Earth coalesced from the stellar nebula some 4.6 billion years ago, at a distance from the sun that was favorable for a wide range of elements to condense. Too near the sun, and our planet would have become a refractory cinder with water and lighter gasses baked away (like the planet Mercury). Too far from the sun, and Earth would have been a cold trap for the collection of methane and hydrogen (like the planet Jupiter).
In the last phases of gravitational coalescence of Earth, our planet was bombarded by the remnants of nebular materials and partially melted,allowing a differentiation of the interior according to density: the heavier elements generally collected at the core (iron and nickel) while the lighter materials such as silicate minerals naturally migrated to the surface. As Earth cooled, water vapor in the atmosphere condensed to form oceans, but the center of the planet remained hot, and continues to be heated by the decay of radioactive elements.
Rocks can be categorized into three main types based upon their origin. Igneous rock crystallizes at great depth in the Earth from molten magma. Sedimentary rock forms when fine-grained materials, often weathered bits of other rock, are deposited in layers by the action of wind or water. Metamorphic rock is produced by the transformation of igneous or sedimentary rock under conditions of heat or pressure, such as when the rock is buried deeply or compressed.
Continental land masses, buoyed by the relatively low density of silicate rock, essentially float above denser rock that forms the upper mantle. Long term large scale convective motion in the viscous mantle beneath the lithosphere pushes and pulls these lightweight crustal plates in a process known as plate tectonics,rearranging the positions of continents and oceans on a time scale of tens to hundreds of millions of years. The collision of oceanic and continental plates results in volcanism, earthquakes and mountain building.
An Introduction to the ABCs of Plate Tectonics (more info)
Geology: Plate Tectonics Animations ( This site may be offline. )
This Dynamic Earth: the Story of Plate Tectonics (more info)
What on Earth is Plate Tectonics? ( This site may be offline. )
How the Earth Works : Dynamic Planet (more info)
Earthquakes can be caused by a number of solid Earth processes, but the largest and most devastating quakes are the result of the movement of tectonic plates as they collide and slide past each other. Plate motion can cause stress to accumulate in the rock on either side of the boundary. When the stress exceeds the strength of the rock, the rock cracks, releasing large amounts of energy as the plate in motion suddenly moves, sending shock waves through the surrounding area, which are felt as an earthquake.
Molten rock from deep within the Earth sometimes makes its way to the surface to form a volcano, one of the most spectacular solid Earth processes to be witnessed. Volcanoes occur in specific regions of the world - at tectonic plate boundaries and intraplate "hot spots." Over 1300 volcanoes are currently active on Earth's surface today, but most of the active volcanism on our planet occurs underwater, at mid-ocean ridge spreading centers. Explosive volcanism that sends large volumes of aerosols and ash into the atmosphere can have long term impacts on Earth's climate.
Erosion and sedimentation
The solid Earth processes of erosion and sedimentation redistribute geologic materials globally, and shape the land surface that interacts with the atmosphere, oceans and life. As tectonic forces build landforms, surface processes driven by mechanical, chemical and biological weathering erode the land surface and return altered rock to ocean basins as sediments.
The sequence of rock uplift, weathering, erosion, deposition, compression and further alteration is known as the rock cycle, and is driven in large part by the dynamic motion of crustal plates. No other planet in the solar system shows evidence of active tectonism. Without the rock cycle, critical building blocks for life such as nutrients and carbon could remain locked or buried in the crust forever with little chance of being exhumed or altered.