[Extrusive] [igneous rocks] form at the earth’s surface from [lava] that cools and hardens relatively quickly.
No [volcanic] [igneous rocks] are forming in Texas now. However, during [Tertiary] time, in the Big Bend area and in other parts of the Trans-Pecos country of west Texas, [lava] came to the surface and hardened. (The [physiographic outline map], [p. 42], shows where these areas are located.)
[Intrusive] [Igneous] Rocks
The cooling and hardening of hot, molten [magma] also takes place below the earth’s surface. Here, the magma cools slowly to form rocks made up of mineral grains that are large enough to be readily visible. These rocks are known as [intrusive] [igneous] rocks. We know that they are present below the surface in Texas because of wells drilled in many areas of the State. In Pecos County, a well reached [granite], an intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas have reached the granite basement rocks at much shallower depths. But not all intrusive [igneous rocks] in Texas are found underground. In the Trans-Pecos country of west Texas, in the [Balcones fault zone], and in the [Llano uplift] of central Texas, some are now seen at the surface. They, like all [intrusive rocks], were formed below the ground, but earth’s processes of uplift and erosion have gradually uncovered them.
[Intrusive] [igneous rocks] form from molten rock material ([magma]) that cools and hardens beneath the earth’s surface.
[SEDIMENTARY ROCKS]
[Sedimentary rocks] are made up of [sediments], which are rock and mineral grains that have come from weathered rocks of all kinds. Rocks are weathered when water, ice, snow, wind, and other agents cause them either to dissolve, as table [salt] does when put in water, or to break apart, as old pavement commonly does.
Soils
Some of the broken-down rocks, along with associated plant and animal matter, develop into soils. When you examine soil with a magnifying glass, you may be able to see some of the small rock and mineral grains that still remain in it. Some soils have formed on top of the rocks from which they came, and some have been moved in from another place.