.
The Crust
The Earth's crust is thin compared to other layers, like the skin of an apple. Oceanic crust is approximately 3-5 miles (8 kilometers) thick, whereas continental crust is approximately 25 miles (32 kilometers) thick.
The Lithospheric Plates
The Crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, semi-rigid asthenosphere.
The Lithosphere
The lithosphere is a zone of rigid, brittle rock comprising the crust and the upper portion of the mantle.
The Mantle
The mantle is the Earth's largest layer. The middle mantle comprises very hot, dense rock that flows like asphalt under heavy weight. The movement of the asthenosphere is the reason that the crustal plates of the Earth move.
The Mantle Composition
The mantle is a rocky layer beneath the crust, composed of silicon, oxygen, magnesium, iron, aluminum, and calcium.
Convection Currents
The middle mantle "flows" because of convection currents, which are caused by hot material at the deepest part of the mantle rising, cooling, and sinking in a continuous cycle.
The Outer Core
The Earth's core is a ball of very hot metals. The outer core is so hot that the metals within it are in a liquid state, composed of melted nickel and iron.
The Inner Core
The inner core has temperatures and pressures so great that its metals are squeezed together and are forced to vibrate in place like a solid.
Land and Water
Photographs from space reveal that Earth is a "watery planet."
Over 70% of the Earth’s surface is covered by water, mainly saltwater oceans and seas.
Continental Drift and Seafloor Spreading
Continental Drift Theory
In the early 1900s, Alfred Wegener proposed the continental drift theory. He suggested that there was once a single "super continent" called Pangaea and believed the continents floated on the oceanic crust. Most scientists rejected his theory due to a lack of evidence.
Pangea
Pangea broke apart over millions of years:
225 million years ago (Permian)
200 million years ago (Triassic)
135 million years ago (Jurassic)
65 million years ago (Cretaceous)
Evidence of Continental Drift
Continents fit together like a jigsaw puzzle.
Fossils match across oceans.
Rock types and mountain ranges match across oceans.
Climate Evidence (Glacial Deposits)
"Puzzle Pieces"
Continents appear to fit together like pieces of a jigsaw puzzle. For example, the shapes of South America and Africa suggest they were once connected.
Distribution of Fossils
Plant and animal fossils found on the coastlines of different continents support the idea that these continents were once joined. Examples include fossils found in Africa, India, South America, Australia, and Antarctica.
Sequence of Rocks
Similar rock patterns are found in South America, India, Africa, Antarctica, and Australia, indicating these landmasses were once connected.
Climate
Tropical plant remains (coal deposits) are found in Antarctica, and glacial deposits are found in Africa, South America, India, and Australia from the same time period, indicating a shared climate.
Sea-Floor Spreading
Ocean floor moves like a conveyor belt, carrying continents with it. New ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle, spreading out and pushing older rocks to the sides of the crack.
New ocean floor is continually added by the process of sea-floor spreading.
Evidence of Sea-Floor Spreading
Sea-Floor Spreading
Evidence from Molten Material – Rocks shaped like pillows (rock pillows) show that molten material has erupted from cracks along the mid-ocean ridge.
Mid-Ocean Ridge
The mid-ocean ridge system is the most extensive chain of mountains on Earth, with over 90% of the range lying in the deep ocean. The ridge wraps around the globe for more than km.
Mid-Ocean Ridge
Mid-ocean ridges occur along divergent plate boundaries where new ocean floor is created as plates spread apart at a rate of cm to cm per year. Molten rock ascends from tens of kilometers deep, causing volcanic eruptions of basalt and building the longest chain of volcanoes in the world.
Sea-Floor Spreading
Evidence from Magnetic Stripes – Rocks that make up the ocean floor lie in a pattern of magnetized stripes, which hold a record of the reversals in Earth’s magnetic field.
Evidence from Drilling Samples – Core samples from the ocean floor show that older rocks are found farther from the ridge, and the youngest rocks are in the center.
Sea-Floor Spreading
Harry Hess proposed sea-floor spreading in the 1960s, describing the process where new material is continually added to the ocean floor, pushing older rocks away from the ridge.
Plate Tectonics
Changes in the Earth’s surface occur so slowly that they are not immediately noticeable.
The idea that the Earth’s landmasses have broken apart, rejoined, and moved forms the plate tectonic theory.
Plate Tectonic Theory
Along the mid-ocean ridge, the seafloor is pulling apart, and the two parts are moving in opposite directions, carrying along continents and oceans. These pieces of Earth’s top layer are called tectonic plates, which move slowly but constantly equivalent to fingernail growth. Earth’s surface layers are divided into nine large plates and several smaller ones.
Plate Tectonics
According to the theory of plate tectonics, the Earth’s outer shell is not one solid piece of rock. Instead, the Earth’s crust is broken into a number of moving plates that vary in size and thickness.
The Earth's Major Plates
These plates are not anchored in place but slide over a hot, bendable layer of the mantle.
Plate Movement
Tectonic plates are always moving:
Pulling away from each other (Divergent)
Crashing head-on (Convergent)
Sliding past each other (Transform)
Divergent Boundaries
Divergent boundaries occur between two plates that are moving apart or rifting. Rifting causes seafloor spreading.
They’re Pulling Apart!
When plates pull away from one another, they form a diverging plate boundary, or spreading zone. Thingvellir, the spreading zone in Iceland, lies between the North American and Eurasian tectonic plates.
Features of Divergent Boundaries
Mid-ocean ridges
Rift valleys
Fissure volcanoes
Convergent Boundaries
Convergent boundaries exist between two plates that are colliding. There are three types.
Ocean to Continent
When an ocean plate collides with a less dense continental plate, a subduction zone forms, where the denser plate slides under the less dense plate. Volcanoes occur at subduction zones.
Continental/Oceanic Crush
Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.
Subduction Zone
Subduction zones occur at deep-ocean trenches, where oceanic crust bends and forms underwater canyons.
Subduction Zone Features
Volcanic arc
Oceanic crust
Continental crust
Lithosphere
Asthenosphere
Benioff Zone
Ocean to Ocean
When an ocean plate collides with another ocean plate, the more dense plate slides under the less dense plate, creating a subduction zone called a trench.
Oceanic-Oceanic Convergence
When both plates are oceanic, one slides under the other, often forming an island group at the boundary.
Continent to Continent
When a continental plate collides with another continental plate, have collision zones. The plates push against each other, creating mountain ranges.
Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about cm a year!
Transform Fault Boundaries
Transform fault boundaries occur between two plates that are sliding past each other, causing earthquakes along faults.
San Andreas Fault, CA
The San Andreas Fault lies on the boundary between the North American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of to centimeters each year, causing frequent earthquakes. These areas are likely to have a rift valley, earthquake, and volcanic action.