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A fine-grained extrusive igneous rock, with a silica content intermediate between that of andesite and rhyolite.

debris flow

A dangerous mixture of water, mud, rocks, trees, and other debris that can move quickly down valleys. Such flows can result from sudden rainstorms or snowmelt that create flash floods. Areas that have experienced a recent wildfire are particularly vulnerable to debris flows, since there is no vegetation to hold the soil.

degrade (energy)

The transformation of energy into a form in which it is less available for doing work, such as heat.

delta, deltaic

A typically wedge-shaped deposit formed as sediment is eroded from mountains and transported by streams across lower elevations. The Mississippi Delta is a modern delta containing sediment being transferred from the Mississippi River into the Gulf of Mexico.

dendritic drainage

A drainage pattern where many smaller streams join and contribute to ever larger streams. The pattern looks somewhat like a tree, in which smaller branches connect to progressively larger branches.


A physical property of minerals, describing the mineral’s mass per volume.


A set of powerful straight-line winds that exceed 94 kilometers per hour (58 miles per hour) and can often approach 160 kilometers per hour (100 miles per hour). These powerful windstorms can travel over 400 kilometers (250 miles) and cause substantial wind damage, knocking down trees and causing widespread power outages. The lightning associated with these intense storms can cause both forest fires and house fires.
Derecho is the Spanish word for "straight ahead."


A lifting device in the form of a framework steel tower that is built over a deep drill hole, typically an oil well. An oil derrick is composed of machinery for hoisting and lowering tools required during the drilling process, and readying the well for extraction of petroleum.


A geologic time period spanning from 419 to 359 million years ago. The Devonian is also called the "age of fishes" due to the diversity of fish that radiated during this time. On land, seed-bearing plants appeared and terrestrial arthropods became established.
The Devonian is part of the Paleozoic.


A dark-gray to black, medium-grained, intrusive igneous rock consisting mainly of labradorite and pyroxene. The crystal size of diabase is medium, between that of a basalt (finely crystalline) and a gabbro (coarsely crystalline).


A mineral form of carbon, with the highest hardness of any material. Most natural diamonds are formed at high temperature and pressure deep in the Earth’s mantle.


A vertebrate animal posessing two holes behind the orbit (eye hole) in each side of its skull. Diapsids are extremely diverse; they arose in the late Carboniferous, and include all dinosaurs, birds, lizards, snakes, crocodiles, and the tuatara.


A unicellular, microscopic, usually planktonic marine alga (plant) belonging to the class Bacillariophyceae, and characterized by cell walls composed of silica usually in two asymmetrical sides.See also plant fossils.

diatomaceous earth

A light-colored porous rock composed of the shells of diatoms. Diatomaceous earth is used today in swimming pool filter systems.


A sheet of intrusive igneous or sedimentary rock that fills a crack cutting across a pre-existing rock body.

dimension stone

The commercial term applied to quarried blocks of rock cut to specific dimensions and used for buildings, monuments, facing, and curbing.


A member of a group of terrestrial reptiles with a common ancestor and thus certain anatomical similarities, including long ankle bones and erect limbs. All of the large reptile groups, including the dinosaurs, disappeared at or before the mass extinction at the end of the Cretaceous.

divergent plate boundary

An active plate boundary where two tectonic plates are pulling apart from one another, causing the mantle to well up at a rift. Mid-ocean ridges are the most common divergent boundary and are characterized by the eruption of bulbous pillow-shaped basalt lavas and hydrothermal fluids.


A carbonate mineral, consisting of calcium magnesium carbonate (CaMg(CO3)2). Dolomite is an important reservoir rock for petroleum, and also commonly hosts large ore deposits.


A rock (also known as dolomitic limestone and once called magnesian limestone) primarily composed of dolomite, a carbonate mineral. It is normally formed when magnesium bonds with calcium carbonate in limestone, forming dolomite.

double refraction

The result of light passing through a material that splits it into two polarized sets of rays, doubling images viewed through that material. For example, a single line on a sheet of paper will appear as two parallel lines when viewed through a clear calcite crystal.


A segment of the Earth’s crust that is broadly bent downward.


Unconsolidated debris transported and deposited by a glacier.

Driftless Area

A region that did not experience glaciation, located in parts of southwestern Wisconsin, eastern Minnesota, and northeastern Illinois and Iowa. This region is known as the Driftless Area since it lacks glacial deposits, which are collectively called drift. Glaciers are known to have reached all sides of the Driftless Area at various times throughout the Quaternary Ice Age, but are not known to have completely encompassed the area at any time.
The Driftless Area is also called the Paleozoic Plateau.
See also: glacier, ice age


A teardrop-shaped hill of till that was trapped beneath a glacier and streamlined in the direction of the flow of the ice moving over it. The elongation of a drumlin is an excellent clue to the direction of flow during an ice sheet ’s most recent advance.

dynamic metamorphism

See regional metamorphism