Inside Earth Continental Drift Sea Floor Spreading Plate Tectonics

The Earth’s Layers The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers. Natural forces interact with and affect the earth’s crust, creating the landforms, or natural features, found on the surface of the earth.

Before we start to look at the forces that contribute to landforms,lets look at the different layers of the earth that play a vital role in the formation of our continents, mountains, volcanoes, etc.

Crust Mantle Outer Core Inner Core

Crust crust - the rigid, rocky outer surface of the Earth, The crust is composed of two rocks. The continental crust is mostly granite. The oceanic crust is basalt. Basalt is much denser than the granite. Because of this the less dense continents ride on the denser oceanic plates.

The Crust The Earth's Crust is like the skin of an apple. It is very thin in comparison to the other three layers. The crust is only about 3-5 miles (8 kilometers) thick under the oceans (oceanic crust) and about 25 miles (32 kilometers) thick under the continents (continental crust).

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 crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere.

The Mantle The Mantle is the largest layer of the Earth. The middle mantle is composed of very hot dense rock that flows like asphalt under a heavy weight. The movement of the middle mantle (asthenosphere) is the reason that the crustal plates of the Earth move.

The Mantle Composition mantle - a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium.

Convection Currents The middle mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling and sinking again --repeating this cycle over and over.

The Outer Core The core of the Earth is like a ball of very hot metals. The outer core is so hot that the metals in it are all in the liquid state. The outer core is composed of the melted metals of nickel and iron.

The Inner Core The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place like a solid.

DID YOU KNOW?

Land and Water Photographs of the earth taken from space show clearly that it is truly a ”watery planet.” More than 70 percent of the earth’s surface is covered by water, mainly the salt water of oceans and seas.

Continental Drift and Seafloor Spreading

Continental Drift Theory In the early 1900s a German explorer and scientist (Alfred Wegener) proposed the continental drift theory. He proposed that there was once a single “super continent” called Pangaea. He believed that the continent floated on the oceanic crust (like an iceberg in the ocean). Most scientist rejected his theory due to lack of evidence.

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 look like they could be part of a giant jigsaw puzzle

Distribution of Fossils Plant and animal fossils found on the coastlines of different continents

Sequence of Rocks Same rock patterns found in South America, India, Africa, Antarctica and Australia

Climate Tropical plant remains (coal deposits) found in Antarctica Glacial deposits in Africa, South America, India, and Australia during the same time

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 1. Evidence from Molten Material – Rocks shaped like pillows (rock pillows) show that molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly

Mid-Ocean Ridge The mid-ocean ridge system is the most extensive chain of mountains on earth, but more than 90% of this mountain range lies in the deep ocean. The mid-ocean ridge wraps around the globe for more than 65,000 km like the seam of a baseball.

Mid-Ocean Ridge Mid-ocean ridges occur along the kind of plate boundary where new ocean floor is created as the plates spread apart. "divergent plate boundary." The plates spread apart at rates of 1 cm to 20 cm per year. As oceanic plates move apart, rock melts and wells up from tens of kilometers deep. Some of the molten rock ascends all the way up to the seafloor, producing enormous volcanic eruptions of basalt, and building the longest chain of volcanoes in the world!

Sea-Floor Spreading 2. 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

Sea-Floor Spreading

Sea-Floor Spreading 3. Evidence from Drilling Samples – Core samples from the ocean floor show that older rocks are found farther from the ridge; youngest rocks are in the center of the ridge

Sea-Floor Spreading Sea-Floor Spreading – Harry Hess in the 1960’s; the process that continually adds new material to the ocean floor while pushing older rocks away from the ridge

Plate Tectonics

Most of these changes in the earth’s surface takes place so slowly that they are not immediately noticeable to the human eye. The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of 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 the continents and oceans that rest on top of them. These pieces of Earth’s top layer are called tectonic plates. They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!) Currently Earth’s surface layers are divided into nine very large plates and several smaller ones.

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. The plates vary in size and thickness.

These plates are not anchored in place but slide over a hot and bendable layer of the mantle.

As mentioned earlier, those tectonic plates are always moving. pulling away from each other (Divergent) crashing head-on (Convergent) or sliding past each other. (Transform)

Divergent Boundaries Boundary 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 between the North American (left side) and Eurasian (right side) tectonic plates. January 2003.

Features of Divergent Boundaries Mid-ocean ridges rift valleys fissure volcanoes

Convergent Boundaries Boundaries between two plates that are colliding   There are 3 types…

Ocean to Continent Ocean plate colliding with a less dense continental plate Subduction Zone: where the more dense plate slides under the less dense plate VOLCANOES occur at subduction zones

Continental/Oceanic Crush Subduction – Process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle; allows part of the ocean floor to sink back into the mantle

Subduction zone Deep-Ocean Trench – Occurs at subduction zones. Deep underwater canyons form where oceanic crust bends downward

Subduction Zone

Andes Mountains, South America

Ocean to Ocean Ocean plate colliding with another ocean plate The more dense plate slides under the less dense plate creating a subduction zone called a TRENCH

And they’re both ocean plates! Converging... They crash! And they’re both ocean plates! When both are oceanic plates, one slides under the other. Often an island group forms at this boundary.

Aleutian Islands, Alaska

Continent to Continent A continental plate colliding with another continental plate Have Collision Zones: The plates push against each other, creating mountain ranges.

They Crash and are both continental plates! Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!

Transform Fault Boundaries Boundary between two plates that are sliding past each other EARTHQUAKES along faults

San Andreas Fault, CA For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California wit hearthquakes. These areas are likely to have a rift valley, earthquake, and volcanic action.

All graphics were taken from Google Images, enchanted learning, boom zone, and other educational sites. All written information was taken from Prentice Hall, World Geography, PBS.org, and other educational websites. A good website for a deeper understanding is www.observe.arc.nasa.gov/ Animations of sea-floor spreading at the mid-oceanic rifts Image: http://www2.nature.nps.gov/geology/usgsnps/animate/pltecan.html