The Earth's Crust - Part 2.

Plate Tectonics.

The term "tectonics" refers to the building and movements of the Earth's crust. According the concept of plate tectonics, the solid brittle lithosphere, which comprises the crust and a thin section of the uppermost mantle, rests on and is carried along by the underlying layer of hot and flowing mantle rock, the asthenosphere. The lithosphere is made up of both continents and ocean basins. It is divided into relatively rigid sections known as "plates", and the movement of these plates relative to one another results in the formations and the modification of the Earth's major surface features.

There are about a dozen large tectonic plates. Some of the plates consist of continental and oceanic crust crust, while others are comprised exclusively of oceanic crust.

Moving Force.

The forces that drive the lithospheric plates across the Earth's surface are as yet not fully understood, but the movement is thought to be caused by convection currents in the mantle, which are driven by heat within the planet. As the plates move relative to each other, diverging, converging or moving past, major geological interactions take place along their boundaries. This is where many of the principal processes that shape the Earth's surface happen - for example, earthquakes, volcanism and the deformation of the Earth's crust that builds mountain ranges.

Divergent Boundaries.

Where plates are moving away from each other along a divergent boundary - typically a mid ocean ridge - magma (molten rock) wells up from below as the release of pressure produces partial melting of the underlying mantle. This magma, generally basaltic in composition, solidifies to create new, primarily oceanic crust. The accompanying plate movements generate a great deal of minor earthquake activity as the crust repeatedly fractures, heals and then fractures again.

Convergent Boundaries.

A convergent boundary occurs where plates are moving towards each other. Where oceanic crust meets oceanic crust, one of the plates descend beneath one another - a process called subduction. The subducted crust is recycled into the mantle, compensating for the new crust being created continually at divergent plate boundaries.

Subduction also happens where the oceanic crust of one plate meets the continental crust of another. The greater buoyancy of the continental crust prevents it from being subducted, so it is the plate carrying the oceanic crust that dives underneath. The subducted oceanic crust triggers melting at depth and molten rock rises up through the crust, erupting to form explosive volcanoes.

The numerous volcanoes found along the upper Pacific coast of the USA, such as Mount Saint Helens, are formed in this manner. If both plates meet along a continental edge, then neither can be subducted. The resulting collision causes the crust to crumple, pushing mountain ranges upwards. The Himalayan mountain range, the highest on the planet, is still being thrust upwards as the Eurasian and Indian-Australian tectonic plates collide.

Transform Faults.

As well as divergent and convergent boundaries, there is a third type of plate boundary, where two plates move past each other without creating or destroying crust, This is called a transform boundary. Large earthquakes are caused by the continuous build up and release of tension where the plates meet. California's San Andreas Fault and the North Anatolian fault system in Turkey are good examples of transform boundaries.

(Source: DK Publishing: Rocks and Minerals. Photo courtesy of Bing images).

Before we go into detail about the different types of rocks and minerals, next week we will be looking at collecting rocks and minerals, and what essential equipment you need. However, here at the Emporium, we've taken the hard work out of collecting your beloved Marble Eggs. All you have to do is click and buy!