Plate Tectonics Many forces cause the surface of the Earth to change over time. However, the largest force that changes our planet’s surface is the movement of Earth's outer layer through the process of plate tectonics. This process causes mountains to push higher and oceans to grow wider.

As shown in this diagram, the rigid outer layer of the Earth, called the lithosphere, is made of plates which fit together like a jigsaw puzzle. These solid but lightweight plates seem to "float" on top of a more dense, fluid layer underneath.

Motions deep within the Earth carry heat from the hot interior to the cooler surface. These motions of material under the Earth's surface cause the plates to move very slowly across the surface of the Earth, at a rate of about 2 inches per year. There are several different hypotheses to explain exactly how these motions allow plates to move.

Interesting things happen at the edges of plates Interesting things happen at the edges of plates. Subduction zones form when plates crash into each other, spreading ridges form when plates pull away from each other, and large faults form when plates slide past each other. Subduction Zone Spreading Ridge Faults

How Plate Tectonics Have Shaped Our Earth

Late Precambrian Supercontinent This map illustrates the  break-up of the supercontinent, Rodinia, which formed 1100 million years ago.   The Late Precambrian was  an "Ice House" World, much like the present-day.

Early Paleozoic Era - 514 M Years Ago Animals with hard-shells appeared in great numbers for the first time during the Cambrian.  The continents were flooded by shallow seas.  The supercontinent of Gondwana had just formed and was located near the South Pole.

Mid Paleozoic Era - 390 M Years Ago By the Devonian the early Paleozoic oceans were closing, forming a "pre-Pangea".  Freshwater fish were able to migrate from the southern hemisphere continents to North America and Europe.  Forests grew for the first time in the equatorial regions of Artic Canada.

Late Paleozoic Era - 255 M Years Ago Vast  deserts covered western Pangea during the Permian as reptiles spread across the face of the supercontinent.  99% of all life perished during the extinction event that marked the end of the Paleozoic Era.

Mesozoic Era - Triassic 237 M Years Ago The supercontinent of Pangea, mostly assembled by the Triassic, allowed land animals to migrate from the South Pole to the North Pole.  By the end of the Triassic Era, Pangea began to split apart.

Mesozoic Era - Early Jurassic 195 M Years Ago By the Early Jurassic, south-central Asia had assembled.  A wide Tethys ocean separated the northern continents from Gondwana.  Though Pangea was intact, the first rumblings of continental break up could be heard. Dinosaurs spread across Pangea.

Mesozoic Era - Late Jurassic 152 M Years Ago The supercontinent of Pangea began to break apart in the Middle Jurassic.  In the Late Jurassic the Central Atlantic Ocean was a narrow ocean separating Africa from eastern North America.   Eastern Gondwana had begun to separate form Western Gondwana.

Mesozoic Era - Cretaceous 94 M Years Ago During the Cretaceous the South Atlantic Ocean opened.  India separated from Madagascar and raced northward on a collision course with Eurasia. Notice that North America was connected to Europe, and that Australia was still joined to Antarctica.

Early Cenozoic Era - 66 Million Years Ago The bull's eye marks the location of the Chicxulub impact site.   The impact of a 10 mile wide comet caused global climate changes that killed the dinosaurs and many other forms of life.  By the Late Cretaceous the oceans had widened, and India approached the southern margin of Asia.

Cenozoic Era - Eocene Period 50 M Years Ago 50 - 55 million  years ago India began to collide with Asia forming the Tibetan plateau and Himalayas.  Australia, which was attached to Antarctica, began to move rapidly northward.

Cenozoic Era - Miocene Period - 14 M Years Ago 20 million years ago, Antarctica was coverd by ice and the northern continents were cooling rapidly.  The world has taken on a "modern" look, but notice that Florida and parts of Asia were flooded by the sea.

Cenozoic Era - The Last Ice Age 18000 Years Ago When the Earth is in its "Ice House" climate mode, there is ice at the poles.  The polar ice sheet expands and contacts because of variations in the Earth's orbit. The last expansion of the polar ice sheets took place about 18,000 years ago. 

Our World 50 Million Years From Now? If we continue present-day plate motions the Atlantic will widen, Africa will collide with Europe closingthe Mediterranean, Australia will collide with S.E. Asia, and California will slide northward up the coast to Alaska.

"Pangea Ultima" will form 250 million years in the Future The next Pangea, "Pangea Ultima" will form as a result of the subduction of the ocean floor of the North and South Atlantic beneath eastern North America and South America.  This supercontinent will have a small ocean basin trapped at its center.

Rocks are not all the same! Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed. Sedimentary Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone.

Metamorphic Rocks Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble. Igneous Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian.

The Rock Cycle Rocks take different forms at different times.  A long time ago our earth was very volcanic.  As these volcanoes cooled and vast oceans swept over the earth, the cooled lava was broken or crushed into small pieces.  These small pieces were cemented together to become sedimentary rocks.  These rocks were buried and the heat and pressure changed them into metamorphic rocks.  They might even have melted and become igneous rocks once more.  As you can tell a rock may change many times and the rock you hold today may look entirely different to someone a long time from now. Look at the diagram below and see if you can trace a rock through the rock cycle.

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